i ASSESSMENT OF DESIGN AND SHIELDING IN SOME SELECTED CONVENTIONAL X-RAY AND COMPUTED TOMOGRAPHY (CT) FACILITIES IN BURKINA FASO BY SAWADOGO, ABDOULAYE (10542172) THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON, IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF MASTER OF PHILOSOPHY IN NUCLEAR SCIENCE AND TECHNOLOGY DEPARTMENT OF MEDICAL PHYSICS OF THE SCHOOL OF NUCLEAR AND ALLIED SCIENCES, UNIVERSITY OF GHANA, LEGON JULY, 2016 University of Ghana http://ugspace.ug.edu.gh ii DECLARATION Candidate’s Declaration This thesis is the result of research work undertaken by ABDOULAYE SAWADOGO in the Department of Medical Physics, School of Nuclear and Allied Sciences, University of Ghana, under the supervision of Prof. CYRIL SCHANDORF and Prof. J.J. FLETCHER. Sign……………………………………. Date…………………………… ABDOULAYE SAWADOGO (Student) Supervisor’s Declaration We hereby declare that the preparation and presentation of this thesis were supervised in accordance with guidelines on supervision of thesis laid down by the University of Ghana. Sign……………………………………. Date…………………………… Prof. CYRIL SCHANDORF (Principal supervisor) Sign……………………………………. Date…………………………… Prof. J.J. FLETCHER (Co-Supervisor) University of Ghana http://ugspace.ug.edu.gh iii ABSTRACT The main objective of this work is to assess the integrity of structural shielding to validate the protection of the staff and the public for two selected general radiography and CT facilities. The assessment for general radiography was based on 729 examinations for facility A and 706 examinations for facility B. For computed tomography (CT) rooms, a total of 293 body and 190 head procedures were used for facility A, and 195 body and 160 head procedures for facility B. For general radiography, the workload distribution and the normalized workload per patient have been determined for each room type (floor and chest-bucky). For facility A, the normalized workload per patient was 0.96 mA min patient-1 for the floor and 1.25 mA min patient-1 for the chest-bucky, the unshielded primary air-kerma per patient for the floor was 2.56 mGy patient-1 and 3.4 mGy patient-1 for the chest-bucky in the general radiography room. The average DLP were 1830 610 mGy cm and 859 438 mGy cm for head and body respectively in the CT room. For facility B, the normalized workload per patient was 1.14 mA min patient-1 for the floor and 0.53 mA min patient-1 for the chest-bucky, the unshielded primary air-kerma per patient was 3.13 for the floor and 1.9 for the chest-bucky in general radiography room. The average DLP were 806 346 mGy cm and 305 154 mGy cm for head and body respectively in the CT room. For both of these facilities, the calculated barrier thicknesses are smaller than the existing barrier thicknesses. Dose rate measurements confirmed that the different barriers thicknesses are enough to maintain the dose received by workers and the public below the recommended dose limits. University of Ghana http://ugspace.ug.edu.gh iv DEDICATION This research work is dedicated to my lovely wife Aoua and my wonderful son Emran Daanish, my sisters Safiatou and Djamila and to my uncle Boukary. University of Ghana http://ugspace.ug.edu.gh v ACKNOWLEDGEMENTS I will give thanks to the International Atomic Energy Agency (IAEA) and the Government of Burkina Faso for giving me the opportunity to undertake this training. I reiterate my utmost gratitude to Prof C. Schandorf and Prof J. J. Fletcher, for their understanding, encouragement and personal guidance that have provided a good basis for this thesis. Thanks to Prof. Yaw Serfor-Armah, the Dean of School of Nuclear and Allied Sciences (SNAS). I thank Dr. Denis Adotey, Head of International Affairs. I acknowledge technical support received from Dr. Issiaka Sankara, Dr. Hamadou Kabore, Dr. Bernard Ouedraogo and Dr. Nina Nde Ouedraogo who allowed me to collect data in their facilities. Thanks to Mr Oubda Ismael for his support. Many thanks to the National Authority of Radiation protection and nuclear safety (Burkina Faso), especially to Mr. Zakaria Yameogo. My thanks to all those medical imaging technicians who have supported me for my data collection, especially to Mr. Aoue Kora and Mrs. Bamba Karima. Thanks to my family, friends, colleagues and lecturers of School of Nuclear and Allied Sciences, for their priceless support and encouragement. Last but not the least, to those who indirectly contributed in this research, your kindness means a lot to me. Thank you very much. University of Ghana http://ugspace.ug.edu.gh vi TABLE OF CONTENTS DECLARATION……………………………………………………………………... ii ABSTRACT………………………………………………………………………….. iii DEDICATION……………………………………………………………………….. iv ACKNOWLEDGEMENTS………………………………………………………….. v TABLE OF CONTENTS……………………………………………………………...vi LIST OF TABLES…………………………………………………………………… x LIST OF FIGURES…………………………………………………………………. xiv LIST OF ABBREVIATIONS AND SYMBOLS…………………………………...xvii CHAPTER ONE……………………………………………………………………… 1 INTRODUCTION……………………………………………………………………. 1 1.1. BACKGROUND ............................................................................................. 1 1.2. STATEMENT OF THE PROBLEM .............................................................. 2 1.3. OBJECTIVES ................................................................................................. 3 1.4. RELEVANCE AND JUSTIFICATION ......................................................... 3 1.5. SCOPE AND LIMITATION .......................................................................... 4 1.6. ORGANISATION OF THE THESIS ............................................................. 5 CHAPTER TWO……………………………………………………………………... 6 LITERATURE REVIEW…………………………………………………………….. 6 2.1. Review of Methods for design and shielding evaluation .................................... 6 2.1.1. Design and Shielding evaluation of conventional x-ray facilities ................ 6 2.1.2. Design and Shielding evaluation of Computed Tomography facilities ...... 40 2.1.3. Design and shielding evaluation of Mammography x-ray facilities ........... 45 2.2. Regulatory requirements for the design and shielding of medical x-ray facilities .................................................................................................................................. 46 2.2.1. X-ray room design ...................................................................................... 46 University of Ghana http://ugspace.ug.edu.gh vii 2.2.2. Considerations about shielding calculation ................................................ 47 CHAPTER THREE…………………………………………………………………. 49 MATERIALS AND METHODS…………………………………………………… 49 3.1. Materials ............................................................................................................ 49 3.1.1. Equipment used for medical imaging ......................................................... 49 3.1.2 Equipment for measurements ...................................................................... 53 3.1.3 Software for calculation ............................................................................... 54 3.2. Methods ............................................................................................................. 54 3.2.1. Data collection ............................................................................................ 54 3.2.2 Design of the radiography and CT rooms ................................................... 55 3.2.3. Determination of the kVp distribution of workload normalized per patient for each radiography installation .......................................................................... 60 3.2.4 Determination of the use factor for the Floor and for the chest Bucky ....... 60 3.2.5. Determination of distances from the source of primary and secondary radiation to the location of the maximally-exposed individual beyond primary and secondary barriers for each radiographic room ............................................. 61 3.2.6. Determination of distances from the isocentre of the gantry to the location of the maximally-exposed individual beyond secondary barriers for CT room in Facilities A and B ................................................................................................. 62 3.2.7 Calculation of the unshielded primary Air Kerma ...................................... 63 3.2.8. Calculation of the primary barriers thicknesses ......................................... 64 3.2.9. Calculation of the unshielded secondary Air Kerma .................................. 66 3.2.11. Calculation of the average DLP for head and for the Body for the facility A and for the facility B ............................................................................. 69 3.2.12. Calculation of the unshielded secondary air-kerma for facility A and facility B ............................................................................................................... 69 3.2.13. Calculation of secondary barriers thicknesses for facility A and for facility B ............................................................................................................... 70 3.2.14. Dose rate measurements ........................................................................ 71 University of Ghana http://ugspace.ug.edu.gh viii CHAPTER FOUR…………………………………………………………………... 73 RESULTS AND DISCUSSIONS…………………………………………………... 73 4.1. Results ............................................................................................................... 73 4.1.1. The kVp distribution of workload and normalized workload per patient .. 73 4.2.2. Unshielded primary air Kerma at 1 meter of the focal spot ....................... 78 4.2.3 Scattered air-kerma at 1 meter of the source of scatter ............................... 83 4.2.4 Leakage air-kerma at 1 meter of the source of tube .................................... 89 4.2.5. Unshielded secondary air-kerma at dS = dL = 1 meter ................................ 93 4.2.6. Primary barriers thicknesses for facility A and for facility B ..................... 94 4.2.7. Secondary barriers thicknesses for facility A and for facility B ................. 95 4.2.8. Average DLP for head and for body scan in facility A and in facility B ... 96 4.2.9. Weekly secondary air Kerma at 1 meter of the isocentre of the gantry for facility A and for facility B ................................................................................ 96 4.2.10. Secondary barriers thicknesses for facility A and for facility B ............... 97 4.2.11. Dose rate measurements ........................................................................... 97 CHAPTER FIVE…………………………………………………………………….102 CONCLUSION AND RECOMMENDATION……………………………………. 102 5.1 Conclusion ........................................................................................................ 102 5.2 Recommendations ............................................................................................ 103 REFERENCES………………………………………………………………………105 APPENDIX………………………………………………………………………... 108 A1: calibration certificate of the EBERLINE RO20 .............................................. 108 A2: Extract of CT data for body examinations in facility A .................................. 110 A3: Extract of CT data for head examinations in facility A .................................. 111 A4: Extract of CT data for body examinations in facility B .................................. 112 A5: Extract CT data for head examinations in facility B ....................................... 113 A6: Extract of data for Rad Rom (chest-bucky) examination in facility A ........... 114 University of Ghana http://ugspace.ug.edu.gh ix A7: Extract of data for Rad Rom (floor) examination in facility A ....................... 115 A8: Extract of data for Rad Rom (chest-bucky) examination in facility B............ 116 A9: Extract of data for Rad Rom (floor) examination in facility B ....................... 117 University of Ghana http://ugspace.ug.edu.gh x LIST OF TABLES Table 2.1: Some recommended occupancy factors provided by the BIR/IPEM working party [6]. ........................................................................................................ 11 Table 2.2: Transmission of primary radiation through a cassette and a film cassette plus grid (adapted from Dixon, 1994). ...................................................................... 16 Table 2.3: Factor required for the calculation of the scatter factor S from kVp and scattering angle using equation (2.6). .......................................................................... 19 Table 2.4: Design goal from NCRP-49 and NCRP-147 .............................................. 23 Table 2.5: The NCRP-147 recommended T ................................................................ 24 Table 2.6: Primary beam use factors (U) for general radiographic room determined from the survey of clinical sites (Simpkin, 1996a). ..................................................... 25 Table 2.7: Half-value layers and tenth-value layers for heavily filtered x-radiation under broad-beam condition ........................................................................................ 31 Table 2.8: The ratio of scattered to incident exposure. ................................................ 32 Table 2.9: Area from where d should be measured ..................................................... 40 Table 2.10: Recommended default DLP values per procedure provided by EC (1999) ...................................................................................................................................... 42 Table 3.1: Use factor for each Rad Room.................................................................... 60 Table 3.2: Distances from the source of primary radiation to the location of the maximally-exposed individual beyond primary barriers. ............................................ 61 Table 3.3: Distances from the source of secondary radiation to the location of the maximally-exposed individual beyond secondary barriers and the corresponding occupancy factors......................................................................................................... 62 Table 3.4: Distances from the isocentre of the gantry to the location of the maximally- exposed individual beyond secondary barriers for CT room in facilities A and B ...... 63 University of Ghana http://ugspace.ug.edu.gh xi Table 3.5: Equivalent thickness of primary beam preshielding ( ) ........................ 65 Table 3.6: Fitting parameters for transmission of broad primary X-ray beam ............ 66 Table 3.7: Fitting parameters for transmission of broad secondary X-ray beam. (Data of Simpkin, 1996). ....................................................................................................... 69 Table 3.8: Fitting parameters for transmission of secondary X-ray beam for 120 kVp (data of Simpkin, 1996). .............................................................................................. 71 Table 4.1: Workload distribution for the floor in facility A. ....................................... 74 Table 4.2: Workload distribution for the chest-Bucky in facility A. ........................... 75 Table 4.3: Workload distribution for floor in facility B. ............................................. 76 Table 4.4: Workload distribution for the chest-Bucky in facility A ............................ 77 Table 4.5: Total normalized workload per patient. ...................................................... 78 Table 4.6: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the floor for facility A. ........................................................................................... 79 Table 4.7: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the chest-Bucky for facility A. ............................................................................... 80 Table 4.8: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the floor for facility B............................................................................................. 81 Table 4.9: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the chest-Bucky for facility B. ............................................................................... 82 Table 4.10: Unshielded primary air-kerma at 1 meter for each of the radiography installation for facility A and facility B compared with standard values from NCRP- 147................................................................................................................................ 83 University of Ghana http://ugspace.ug.edu.gh xii Table 4.11: Scatter air-kerma (mGy) at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the floor of the facility A. ...................... 84 Table 4.12: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the floor of the facility A. ................................................................ 84 Table 4.13: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each interval for the chest-Bucky of the facility A. ........................ 85 Table 4.14: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the chest-bucky of the facility A. .................................................... 85 Table 4.15: scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each interval for the floor of the facility B. ..................................... 86 Table 4.16: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the floor of the facility B. ................................................................ 87 Table 4.17: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the chest-Bucky of the facility B. ................. 87 Table 4.18: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the chest-bucky of the facility B. .................................................... 88 Table 4.19: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each radiography installation within facility A and facility B. ....... 88 Table 4.20: Leakage air-kerma at 1 meter of the tube for the floor of the facility A. . 89 Table 4.21: Leakage air-kerma at 1 meter of the tube for the chest-Bucky of the facility A. ..................................................................................................................... 90 Table 4.22: Leakage air-kerma at 1 meter of the tube for the floor of the facility B. .. 91 Table 4.23: Leakage air-kerma at 1 meter of the tube for the chest-Bucky of the facility B....................................................................................................................... 92 University of Ghana http://ugspace.ug.edu.gh xiii Table 4.24: Leakage air-kerma at 1 meter of the focal spot of the tube for each radiography installation for facility A and for facility B compared with standard values from NCRP-147. ............................................................................................... 93 Table 4.25: Leakage and side-scatter and leakage and forward/backward-scatter for facility A and for facility B .......................................................................................... 94 Table 4.26: Calculated chest-Bucky wall thickness for facility A and for facility B .. 94 Table 4.27: Secondary barriers thicknesses for facility A and for facility B ............... 95 Table 4.28: Average DLP for head and for body scan in facility A and in facility B. 96 Table 4.29: Weekly secondary air-kerma at 1 meter of the isocentre of the gantry for facilities A and B. ........................................................................................................ 97 Table 4.30: Secondary barriers thicknesses for facility A and for facility B. .............. 97 Table 4.31: Dose rate at some given location for the radiography room of facility A 98 Table 4.32: Dose rate at some given location for the CT room of facility A. ............. 99 Table 4.33: Dose rate at some given location for the radiography room of facility B. .................................................................................................................................... 100 Table 4.34: Dose rate at some given location for the CT room of facility B............. 101 University of Ghana http://ugspace.ug.edu.gh xiv LIST OF FIGURES Figure 2.1: Typical design for a control booth in a radiographic X-ray room surrounded by occupied areas. Dashed lines indicated the required radiographer’s line of sight to the X-ray table and wall Bucky. The exposure switch is positioned at least 1 m from the edge of the control booth.......................................................................... 8 Figure 2.2.: Transmission of primary radiation through lead ...................................... 13 Figure 2.3: Transmission of primary radiation through concrete ................................ 13 Figure 2.4: The ratio of scatter to leakage radiation at 90 degree scattering angle. .... 18 Figure 2.5: Scatter angle and distance r from a barrier which is parallel to the central ray of the X-ray beam. ................................................................................................. 20 Figure 2.6: Variation of scatter with angle from Williams (1996). ............................. 21 Figure 2.7: flow diagram indicating steps required to adopt the BIR/IPEM working party. ............................................................................................................................ 22 Figure 2.8: Distances in which the air kerma should be calculated ............................. 26 Figure 2.9: Attenuation in lead of x-ray from 50 to 400 kVp ...................................... 28 Figure 2.10: Attenuation in concrete of x-ray generated at 50 to 300 kVp ................. 29 Figure 2.11: Relationship between the transmission factor B and the number of half- value layers, N, or tenth-value layers, n. ...................................................................... 30 Figure 2.12: Primary, scattered and leakage radiation ................................................. 33 Figure 2.13: Primary beam air-kerma per unit workload at 1 m ............. 33 Figure 2.14: The scatter fraction 10-6 per cm2 of primary beam area at 1 m. [Data of Trout and Kelley (1972) reanalysed by Simpkin and Dixon (1998) for tungsten anode, aluminium-filtered beams. Data of Simpkin (1996b) for molybdenum anode, molybdenum filtered mammographic beams]. ............................................................ 35 University of Ghana http://ugspace.ug.edu.gh xv Figure 2.15: Representative radiographic room........................................................... 39 Figure 2.16: Isodose contour map showing the form of the scatter dose distribution in the horizontal plane for a 10 mm slice through a head phantom using 120 kVp and 350 mAs. ...................................................................................................................... 43 Figure 2.17: Plan of CT room with isodose contours sketched in for a 10 mm slice using 120 kVp and 250 mAs for a phantom. ............................................................... 44 Figure 3.1: Radiographic system ................................................................................. 49 Figure 3.2 (a): The cassette reader Figure 3.2 (b): X-ray tube .... 50 Figure 3.3 (a): The control panel Figure 3.3 (b): Lead glass windows ..................... 51 Figure 3.4 (a): The radiographic system Figure 3.4 (b): Control panel ................ 52 Figure 3.5 (a) Siemens CT Machine Figure 3.5 (b) General Electric CT Machine ... 53 Figure 3.6: EBERLINE RO20 ..................................................................................... 54 Figure 3.7: General view plan of facility A ................................................................. 55 Figure 3.8: Plan drawing of the radiographic room for facility A ............................... 56 Figure 3.9: Plan drawing of CT room for facility A .................................................... 56 Figure 3.10: Elevation drawing of the radiographic room for facility A ..................... 57 Figure 3.11: General view plan for the floor of facility B ........................................... 57 Figure 3.12: General view plan for the first floor of facility B.................................... 58 Figure 3.13: Plan drawing of the radiographic room for facility B ............................. 58 Figure 3.14: Plan drawing of CT room for facility B .................................................. 59 Figure 3.15: Elevation drawing of the radiographic room for facility B ..................... 59 Figure 3.16: Water gallon on the table at 1 m of the source ........................................ 72 Figure 3.17: Water gallon on the chest-Bucky at 1 m of the source. ........................... 72 University of Ghana http://ugspace.ug.edu.gh xvi Figure 4.1: The kVp distribution of workload plot for floor (facility A). ................... 74 Figure 4.2: The kVp distribution of workload plot for chest Bucky (facility A)......... 75 Figure 4.3: The kVp distribution of workload plot for floor (facility B) ..................... 76 Figure 4.4: The kVp distribution of workload plot for chest Bucky (facility B). ........ 77 University of Ghana http://ugspace.ug.edu.gh xvii LIST OF ABBREVIATIONS AND SYMBOLS AAPM American Association of Physicists in Medicine AutoCAD Computer-aided design Scatter fraction per primary beam area at 1 m primary distance B Broad beam transmission Transmission of leakage radiation through x-ray tube housing BIR British Institute of Radiology CT Computed Tomography Computed Tomography Dose Index Volume d Distance from a radiation source to an occupied area DAP dose-area product DLP Dose-Length Product Primary distance Secondary distance ESD entrance surface dose FFD Focus-Film-Distance F Primary beam field area at primary beam distance Gy gray University of Ghana http://ugspace.ug.edu.gh xviii HVL Half-Value Layer ICRP International Commission on Radiological Protection IPEM Institute of Physics and Engineering in Medicine ISL Inverse Square Law Kerma kinetic energy released per unit mass kVp kilovolt peak Leakage air-kerma at 1 m from source Air-kerma in an occupied area due to scattered radiation Unshielded primary air kerma per patient at 1 m from the source Air-kerma at 1 m per unit workload due to primary beam mAs milliampere-seconds NCRP National on Radiation Protection and Measurements N Number of patients per week undergoing x-ray procedures P Shielding design goal QA Quality Assurance QC Quality Conrol Sv sievert SID Source to Image Distance T Occupancy (factor) University of Ghana http://ugspace.ug.edu.gh xix TVL Ten-Value Layer U Use factor W Workload Normalized workload per patient University of Ghana http://ugspace.ug.edu.gh 1 CHAPTER ONE INTRODUCTION 1.1. BACKGROUND X-rays are an important tool in medical diagnosis and therapy. However, if the x-rays are not shielded such that they only interact with the intended locations, they are potentially hazardous to the workers, patients and members of the public [1]. Often the x-ray facilities used for diagnostic purposes in several third world countries are very few compared to the demand [2]. Because of high patient workload and less preventive maintenance, breakdowns are common. In addition to that most rooms used to host x-ray facilities were not originally intended for the purposes and are often smaller than the recommended standard floor area of 38 m2 (Heath Building Note (Scottish office, 1994)) for general-purpose x-ray machines. Furthermore, the locations of x-ray machines are not optimized relative to the layouts of the rooms. The review for optimal shielding requirements in such cases is further complicated because the composition of the building materials used to construct most walls are precisely unknown. The review of radiation shielding conditions is necessary when the designing assumptions change [3]. Of more interest is the review of the secondary barriers because of the higher occupancy than previously predicted. The National Council on Radiation Protection and measurements report number 49 (NCRP49) provides the widely accepted traditional methodology for radiation shielding designing [3]. This traditional technique for designing radiation barriers may be unrealistic because the assumptions made in shielding designing do not reflect the existing situations. It has University of Ghana http://ugspace.ug.edu.gh 2 for example been established that these methods may underestimate or overestimate the scattered and leakage radiation respectively from modern x-ray units. As a result of the inherent limitations in the traditional methods, a new shielding concept recommended by NCRP-147 is therefore adopted to derive the radiation dose levels at selected points. Further to that, the NCRP-147 is chosen for use because it is one of the latest recommended shielding concepts for typical modern diagnostic x-ray and computed tomography units. The aim of this study is therefore to compare the radiation doses calculated using the NCRP-147 model and area monitoring data prior to assess the shielding adequacy of the existing barriers. 1.2. STATEMENT OF THE PROBLEM In BURKINA FASO there are currently 77 conventional diagnostic x ray machines and 18 CT machines for a population around 17000000 residents. These machines operate in public and private hospitals and most of them are more than 5 years old. Changes may occur because of the following reasons:  high patient workload  un-attenuated primary and secondary radiation paths into control areas and other areas outside the X-ray facility;  changes in equipment layout, room configuration or adjacent areas that would affect the shielding design;  Impaired shielding due to the installation of electrical outlets plumbing or air conditioning ducts and so on [4]. University of Ghana http://ugspace.ug.edu.gh 3 There is therefore the need to assess the adequacy of the shielding at the design state of general Radiography and CT facility and periodically when any of the factors mentioned above changes. Such shielding evaluations of general Radiography and CT facilities have not be done in Burkina Faso .This thesis work is intended to do a retrospective evaluation of two selected general radiography and CT facilities in Burkina Faso. The data from this study may prompt similar studies to be carried out on the facilities in country. 1.3. OBJECTIVES The main objective of this work is to assess the integrity of structural shielding in order to validate the protection of the staff and the public at two selected general radiography and CT facilities in Burkina Faso. The specific objectives will be to:  Estimate the workload spectra for the facilities selected;  Estimate shielding thicknesses for the primary and secondary barriers;  Verify adequacy of shielding design to protect staff and the public;  Make appropriate recommendations from the findings. 1.4. RELEVANCE AND JUSTIFICATION Exposure to X-ray emitted during the diagnostic imaging process presents a risk to patients, hospital staff, and the public. The effects of this exposure are known as stochastic effects and may induce cancer. The stochastic effect do not have a threshold of dose, but the probability of occurrence increases with the dose. To optimize the dose received by workers, the public and the patient, NCRP-147 [5] provides the methodology for assessing the structural shielding design for medial University of Ghana http://ugspace.ug.edu.gh 4 X-ray Imaging facilities to verify the adequacy of the shielding that the systems provided. However deficiencies in radiation shielding designs may arise due to insufficient or incorrect information available to the person preparing the design, complex operating techniques of the radiation apparatus, the repair or reinstallation of electrical outlets, plumbing or air conditioning ducts and so on can impaired the shielding integrity. For these reasons it is strongly recommended to assess the design and the shielding that will allow to optimize the dose received by the worker and the member of the public. The results of this assessment will verify the structural shielding integrity, the average equivalent dose rate for controlled and supervised areas in line with NCRP-147 Report. This is important for the purpose of optimization of protection of workers and the public. 1.5. SCOPE AND LIMITATION This study is based on theoretical approach by calculating the required barriers thicknesses for two general radiography and two computed tomography rooms and compare them with the existing barriers thicknesses. The study is also aimed to assess the measures in place to protect staff, patients and the public against the harmful effect of ionizing radiation. This work is the first attempt for the assessment of the design and shielding of diagnostic medical facility in Burkina Faso. University of Ghana http://ugspace.ug.edu.gh 5 1.6. ORGANISATION OF THE THESIS Chapter one introduces the background, the objectives, relevance and scope and limitation of the work. Chapter two presents the literature review on the methodology of radiation shielding for diagnostic imaging, based on British Institute of Radiology and the Institute of Physics and Engineering in Medicine (BIR/IPEM) works, NCRP- 49 and NCRP-147 methods. The materials and methods are described in chapter three. Chapter four summarizes the results of this thesis and the discussion on the comparatives values of the normalized workload, the primary and secondary unshielded air-kerma. The thesis ends with chapter five which provides the conclusion and some recommendations to the relevant stakeholders. University of Ghana http://ugspace.ug.edu.gh 6 CHAPTER TWO 2.0. LITERATURE REVIEW This chapter reviews the literature relevant on the methodology of radiation shielding for diagnostic imaging, based on BIR/IPEM works, NCRP-49 and NCRP-147 methods. The use of radiation apparatus for health purposes may give rise to radiation dose or dose rates above the design limits. For this reason it is required to undertake an assessment to establish if any shielding or additional shielding is required to limit radiation exposures to employees and members of the public to an acceptable level. The proper shielding of medical diagnostic x-ray rooms play a key role in protecting the workers and the members of the public. British Institute of Radiology (BIR) and the National Council on Radiation Protection and Measurements (NCRP), present guidelines for medical diagnostic x-ray shielding that form the basis for most countries in designing x-ray rooms and shielding. This chapter discuss first the review of methods for design and shielding evaluation in conventional radiography, in computed tomography (CT), and in mammography room and ends by presenting the regulatory requirements for design and shielding in medical X-ray facilities. 2.1. Review of Methods for design and shielding evaluation 2.1.1. Design and Shielding evaluation of conventional x-ray facilities University of Ghana http://ugspace.ug.edu.gh 7 2.1.1.1 Design of the conventional X-ray room Radiographic rooms are used in a more versatile manner than most other x-ray installations. Generally there are two main imaging stations: a table and a vertical Bucky stand used principally for chest radiography. Examinations on the table most commonly involve the radiation beam firing downwards with the cassette either in the Bucky tray or on the table top (generally for extremities). For these locations protection is required for the secondary radiation and for the transmitted primary radiation. The area of the wall or floor exposed to the latter component being both limited and predictable. However, other parts of the floor or walls may be exposed to transmitted primary beam, for example, to lateral views taken with the patient on the table and examination of patients on trolleys. It is likely that protection will be required from both the transmitted primary and secondary radiation component. Certain radiographic techniques may make protection from unattenuated primary radiation necessary [6]. Provision shall be made for the operator to observe and communicate with the patient on the table or at the vertical cassette assembly. The operator of a radiographic unit shall remain in a protected area (control booth) or behind a fixed shield that will intercept the incident radiation. The control booth should not be used as a primary barrier. The exposure switch shall be positioned such that the radiographer cannot make an exposure with his or her body outside of the shielding area. This is generally accomplished if the X-ray exposure switch is at least 1 m from the edge of the control booth [5]. The control booth shall consist of a permanent structure at least 2.1 m high from the floor and should contain unobstructed floor space sufficient to allow safe operation of University of Ghana http://ugspace.ug.edu.gh 8 the equipment. The booth shall be positioned so that no unattenuated primary or unattenuated single-scattered radiation will reach the operator’s position in the booth. The control booth shall have a window or viewing device that allows the operator to view the patient during all X-ray exposure performed in the room. The operator must be able to view the wall Bucky and X-ray table, as well as patients confined to stretchers. When an observation window is used, the window and frame shall provide the necessary attenuation required to reduce the air-kerma to the shielding design goal. The window(s) should be at least 45 45 cm and centred 1.5 m above the finished floor [5]. A typical design for a control booth is illustrated in Figure 2.1: Figure 2.1: Typical design for a control booth in a radiographic X-ray room surrounded by occupied areas. Dashed lines indicated the required radiographer’s line of sight to the X-ray table and wall Bucky. The exposure switch is positioned at least 1 m from the edge of the control booth. 2.1.1.2 Shielding evaluation of conventional x-ray facilities Distance from the table to the control booth Distance from the chest- bucky to the control booth University of Ghana http://ugspace.ug.edu.gh 9 2.1.1.2.1 Methodology adopted by the joint BIR/IPEM working party i. Design criteria The Ionising Radiations Regulations 1999 (IRR99) require that work involving exposure to external radiations should be performed in rooms which are provided with adequate shielding. Because of public access to the surrounding area or access of employees who are not directly involved in the work in x-ray rooms, the shielding should be design to reduce dose rates to the lowest level that is reasonably practicable. It is therefore necessary to formulate design criteria to ensure that this requirement is met [6]. The National Radiation Protection Board (NRPB) has based on [7] to recommend that the constraint on optimisation for a single source of radiation should not exceed 30% of the dose limit to public, in this case 0.3 mSv. They also emphasised that the introduction of dose constraints does not replace the requirement on operators to optimise their use of sources or their management of practices to ensure that exposures to members of the public are kept as low as reasonably achievable. Dose constraints, therefore, represent an upper bound on the outcome of any optimisation procedure. However, there is also an acceptance that on-site exposures to members of the public should not be assessed in terms of either continuous occupancy or exposure. In terms of shielding, therefore, the application of the dose constraint must be made using realistic assumptions. The Working Party considered the application of alternatives to the 0.3 mSv per annum dose constraint. If fewer than 10 exposures are made on each working day, or University of Ghana http://ugspace.ug.edu.gh 10 if the equipment is used for a restricted number of days in the year, the limitation to a time averaged dose rate 7.5 Sv h-1 would represent the design constraint [6]. ii. Occupancy factor The application of the dose constraint must be made using realistic assumptions regarding the occupancy of areas which are relevant in terms of the shielding problem. To be realistic, the occupancy factor for an area should not be considered as being an indication of the time during which it is occupied by a generic group of people (such as patients in a waiting room). Instead, it is the fraction of time spent by the single person who is there the longest. In this context, it is most likely that the critical groups for shielding purposes will not be patients or patients' visitors but non radiation workers employed by the hospital. Given this assumption, the occupancy factor is best defined as being the fraction of an 8 hours day or 2000 hour year for which a particular area is occupied by a single person. The working party provides the values specifies in Table 2.1. University of Ghana http://ugspace.ug.edu.gh 11 Table 2.1: Some recommended occupancy factors provided by the BIR/IPEM working party [6]. Location Occupancy factor Adjacent X-ray room 1 Reception area 1 Film reading area 1 Offices, shop, living quarter, children’s indoor play area, occupied space in nearby buildings 1 Staff room 1 X-ray control room 1 Nurses’ station 1 Patient examination and treatment rooms ½ Corridors 1/5 Wards, patient rooms 1/5 Toilets or bathrooms 1/10 Outdoor areas with seating 1/10 Storage rooms 1/20 Unattended vending areas 1/20 Patient changing room 1/20 Stairways 1/20 Unattended car parks 1/20 Unattended waiting rooms 1/20 iii. Workload A prerequisite to designing shielding for any x-ray facility is a knowledge of the use to which the room is going to be put and of the number of patients that are expected to be imaged in a year. This information will allow estimates of workload to be made. Without doubt, the best estimates of workload are those which take into account local practice, rather than generic figures which represent 'busy departments'. For example, the working party recommended that Dose Area Product (DAP) is used as the measure of workload. DAP can be estimated from the Entrance Surface Dose (ESD) measurement or calculation provided that relativistic field sizes and backscatter factors are available. University of Ghana http://ugspace.ug.edu.gh 12 iv. Primary radiation For primary radiation it is necessary to know which examination may involve the use of the horizontal beam. Most commonly, horizontal beams are used with the vertical Bucky with the patient standing or lying on a trolley. The vertical Bucky may be used for the following examinations:  Chest (at between 60 and 150 kVp);  Shoulder, cervical spines (at between 60 and 75 kVp);  Abdomens (70-90kVp);  Standing knees, femur (60-70 kVp);  Spines (70-90 kVp). v. Transmission calculation method The maximum transmission (B) is given by B = (2.1) In which is the annual dose constraint, is the kerma incident on the wall per week, and T is the occupancy factor. Film dose method and Entrance Surface Dose method are used for the calculation of the kerma incident on the wall per week ( ). Having the transmission, the required primary barrier thickness of lead and concrete are given through figure 2.2 and figure 2.3 respectively. University of Ghana http://ugspace.ug.edu.gh 13 Figure 2.2.: Transmission of primary radiation through lead Figure 2.3: Transmission of primary radiation through concrete University of Ghana http://ugspace.ug.edu.gh 14 a) Film dose method The first of the clinical situations is that in which the X-ray beam is entirely intercepted by the patient. The shielding calculation can then be based on the incident film kerma. 400 speed radiographic film requires, by definition, a dose of 2.5 μGy to produce a density of 1.0 plus base plus fog. Some areas of the film will have higher densities because [6]:  radiologists generally prefer somewhat darker films;  the beam may be larger than the body part (for example for extremities), and  There are density variations across the film. In addition slower films may be used for extremities (although for protection purposes, these are associated with low kVps and are therefore of much less relevance). When 400 speed film is used, it is proposed that a value of 10 μGy is taken as the maximum air-kerma incident on any part of the film cassette. If 200 speed class film is used, 20 μGy should be used. The use of 10 μGy as the kerma to the film is indicative of significant attenuation of the primary beam through the patient. Patient attenuation will vary from about 10-3 for a lateral view of the lumbar spine with an Entrance Surface Dose (ESD) of 10 mGy to 0.1 for a chest radiograph with an ESD of 0.1 mGy. Making the assumption that the beam is fully collimated to the area of the cassette, there is then further attenuation produced in the cassette itself and in the structure of the cassette holder and the table base or vertical Bucky stand. The kerma at the wall is given by: University of Ghana http://ugspace.ug.edu.gh 15 (2.2) In which n is the number of films per week, (=10 ) is the film kerma, is the transmission through the film and cassette, d is the distance from the film to the wall, and FFD is the focus-film distance. b) Entrance Surface Dose method The second situation occurs where the beam passes outside the patient in, for example, skull radiography or extremity examinations. In this case it is recommended that the Kerma is calculated from ESD, taking into consideration primary attenuation factors for the cassette, table assembly, etc. The primary Kerma at the wall per film (K) in the absence of the Bucky can be calculated from the inverse square law (ISL): (2.3) In which ESD is the entrance surface dose, is the entrance surface to film distance and is the film-to-wall distance. Knowing the number of radiographic film in a week, the kerma incident in the wall per week ( ) can easily be calculated by multiplying the kerma at the wall per film (K) by the number of radiographic film per week. The transmission of the primary beam through a cassette and a film cassette is show in Table 2.2. University of Ghana http://ugspace.ug.edu.gh 16 Table 2.2: Transmission of primary radiation through a cassette and a film cassette plus grid (adapted from Dixon, 1994). 60 kVp 80 kVp 100 kVp 125 kVp Primary transmission (cassette) 0.055 0.11 0.16 0.21 Lead equivalence of cassette (mm) 0.14 0.17 0.19 0.21 Primary transmission (cassette + grid) 0.028 0.068 0.10 0.14 Lead equivalence of cassette + grid (mm) 0.19 0.23 0.26 0.29 1. Geometries Attenuation of primary beam needs to be estimated for three geometries: - Table radiography with attenuation in the cassette plus table assembly; - Cross table radiography with attenuation in the cassette alone; - Vertical Bucky radiography with attenuation in the cassette plus Bucky assembly. 2. Quality Assurance (QA) Some caution is required in adopting the film dose method. Performance testing procedure, as part of an overall quality assurance programme, are carried out at increasingly frequent intervals. Many test e.g. radiographic output, can involve relatively high doses, possibly several tens of mGy, delivered directly on the table top at possibly monthly intervals. This could be the largest contribution to the dose to the floor. It is therefore important to consider whether such measurements might have any impact on the floor shielding. University of Ghana http://ugspace.ug.edu.gh 17 vi. Secondary radiation Secondary radiation comprises a scatter component and a leakage component. Both components must be taken into account when considering the transmission of the secondary radiation. It is often assumed that the scattered radiation will have the same transmission properties as the primary beam whilst the leakage component will be harder. 1) Leakage radiation In the traditional treatment of leakage radiation, it is usual to assume that all is generated at the maximum potential of the generator/tube combination. This can lead to extremely conservative design parameters given that much radiography is performed at potentials below 100 kVp whilst leakage parameters are frequently specified at 150 kVp. Simpkin and Dixon (1998) have reworked the issue of the transmission of secondary radiation to take this fact into account. In doing so, they have demonstrated that the NCRP-49 approach to leakage radiation can result in solutions which are up to 8300 times too conservative. The Figure 2.4 is a graph of data extracted from the work of Simpkin and Dixon and demonstrates the ratio of leakage to scatter at 90o for a range of accelerating potentials. The data are for a field size of 1000 cm2 and are specified at 1 metre from the sources of the scatter and leakage. The assumption is also made that protection against leakage radiation is only sufficient to ensure an air-kerma of 1 mGy/hour at 1 metre with leakage factors of 150 kVp and 3.3 mA. As has been pointed out previously the majority of x-ray tubes have more protection than this in place. In making this conservative assumption it is evident that there will be considerably less leakage radiation than scatter at commonly used energies. Consequently, the ‘add University of Ghana http://ugspace.ug.edu.gh 18 one half value layer’ approach is rejected. Instead, secondary radiation transmission curves which take into account the variation of leakage radiation are provided. Figure 2.4: The ratio of scatter to leakage radiation at 90 degree scattering angle. 2) Scatter radiation Scatter dose is a function of kVp, scattering angle, entrance surface dose, and the area of the x-ray beam. This principle was the basis of the standard equation used for scatter dose calculation given in [8]. This equation can be written as follow: (2.4) In which is the scatter air-kerma at 1 m, is the kerma free in air at the beam entry point, F cm2 is the x-ray beam area at the image receptor and a is an experimentally determined constant which is a function of kV and scattering angle. Values for a given in [8] were based on data from Trout and Kelley (1972). More recently, Williams (1996) proposed that DAP be used for scatter dose estimation DAP meters are calibrated in terms of air-kerma product which is essentially the same as KUF in equation (2.4). The only difference is that F is defined University of Ghana http://ugspace.ug.edu.gh 19 at the image receptor and not at the position at which air-kerma is specified. Williams measured scatter dose as a function of angle and kV. He defined the scatter factor, S, as: (2.5) Experimental values of S (obtained from a RANDO phantom) are plotted below as a function of angle for a range of kVps in the Figure 2.4. The S values can be approximated by the equation: (2.6) For which the fitting constants are given in the Table 2.3. Table 2.3: Factor required for the calculation of the scatter factor S from kVp and scattering angle using equation (2.6). constant value a -1.042 10-7 b 2.265 10-5 c -2.751 10-2 d 8.371 10-2 e 1.578 f 5.987 10-3 These data were derived from measured values over a kV range of 50-125 kV and scattering angles between 30o and 150o. The equation is valid for scattering angle between 30o and 150o and tube potentials between 50 kVp and 125 kVp. University of Ghana http://ugspace.ug.edu.gh 20 It is recommended that where possible, scatter dose is calculated from DAP using the scatter factor given by equation (2.6). When the central ray of the X-ray beam is parallel to a shielding barrier, the angle of scatter which is directed to the point on the barrier closest to the patient is 90o. This is the commonest geometry in the X-ray room when considering the layout of the room walls. At greater angles, the scatter kerma increases as shown in the Figure 2.6 and the distance from the patient to the barrier also increases (see figure 2.5), which introduces a compensatory decrease in kerma owing to Inverse Square Law (ISL). Combining equation (2.6) with the ISL correction as the distance r increases with , it can be shown that the maximum kerma to the barrier is at a scattering angle of 117o. By substituting this angle in equation (2.6) with the values of the fitting constants in the Table 2.3, it can be shown that the maximum scatter kerma (Smax) at a wall 1 m from the patient is given by: Smax = [(0.031 kVp) + 2.5] μGy (Gy cm2)-1 (2.7) For distances greater than 1 m, the ISL can be applied. Figure 2.5: Scatter angle and distance r from a barrier which is parallel to the central ray of the X-ray beam. University of Ghana http://ugspace.ug.edu.gh 21 Figure 2.6: Variation of scatter with angle from Williams (1996). 3) Calculation involving a mix of primary and secondary radiation On some occasions the barrier to be shielded is subject to both primary and secondary radiation. The problem becomes one of ensuring that the sum of the primary and secondary radiation transmissions through the barrier is less than 0.3 mGy. In this situation the shielding requirement for one component does not dwarf, the BIR/IPEM working party recommends that the design criterion is halved to 0.15 mGy for each component. A calculation of the amount of shielding required for each component can then be performed and the larger value chosen as the final result. This approach is inherently conservative and will tend to overprotect when one component is considerably different to the other. Figure 2.7 shows the flow diagram indicating steps required to adopt the BIR/IPEM working party methodology. University of Ghana http://ugspace.ug.edu.gh 22 No Yes Yes No Yes No Figure 2.7: flow diagram indicating steps required to adopt the BIR/IPEM working party. Primary beam? Barrier exposed to secondary? For primary (DC=0.3 mGy) Beam fully intercepted by patient? Secondary barrier (DC=0.15 mGy) Film dose method Secondary barrier (DC=0.3 mGy) Barrier dose (ISL) Barrier dose (ISL) ESD method For primary (DC=0.15 mGy) Barrier thickness (primary beam) Subtract lead equivalent of cassette or table or vertical Bucky Barrier thickness (limiting HVLS) University of Ghana http://ugspace.ug.edu.gh 23 2.1.1.2.2 Methodology adopted by the NCRP The NCRP concepts of shielding calculation depend on: • Shielding design goals (P), • Distance (d) to occupied areas, • The occupancy factor (T), • Workload and its distribution (W), • Use factor (U), • Shielding materials. 1. Shielding design goal Shielding design goals are used in the design or evaluation of barriers constructed for the protection of occupationally exposed persons and members of the public. These are based on the type of area, the occupancy and distance to the occupied area. Table 2.4 presents the shielding design goal from NCRP-49 and NCRP-147. Table 2.4: Design goal from NCRP-49 and NCRP-147 Controlled area Uncontrolled area NCRP-49 1976 50 mGy/y or 1 mGy/wk 5 mGy/y or 0.1 mGy/wk NCRP-147 2004 5 mGy/y or 0.1 mGy/wk 1 mGy/y or 0.02 mGy/wk Effect Factor of 10 decrease Factor of 5 decrease University of Ghana http://ugspace.ug.edu.gh 24 2. Occupancy (T) Occupancy is a fraction of time a particular place is occupied by staff, patients or public and has to be conservative. Table 2.5: The NCRP-147 recommended T Offices, labs, pharmacies, receptionist areas, attended waiting rooms, kids’ play areas, x-ray rooms, film reading areas, nursing stations, x-ray control rooms 1 Patient exam & treatment rooms Corridors, patient rooms, employee lounges, staff restrooms Corridor doors Public toilets, vending areas, storage rooms, outdoor areas w/ seating, unattended waiting rooms, patient holding Outdoors, unattended parking lots, attics, stairways, unattended elevators, janitor’s closets 3. Workload The workloads is calculated from the knowledge of local situations. Workload can also be expressed in the form of procedures per week. The workload for a given facility is defined as the total number of milliamperes-minutes per week that the x-ray tube is in operation. The NCRP-49 had assumed that the entire workload in an installation was performed at a single kVp, 1000 mA min wk-1 at 100 kVp. In NCRP- 147, the average workload per patient, which may include multiple exposure due to several different radiological modalities, is called the normalized workload ( ), University of Ghana http://ugspace.ug.edu.gh 25 and the total workload for a given installation is the product of the normalized workload and the weekly number of patient, N. W = N (2.8) 4. Use factors (for conventional diagnostic X-ray) The use factor (U) is the fraction of the primary beam workload that is directed toward a given primary barrier. The value of U will depend on the type of radiation installation and the barrier of concern. Table 2.6: Primary beam use factors (U) for general radiographic room determined from the survey of clinical sites (Simpkin, 1996a). Barrier Use factor (U) Apply to workload distribution Floor 0.89 Rad Room (floor or other barriers) Cross-table wall 0.09 Rad Room (floor or other barriers) Wall opposite to the cross- table wall 0.02 Rad Room (floor or other barriers) Chest image receptor 1 Rad Room (chest-bucky) The shielding requirements for the ceiling of a radiographic facility is determined by the secondary barrier rather than by the use factor, which is generally extremely low. 5. Distances The distances measured are in metres from either the primary (dpri) or secondary (dsec) radiation source to the occupied area. NCRP Report 147 recommends the following default distances: – for areas above the source (storey building or room above source): 0.5 m; – for areas behind the barrier wall: 0.3 m; and University of Ghana http://ugspace.ug.edu.gh 26 – For areas below the source: 1.7 m. These distances are chosen because it is presumed to be closest to sensitive organs of the person who normally occupy these areas. The Figure 2.8 below shows the occupied area where the kerma should be calculated. Figure 2.8: Distances in which the air kerma should be calculated 6. Formula for calculation of shielding requirements The objective of a shielding calculation is to determine the thickness of the barrier that is sufficient to reduce the air kerma in an occupied area to a value , the weekly shielding design goal modified by the occupancy factor for the area to be shielded, [5]. Broad beam transmission, B(x, m), of X rays through a shielding barrier of thickness x of a given material m is defined as the ratio of the air kerma from a broad beam to an occupied area when shielded, k(x), to that in the unshielded condition k (0): (2.9) University of Ghana http://ugspace.ug.edu.gh 27 Where k (0) is the unshielded air kerma rate; k(x), shielded air kerma rate. Transmission depends on the energies of the x rays, the thickness, the material of the shielding barrier. The transmission, B , of broad X-ray beams through a variety of shielding materials in medical x ray imaging applications has been found to be well described by a mathematical model published by Archer et al.,1983 [1]. This model has the form where x is the thickness of shielding material, and , , are fitting parameters: (2.10) This equation may be solved for the thickness x as a function of transmission B. (2.11) a) NCRP-49 method This method requires knowing the workload W, in mA-minutes per week, the use factor U, the occupancy factor T and the distance d, in metres, from the source to the occupied area. The method involve computation of an average value for the exposure per unit workload at unit distance, K, (in mGy/mA-min at 1 metre) and then using the curves to determine the thickness of lead or concrete required to reduce radiation level to the required value. i. Primary protective barriers For primary protective barriers, the value K can be calculated from the following equation: University of Ghana http://ugspace.ug.edu.gh 28 (2.12) Where: P = maximum permissible weekly exposure expressed in R/week or mGy/week. For controlled areas P = 0.04 R/week or 1 mGy/week; for uncontrolled area P = 0.002 R/week or 0.1 mGy/week. d = distance in metres from the target to the primary area. W = workload in mA-min/week U = use factor T = occupancy factor K = exposure per unit workload at unit distance, in mGy/mA-min at 1metre. Figure 2.9, 2.10 represents the attenuation curve in lead and concrete of x-ray from 50 to 400 kVp. Figure 2.9: Attenuation in lead of x-ray from 50 to 400 kVp University of Ghana http://ugspace.ug.edu.gh 29 Figure 2.10: Attenuation in concrete of x-ray generated at 50 to 300 kVp ii. Secondary protective barriers Secondary protective barriers are required to provide shielding against both leakage and scattered radiation. Since these two types of radiation are of different qualities, it is necessary to determine the barrier thickness requirements for each separately. If the computed barrier thicknesses for leakage and scatter radiation are about the same, one half-value layer should be added to the larger one to obtain the total secondary barrier thickness. If the computed leakage and scattering thicknesses differ by at least three half-value layers, the larger of the two will be adequate.  Barrier against leakage radiation To determine the barrier thickness required to protect against leakage radiation it is necessary to calculate the transmission factor, B, required to reduce the weekly University of Ghana http://ugspace.ug.edu.gh 30 exposure to p. for a diagnostic- type tube housing, where the maximum allowable leakage from the housing is 0.115 R/h at 1 m. The transmission factor is given by the following formula: (2.13) Where: d = distance in metres from the tube housing to the secondary barrier. I = tube current in milliamperes. Having calculated the transmission factor, B, the barrier thickness, as a number of half-value layers or tenth-value layers, can be determined from the Figure 2.11 below. The required barrier thickness in millimetres of lead or centimetres of concrete can be obtained from the table 2.7 below, for the appropriate energy. Figure 2.11: Relationship between the transmission factor B and the number of half-value layers, N, or tenth-value layers, n. University of Ghana http://ugspace.ug.edu.gh 31 Table 2.7: Half-value layers and tenth-value layers for heavily filtered x- radiation under broad-beam condition Tube potential Attenuation material Lead (mm) Concrete (cm) kVp HVL TVL HVL TVL 50 0.06 0.17 0.43 1.5 70 0.17 0.52 0.84 2.8 85 0.22 0.73 1.25 4.5 100 0.27 0.88 1.60 5.3 125 0.28 0.93 2.00 6.6 150 0.30 0.99 2.24 7.4 200 0.52 1.70 2.50 8.4 250 0.88 2.90 2.80 9.4 300 1.47 4.80 3.10 10.4  Barrier against scatter radiation Scattered radiation has a much lower exposure rate than that of the incident beam and usually is of lower energy. However, for X-ray equipment operating below 500 kVp it is usually assumed that the scattered X-rays have the same barrier penetrating capability as the primary beam. For X-rays generated at of less than 500 kV, the values for K can be determined from the following formula: (2.14) Where: p = design goal; d = distance in metres from the target to the scattered; D = distance in metres from the scattered to the secondary barrier; University of Ghana http://ugspace.ug.edu.gh 32 F = field area, in cm2; a = ratio of scattered to incident exposure has provided in Table 2.8. Table 2.8: The ratio of scattered to incident exposure. Tube Potential kVp Scattering angle (from central axis of beam 30o 45o 60o 90o 120o 135o 50 0.0005 0.0002 0.00025 0.00035 0.0008 0.0010 70 0.00065 0.00035 0.00035 0.0005 0.0010 0.0013 85 0.0012 0.0007 0.0007 0.0009 0.0015 0.0017 100 0.0015 0.0012 0.0012 0.0013 0.0020 0.0022 125 0.0018 0.0015 0.0015 0.0015 0.0023 0.0025 150 0.0020 0.0016 0.0016 0.0016 0.0024 0.0026 200 0.0024 0.0020 0.0019 0.0019 0.0027 0.0028 250 0.0025 0.0021 0.0019 0.0019 0.0027 0.0028 300 0.0026 0.0022 0.0020 0.0019 0.0026 0.0028 b) NCRP-147 methods The NCRP-147 distinguishes three models for diagnostic X-ray shielding:  First-principle: extension to NCRP-49  Calculation of the kerma per patient and use of transmission curves designed for particular room type.  NT/Pd2 model i) First principle extension to NCRP-49 The kerma in occupied area may have contribution from: - Primary radiation, - Scattered radiation and - Leakage radiation. University of Ghana http://ugspace.ug.edu.gh 33 Figure 2.12: Primary, scattered and leakage radiation  Primary beam model The calculation of the primary beam kerma starts with the knowing of the air-kerma per unit workload at 1 m , for three phase unit (data of Archer et al. 1994). Figure 2.13: Primary beam air-kerma per unit workload at 1 m University of Ghana http://ugspace.ug.edu.gh 34 Calculation of unshielded primary beam kerma The unshielded primary air-kerma (2.14) Calculation of the primary air-kerma due to use factor-corrected workload At distance from the focal spot of an X-ray tube, the total primary air-kerma due to use factor-corrected workload [UW (kVp)] is: (2.15) Calculation of the shielded air-kerma Behind a barrier of total thickness , whose transmission to primary X rays at this operating potential is , the shielded air-kerma is: (2.16) For the all distribution of workloads, total kerma is: (2.17)  Scattered radiation The intensity of X-rays scattered off the patient is dependent on the scattering angle (defined from the direction of the centre of the primary beam to a ray pointing to the occupied area), the number of primary photons incident on the patient, the primary photon beam energy, and the location of the X-ray beam on the patient. It is assumed that the number of primary photon incident on the patient varies linearly with the X- University of Ghana http://ugspace.ug.edu.gh 35 ray beam field size. The ratio of scattered to primary air-kerma, when divided by the primary beam field size at 1 m primary distance, defines the scatter fraction ( ). The scatter fraction is given by equation 2.18. (2.18) The scatter fraction is broadly distributed over a range of beam sizes with coefficients of variation on the order of 30 percent. Figure 2.14 shows scaled by 10-6 (i.e., values taken from Figure 2.14 need to be multiplied by 10-6) determined from Trout and Kelley (1972) at the mean plus one standard deviation level, as a function of scattering angle and operating potential. Figure 2.14 also shows for mammographic beams measured by Simpkin (1996b). Figure 2.14: The scatter fraction 10-6 per cm2 of primary beam area at 1 m. [Data of Trout and Kelley (1972) reanalysed by Simpkin and Dixon (1998) for tungsten anode, aluminium-filtered beams. Data of Simpkin (1996b) for molybdenum anode, molybdenum filtered mammographic beams]. University of Ghana http://ugspace.ug.edu.gh 36 Calculation of the scattered shielded air-kerma The scattered shielded air-kerma is calculated using equation (2.19).  Leakage radiation The leakage air-kerma rate at 1 m from the x-ray tube operated at potential kVp and tube current I is then: The unshielded leakage kerma KL (at 1 m) at a given kVp is given by equation (2.20): Applying inverse square to distance from tube to shielded area and putting a barrier with transmission Expo (-ln (2) x /HVL) between tube and area yields is. The total leakage air-kerma equal to the sum of over the operating potentials in the workload: University of Ghana http://ugspace.ug.edu.gh 37 ii) NCRP-147 shielding model no 2 For each clinical workload distribution, of total workload per patient, for both primary and secondary barriers, NCRP-147 provided:  , the kerma per patient at 1 m distance  , the transmission of the radiation generated by this workload distribution for primary or secondary barriers.  Calculation of the air-kerma The weekly unshielded primary air-kerma [ (0)] in the occupied area due to patients examined per week is: (2.23) Where is the unshielded primary air-kerma per patient at 1 m for each of the workload distributions and is the distance (in meters) from the x-ray tube to the occupied area.  Calculation of the transmission (2.24) Where is the thickness of shielding material, is the equivalent thickness of preshielding material, P is the Maximum Permissible Dose, is the distance from source to the primary protected barrier, N is number of patients examined in a week, University of Ghana http://ugspace.ug.edu.gh 38 T is the occupancy factor, U is the use factor and is the unshielded primary air- kerma per patient at a distance of 1 meter. The thickness is then: (2.25) For the secondary barrier: The transmission is: (2.26) is the unshielded secondary air-kerma The required thickness is then: (2.27) iii. NCRP-147 shielding model no 3 Using XRAYBARR NCRP shows barrier thickness requirement calculated for representative rooms: - Assume conservatively small room layout - Presumes that the kinds of exposure made amongst the various X-ray tubes/position follow those observed by the AAPM TG-9 survey. University of Ghana http://ugspace.ug.edu.gh 39 Figure 2.15: Representative radiographic room. From model 2 transmission requirement is: (2.28) The barrier thickness requirement must scale as:  Description of the method  Calculate  Look up the required barrier thickness on the appropriated graph for that workload distribution, barrier and barrier material. University of Ghana http://ugspace.ug.edu.gh 40 From where is d measured? Table 2.9: Area from where d should be measured Primary Barriers Floor Overhead radiographic tube Chest Bucky wall Chest tube (72`` SID) Cross-table Lateral wall Cross-table tube (40`` SID) 2% U wall Centre of table Secondary Barriers Floor Patient on table Chest Bucky secondary wall Chest tube (72`` SID) Secondary wall Patient on table Ceiling Patient on table 2.1.2. Design and Shielding evaluation of Computed Tomography facilities 2.1.2.1. Design of Computed Tomography unit Computed tomography (CT) employs a collimated X-ray fan-beam that is intercepted by the patient and by the detector array. Consequently, only secondary radiation is incident on protective barriers. The operating potential, typically in the range of 80 to University of Ghana http://ugspace.ug.edu.gh 41 140 kVp, as well as the workload are much higher than for general radiography or fluoroscopy. Due to the potential for a large amount of secondary radiation, floors, walls and ceilings need special consideration. Additionally, scattered and leakage radiation levels along the axis of the patient table, the model used in the NCRP-147 assumes a conservatively safe isotropic scattered-radiation distribution. This is an important consideration if a replacement unit has a different orientation. 2.1.2.2. Calculation of the barriers thicknesses  Dose-Length Product Method Computed tomography is currently undergoing rapid and significant change. Several CT manufactures now display DLP values or CTDIvol for a given scan series on the scanner monitor (Nagel, 2002) where:  DLP (Dose-Length Product) = CTDIVOL * L (2.29)  (2.30) k1sec(head) = khead DLP(head) (2.31) k1sec(body) = 1.2 kbody * DLP (body) (2.32) k1sec is the secondary air-kerma at 1 m from the isocentre. The table below gives the currently recommended default DLP values per procedure for use as a guide in planning shielding in cases where facility-specific DLP values are not available. University of Ghana http://ugspace.ug.edu.gh 42 Table 2.10: Recommended default DLP values per procedure provided by EC (1999) Procedure DLP(mGy) Head 1200 Chest 525 Abdomen 625 Pelvis 500 Body average(chest, abdomen or pelvis) 550  The isodose Map Method Information on scatter levels provided by manufacturers usually takes the form of isodose curves for a single slice using particular scan parameters and phantoms. Two drawing are required, one in the horizontal plane (floor plan) and one in the vertical plane (elevation). Sometimes only a single contour at a particular dose level is provided or a sequence of doses measured at a range of positions at the same distance from the isocentre according to an ISL at distances beyond the limit of dose contour plots. The decline in scatter with distance from the isocentre may be plotted for critical directions to determine the dose level at relevant boundaries. Scatter diagrams provided by X-ray companies would normally have been produced using standard PMMA phantoms representing the head (16 cm diameter) and body (32 cm diameter). Scatter from a body phantom may be 20-100% higher than that from a head phantom when the same exposure factors are used, as a result of the greater volume of tissue irradiated and the use of different filter options. Self- shielding by the body is generally not included and may reduce the dose by 50% in certain directions. However, it is not recommended that adjustments are made to University of Ghana http://ugspace.ug.edu.gh 43 allow for this, since it will depend on body size and the scanner will be used to scan phantoms during QA tests. In order to estimate dose levels from scatter plots, the workload in the Department must be predicted in terms of the scan parameters used. Figure 2.16: Isodose contour map showing the form of the scatter dose distribution in the horizontal plane for a 10 mm slice through a head phantom using 120 kVp and 350 mAs. University of Ghana http://ugspace.ug.edu.gh 44 Figure 2.17: Plan of CT room with isodose contours sketched in for a 10 mm slice using 120 kVp and 250 mAs for a phantom. Figure 2.18: elevation of CT room with isodose contours sketched in for a 10 mm slice using 120 kVp and 250 mAs for a phantom. University of Ghana http://ugspace.ug.edu.gh 45 2.1.3. Design and shielding evaluation of Mammography x-ray facilities 2.1.3.1 Design of Mammography unit Mammography is radiographic imaging of the breast. Specially- designed equipment, consisting of an X-ray tube with a molybdenum, rhodium or tungsten anode and molybdenum, rhodium or aluminium filtration. X-ray mammography possesses a number of unique properties when compared to other x-ray modalities: • The maximum energy of the primary x-ray beam is in the range 25 - 35 kV, and will typically be <30kV; • The typical unit, with a molybdenum anode and molybdenum filter is lightly filtered (0.3 - 0.4 mm Al), giving a very low average x-ray energy; • Equipment is designed so that the primary beam is constrained to fall within the area of the image receptor, and therefore in practice only scattered radiation needs to be considered. 2.1.3.2 Shielding evaluation for Mammography To determine the barrier thickness the following method can be used:  Calculate  Look up the required barrier thickness on the graph appropriate for that workload distribution, barrier and barrier material. University of Ghana http://ugspace.ug.edu.gh 46 2.2. Regulatory requirements for the design and shielding of medical x-ray facilities 2.2.1. X-ray room design Provisions for the incorporation of safety features are best made at the facility design stage (X-ray rooms and other related rooms). The three factors relevant to dose reduction (time, distance and shielding) can be combined in the design to optimize protection. Larger rooms are preferable to allow easy access for patients on a bed trolley and to reduce exposure of the staff as well as the public, and at the same time allow for patient positioning and easy movement during the procedure, which in the case of fluoroscopy helps reduce time and exposure [9]. The following are examples of safety features:  A protective barrier should be placed at the control console to shield staff, who should not need to wear protective clothing while at the console.  The design of the room should be such that the X-ray beam cannot be directed at any area which is not shielded, i.e. the dose received in this area would be unacceptable.  The X-ray room should be designed so as to avoid the direct incidence of the X- ray beam on the access doors. The doors should be made to act as a protective shield for scattered radiation and be shut when the X-ray beam is on.  The operator needs to be able to clearly observe the patient at all times during an X-ray diagnostic procedure.  A radiation warning sign should be posted on each entrance to an X-ray room as an indicator of radiation. A sign should also be posted to indicate that the X-ray room is a controlled area. University of Ghana http://ugspace.ug.edu.gh 47  A warning light should be placed at the entrance to any room where x-ray fluoroscopy or computed tomography (CT) equipment is in use. The light should be illuminated when the X-ray beam is energized.  In rooms with a heavy workload using fluoroscopy with staff close to the patients, such as rooms for interventional procedures, it is advisable that ceiling mounted protective screens and table mounted leaded curtains be installed. 2.2.2. Considerations about shielding calculation The nominal design dose (shielding design goal) parameters in occupied areas is derived by the process of constrained optimization, i.e. selecting a source related dose constraint, with the condition that the individual doses from all relevant sources is well below the dose limits for the persons occupying the area to be shielded. Shielding barriers are calculated by the attenuation they have to provide. The shielding thickness is obtained from the attenuation factor, which is required to reduce the dose that would be received by staff and the public if shielding were not present to a dose value that can be considered as acceptable, as a result of an optimization process, i.e. a nominal design dose derived by a process of optimization: (a) Doses that would be received without shielding are calculated by using tabulated workload values (mAmin per week for the most relevant beam qualities, i.e. kV and filtration), tabulated ‘use factors’ for a given beam direction (fraction of the total amount of radiation emitted in that direction) and tabulated ‘occupancy factors’ (fraction of the total exposure that will actually affect individuals at a place, by virtue of the time permanence in that place). For secondary barriers, the ‘use factor’ is always unity, since scatter and leakage radiation is propagated in all directions all the time. University of Ghana http://ugspace.ug.edu.gh 48 (b) Once the dose that would be received without shielding is known, it is necessary to calculate the attenuation that is necessary to reduce this dose to a design level or to a level that can be considered ‘optimized protection’, i.e. a dose below which additional cost and effort in shielding is not warranted by the dose being averted. This would require successive calculations to determine where this level lies. The nominal design dose in occupied areas is derived by the process of constrained optimization, i.e. selecting a source related dose constraint, with the condition that the individual doses from all relevant sources are well below the dose limits for the individuals occupying the area to be shielded. However, when using constraints for shielding calculations, consideration should be given to the remark made in ICRP Publication 33, that actual dose values to individuals are 1/10 (for equivalent dose) to 1/30 of dose values of effective dose used as shielding design parameters. This is due to a number of conservative assumptions made in the calculation. Typical conservative assumptions used in shielding design are: – Attenuation by the patient and image receptor is usually not considered; – Workload, Use and Occupancy factors are overestimated; – Staff members are always in the most exposed place of the room – Distances are the minimum possible all the time; – Leakage radiation is the maximum all the time (corresponding to the least favourable exposure factors); – Field size used for the calculation of scatter radiation is usually overestimated. – The numerical value of calculated air-kerma (in mGy) is directly ‘used’ to compare with dose limits or constraints (mSv), which are given in terms of effective dose. However, the actual effective dose is substantially lower than the air-kerma, given the dose distribution within the body for the beam qualities used in diagnostic radiology. University of Ghana http://ugspace.ug.edu.gh 49 CHAPTER THREE 3.0. MATERIALS AND METHODS This chapter presents materials and methods used for the research work. The method used is based on the NCRP-147 methodologies and the materials include those used for the medical imaging and those used for measurements. 3.1. Materials 3.1.1. Equipment used for medical imaging 1. Equipment used for radiography in facility A The equipment used for radiographic imaging in the facility A is the FUJIFILM Medical solution (FDR Smart), model E7884X. The system is composed of the radiographic table, the chest Bucky, the X-ray tube (TOSHIBA), the film printer devise and the control panel. The maximum selectable tube current is 500 mA and the maximum selectable tube voltage is 150 kVp. Figures 3.1, 3.2 (a), 3.2 (b), 3.3 (a) and 3.3 (b) show the Radiographic system and accessories for facility A. Figure 3.1: Radiographic system University of Ghana http://ugspace.ug.edu.gh 50 Figure 3.2 (a): The cassette reader Figure 3.2 (b): X-ray tube University of Ghana http://ugspace.ug.edu.gh 51 Figure 3.3 (a): The control panel Figure 3.3 (b): Lead glass windows 2. Equipment used for radiography in facility B The equipment used for radiographic imaging in the facility B is PHILIPS (MEDIO 50 CP), series 65449. The system is composed of the radiographic table, the chest Bucky, the X-ray tube (PHILIPS), the film printer device and the control panel. The maximum selectable tube current is 500 mA and the maximum selectable tube voltage is 150 kVp. Figure 3.4 (a) and (b): show the radiographic system and accessories for Facility B University of Ghana http://ugspace.ug.edu.gh 52 Figure 3.4 (a): The radiographic system Figure 3.4 (b): Control panel 3. Equipment used for CT scanner in facility A The equipment used for the CT imaging in the facility A is a product of SIEMENS, with model number 10891400 and the serial number 88020. The maximum tube voltage is 130 kVp and the maximum current is 345 mA. After each patient scan the DLP, the kVp, the mAs are recorded in the computer. 4. Equipment used for CT in facility B The device used for CT scanner is made by General Electric, the model is BRIVO CT 325, the maximum tube voltage of the equipment is 140 kVp and the maximum current is 200 mA. University of Ghana http://ugspace.ug.edu.gh 53 Figures 3.5 (a) and (b) show Siemens CT machine for facility A and General Electric CT machine for facility B respectively. Figure 3.5 (a) Siemens CT Machine Figure 3.5 (b) General Electric CT Machine 3.1.2 Equipment for measurements For the measurements of the different room dimensions the tape measure were used and for the measurement of the ambient dose behind each barrier a calibrated EBERLINE RO20 SI Ion Chamber: RO20UK-SI SN: 01092 was used. University of Ghana http://ugspace.ug.edu.gh 54 Figure 3.6: EBERLINE RO20 3.1.3 Software for calculation Excel spreadsheet was used for each calculation. AutoCAD have been used for plan and elevation drawing of each radiography and each CT room. 3.2. Methods 3.2.1. Data collection In this study, for each patient examination, the kVp, the mAs, the number of exposures (including repetitions), and the direction of the primary beam (chest Bucky or table) were recorded manually for three (3) months in radiography rooms in two (2) private medical imaging facilities. For facility A, a total of 440 patients was surveyed from the chest Bucky and 289 from the table, which gives a total of 729 patients. For facility B, a total of 365 patients was surveyed from the chest Bucky and 341 patients from the table, which gives a total of 706 patients. For these same facilities, the type of procedure (head or body), and the DLP were collected in CT rooms within the same period of three (3) months. A total of 293 body procedures and 190 head University of Ghana http://ugspace.ug.edu.gh 55 procedures were done in facility A. A total of 195 body procedures and 160 head procedures were surveyed in facility B. 3.2.2 Design of the radiography and CT rooms For the determination of occupancy factors and the primary and secondary distances, the elevation and plan drawing were done. These drawings were done by using a tape measure for the measurements of the dimensions of the rooms, the distances from the focal spot to critical points, walls thicknesses etc. After that a computer programme named AutoCAD was used to design each type of drawing, shown in Figures 3.7, 3.8, 3.9, 3.10, 3.11, 3.12, 3.13 and 3.14. Figure 3.7: General view plan of facility A University of Ghana http://ugspace.ug.edu.gh 56 Figure 3.8: Plan drawing of the radiographic room for facility A Figure 3.9: Plan drawing of CT room for facility A Wall 1 Wall 2 Wall 3 Wall 4 University of Ghana http://ugspace.ug.edu.gh 57 Figure 3.10: Elevation drawing of the radiographic room for facility A Figure 3.11: General view plan for the floor of facility B University of Ghana http://ugspace.ug.edu.gh 58 Figure 3.12: General view plan for the first floor of facility B Figure 3.13: Plan drawing of the radiographic room for facility B University of Ghana http://ugspace.ug.edu.gh 59 Figure 3.14: Plan drawing of CT room for facility B Figure 3.15: Elevation drawing of the radiographic room for facility B Wall 1 Wall 4 Wall 2 Wall 3 University of Ghana http://ugspace.ug.edu.gh 60 3.2.3. Determination of the kVp distribution of workload normalized per patient for each radiography installation For each radiography installation, the kVp, the mAs, the number of exposures (including repetitions) were collected manually. The mAs was accumulated in a 5 kVp wide bin. For the conservative safe assumption, the higher value of the kVp was considered for each bin due to the direct proportion that exists between the kVp and the kerma. The average workload for each kVp bin was calculated and the sum of the workload for each kVp distribution provided the normalized workload per patient for the radiography installation. 3.2.4 Determination of the use factor for the Floor and for the chest Bucky To determine the use factor for each direction (floor and wall supporting the chest Bucky), the total normalized workload per patient of the room was determined first, and for each direction the use factor was obtained by dividing the normalized workload per patient for the radiography installation by the total normalized workload per patient of the radiography room. The Table 3.1 provides the use factor for each Radiography (Rad) Room at facility A and B. Table 3.1: Use factor for each Rad Room. Rad Room Facility A Facility B Normalized workload Use factor Normalized workload Use factor Floor 0.96 0.43 1.14 0.68 Chest Bucky wall 1.25 0.57 0.53 0.32 University of Ghana http://ugspace.ug.edu.gh 61 3.2.5. Determination of distances from the source of primary and secondary radiation to the location of the maximally-exposed individual beyond primary and secondary barriers for each radiographic room For the wall1 (chest Bucky), the primary distance was determined by measuring the distance between the focal spot of the X-ray tube and the wall supporting the chest assembly, by adding the existing wall thickness, plus 0.3 m (distance from the wall to the occupied area). For the floor, the primary distance was determined by measuring the distance between the focal spot of the X-ray tube and the floor, by adding the existing floor thickness, plus (HC-1.7m) where HC is the height of the radiographic room in meter. Secondary barriers consist of the wall1 (chest Bucky), wall2, wall3, wall4, floor, the plate glass, and the ceiling. For each of these barriers, two component have to be taken into account; scattered and leakage radiation. Assuming as the distance from the source of scattered radiation to the secondary barrier, the distance from the tube to the secondary barrier. For the conservative safe assumption, = minimum ( ) was considered (confer figures 3.8, 3.10, 3.13, 3.14 and 3.15). Table 3.2: Distances from the source of primary radiation to the location of the maximally-exposed individual beyond primary barriers. Facility A B Floor 3.1 m 3.1 m Chest Bucky 2.1 m 2.1 m University of Ghana http://ugspace.ug.edu.gh 62 Table 3.3: Distances from the source of secondary radiation to the location of the maximally-exposed individual beyond secondary barriers and the corresponding occupancy factors. Facility A B (m) occupancy (m) occupancy Floor 2.40 1.00 2.40 1.00 Chest Bucky (wall1) (d1) 3.10 1.00 3.1 1.00 Room opposite to chest Bucky (wall2) (d2) 2.65 1.00 3.50 1.00 Cross-table lateral wall (wall4) (d4) 4.15 0.25 1.80 0.25 Secondary wall opposite to the cross-table lateral wall (wall3) (d3) 2.05 0.20 3.30 1.00 Ceiling (d5) 2.30 1.00 2.30 1.00 Control wall 1.90 1.00 2.00 1.00 3.2.6. Determination of distances from the isocentre of the gantry to the location of the maximally-exposed individual beyond secondary barriers for CT room in Facilities A and B The distances determined are shown in Table 3.4 University of Ghana http://ugspace.ug.edu.gh 63 Table 3.4: Distances from the isocentre of the gantry to the location of the maximally-exposed individual beyond secondary barriers for CT room in facilities A and B Facility A B (m) occupancy (m) Occupancy Wall 1 4.85 1.000 2.90 1.000 Wall 2 2.60 1.000 5.60 0.500 Wall 3 2.75 0.200 2.10 0.125 Wall 4 3.40 0.200 2.80 0.200 3.2.7 Calculation of the unshielded primary Air Kerma 3.2.7.1 Calculation of the primary beam Air kerma at unit workload at 1 m [ ] The primary Air kerma per unit workload for each kVp were calculated from the equation 3.1, Archer et al. (1994). [ ] (3.1) 3.2.7.2 Calculation of the primary air-kerma at 1 m for each kVp The equation 3.2 was used to calculate the primary air-kerma at 1 m for each kVp distribution. [ ] (3.2) University of Ghana http://ugspace.ug.edu.gh 64 The unshielded primary air-kerma values at 1 m from the tube were calculated using equation 3.3. (3.3) 3.2.8. Calculation of the primary barriers thicknesses For the barrier supporting the chest Bucky, the image receptor is available to provide attenuation of the primary beam before it strikes the structural barrier. The barrier thickness is then calculated using equation 3.4. (3.4) Where:  N is the number of patient examined in a week,  T is the occupancy factor,  is the distance from the focal spot of the r-ray tube to the primary barrier,  is the shielding design goal,  are fitting parameters,  is the image receptor preshielding thickness. Table 3.5 shows the minimum equivalent value of that may be used with any of the workload distribution. University of Ghana http://ugspace.ug.edu.gh 65 Table 3.5: Equivalent thickness of primary beam preshielding ( ) Application (in mm) Lead Concrete Steel Image receptor in radiographic table or wall-mounted cassette holder (attenuation by grid, cassette, and image-receptor supporting structures 0.85 72 7 Cross-table lateral (attenuation by grid and cassette only 0.30 30 2 For the floor some examination are done with attenuation device and other without attenuation. For the conservatively safe assumption it was assumed that all examination were done without preshielding material. Then the thickness is given through the equation 3.5. (3.5) Table 3.6 shows fitting parameters for transmission of broad primary X-ray beam. University of Ghana http://ugspace.ug.edu.gh 66 Table 3.6: Fitting parameters for transmission of broad primary X-ray beam Workload distribution Lead Concrete (mm-1) (mm-1) (mm -1) (mm-1) Rad Room (chest Bucky) 2.264 13.08 0.56 0.03552 0.1177 0.6007 Rad Room (floor) 2.651 16.56 0.4585 0.03994 0.1448 0.4231 3.2.9. Calculation of the unshielded secondary air-kerma 3.2.9.1. Calculation of the unshielded scatter air-kerma The unshielded scattered air-kerma at a given scatter angle at a given kVp and at a scattered radiation distance from the patient is calculated using the equation 3.6. (3.6) Where:  is the scatter fraction for a given scatter angle and a given kVp  is the primary beam field size measured in cm2  is the Source-to-image-receptor distance (SID) University of Ghana http://ugspace.ug.edu.gh 67 The scatter fraction were calculated using the equation 3.7. This equation is a work of Trout and Kelley (1972) and reanalysed by Simpkin and Dixon (1998) for tungsten anode, aluminium-filtered beams. (3.7) The total unshielded scattered air-kerma is the sum over the operating potentials (equation 3.8). (3.8) 3.2.9.2. Calculation of the unshielded leakage air-kerma The air-kerma from leakage radiation was estimated by assuming that the leakage radiation intensity with no housing is equal to that of the primary beam. For each of the two facilities under study, the chosen leakage technique factors was 150 kVp and 3.3 mA. Then the tube housing thickness required to reduce leakage radiation to the regulatory limit (0.876 mGy h-1) is 2.32 mm of lead. For each kVp interval the unshielded air-kerma is attenuated by the transmission factor and summed to obtain the unshielded leakage air-kerma. The values of the attenuation coefficient ( ) are provided in the Table 3.7. The unshielded leakage air-kerma at a given kVp and at leakage radiation distance = 1 m from the X-ray tube was calculated using the equation 3.9. (3.9) University of Ghana http://ugspace.ug.edu.gh 68 The unshielded leakage air Kerma at leakage radiation distance from the X- ray tube was obtained by summing for each kVp range (Equation 3.10). (3.10) 3.2.9.3. Calculation of the total unshielded secondary air-kerma The total unshielded secondary air-kerma was obtained using equations 3.8 and 3.10 to obtain equation 3.11. (3.11) 3.2.10. Calculation of secondary barriers thicknesses Having the secondary unshielded air-kerma at 1 m, the secondary barriers thicknesses was calculated through the equation 3.12. (3.12) Where Table 3.7 provides the fitting parameters for transmission of broad secondary X-ray beam. University of Ghana http://ugspace.ug.edu.gh 69 Table 3.7: Fitting parameters for transmission of broad secondary X-ray beam. (Data of Simpkin, 1996). Workload distribution Lead Concrete (mm-1) (mm-1) (mm -1) (mm-1) Chest-bucky 2.256 13.8 0.8837 0.03560 0.1079 0.7705 Floor 2.513 17.34 0.4994 0.03920 0.1464 0.4486 all barriers 2.298 17.38 0.6193 0.03610 0.1433 0.56 3.2.11. Calculation of the average DLP for head and for the Body for the facility A and for the facility B For each of these two facilities, patients’ examinations were divided into two:  Head scan and  Body scan. For each examination, the total DLP, the type of examination (head or body) was provided on the computer screen. For each type of examination, the DLPs recorded for three months were summed and divided by the total number of patients to obtain the average DLP. 3.2.12. Calculation of the unshielded secondary air-kerma for facility A and facility B The unshielded secondary air-kerma at 1 meter from the isocentre for head and for body were determined using the following equations: University of Ghana http://ugspace.ug.edu.gh 70  k1sec(head) = khead DLP (head) (3.13)  k1sec(body) = 1.2 kbody * DLP (body) (3.14) Where:  khead = 9x10-5 1/cm.  kbody = 3x10-4 1/cm. The total unshielded weekly secondary air Kerma at 1 meter of the isocentre was calculated through equation 3.15 (3.15) Where:  is the weekly number of patients undergoing body scan and  is the weekly number of patients undergoing head scan. 3.2.13. Calculation of secondary barriers thicknesses for facility A and for facility B The secondary barriers thicknesses was calculated through equation 3.16 (3.16) University of Ghana http://ugspace.ug.edu.gh 71 Table 3.8: Fitting parameters for transmission of secondary X-ray beam for 120 kVp (data of Simpkin, 1996). Lead concrete (mm-1) (mm-1) (mm -1) (mm-1) 2.246 5.73 0.547 0.0383 0.0142 0.658 3.2.14. Dose rate measurements The dose rate was measured using a filled 20 litres water gallon as a phantom. The gallon was located at 1 meter from the source of the radiography rooms (Figure 3.14 and 3.15). The parameters (kVp, mAs) for the most frequently used procedure were used. The measurements were done with the gallon on the table and with the gallon on the chest-Bucky. The maximum dose rate was considered for each barrier. For CT rooms the filled water gallon was placed on the table and the parameters (kVp, mAs) for the most frequently used procedure was chosen. University of Ghana http://ugspace.ug.edu.gh 72 Figure 3.16: Water gallon on the table at 1 m of the source Figure 3.17: Water gallon on the chest-Bucky at 1 m of the source. University of Ghana http://ugspace.ug.edu.gh 73 CHAPTER FOUR 4.0. RESULTS AND DISCUSSIONS This chapter presents the results on estimated normalized workload per patient for each radiographic installation (floor and chest Bucky), the corresponding primary air- kerma at 1 meter from the X-ray tube, scattered and leakage air-kerma at 1 meter from the source of scatter and leakage radiation, primary and secondary barriers thicknesses. For Computer Tomography facilities the chapter presents the average DLP values for the head and for the body as well as secondary barriers thicknesses for facilities A and B. It also discusses the results found from this work and inter- comparison with corresponding values from NCRP-147 and American Association of Physicists in Medicine (AAPM-TG9). 4.1. Results 4.1.1. The kVp distribution of workload and normalized workload per patient 4.1.1.1. The kVp distribution of workload The kVp distribution of workload shows ranges of kVp the most used for examination within each radiographic installation. Tables 4.1, 4.2, 4.3, and 4.4, and Figures 4.1, 4.2, 4.3 and 4.4 present the workload distribution, and the workload distribution plot for floor and for the chest-bucky for facility A and facility B. University of Ghana http://ugspace.ug.edu.gh 74 Table 4.1: Workload distribution for the floor in facility A. kVp Workload per kVp (W(kVp)) 40 0.00000 45 0.00878 50 0.08035 55 0.02055 60 0.03195 65 0.02652 70 0.34095 75 0.08171 80 0.13947 85 0.02560 90 0.06858 95 0.07087 100 0.01280 105 0.05213 Figure 4.1: The kVp distribution of workload plot for floor (facility A). University of Ghana http://ugspace.ug.edu.gh 75 Table 4.2: Workload distribution for the chest-Bucky in facility A. kVp Workload per kVp (W(kVp)) 50 0.00000 55 0.00557 60 0.11950 65 0.12361 70 0.31837 75 0.23557 80 0.21463 85 0.07749 90 0.02805 95 0.02802 100 0.03456 105 0.00195 110 0.01044 115 0.00787 120 0.02666 125 0.01237 130 0.00179 135 0.00000 Figure 4.2: The kVp distribution of workload plot for chest Bucky (facility A) University of Ghana http://ugspace.ug.edu.gh 76 Table 4.3: Workload distribution for floor in facility B. kVp Workload kVp (W(kVp)) 40 0.00000 45 0.00030 50 0.00828 55 0.06135 60 0.03886 65 0.32459 70 0.51168 75 0.02497 80 0.00906 85 0.00000 90 0.00406 95 0.00000 100 0.00000 105 0.00047 110 0.00000 115 0.00000 120 0.00000 125 0.15911 130 0.00000 Figure 4.3: The kVp distribution of workload plot for floor (facility B) University of Ghana http://ugspace.ug.edu.gh 77 Table 4.4: Workload distribution for the chest-Bucky in facility A kVp Workload kVp (W(kVp)) 50 0.00000 55 0.00000 60 0.00000 65 0.00665 70 0.28290 75 0.00259 80 0.01480 85 0.01473 90 0.03666 95 0.03753 100 0.04423 105 0.00155 110 0.00106 115 0.00094 120 0.00000 125 0.08923 130 0.00000 Figure 4.4: The kVp distribution of workload plot for chest Bucky (facility B). University of Ghana http://ugspace.ug.edu.gh 78 4.1.1.2. The total normalized workload per patient Table 4.5 presents the normalised workload per patient for each radiography installation (in mA min patient-1) for facility A and B, and standard values provided by the NCRP-147. Table 4.5: Total normalized workload per patient. Workload distribution Wnorm (mA min patient-1) For facility A Wnorm (mA min patient-1) For facility B Wnorm (mA min patient-1) For NCRP-147 Floor 0.96 1.14 1.90 Chest Bucky 1.25 0.53 0.60 All barriers 2.21 1.67 2.50 Table 4.5 shows that the normalized workload per patient for all barriers in facility A is lower than those for standard values from NCRP-147 but the normalized workload per patient for the chest-bucky is higher than those from standard. For facility B the normalized workload for the floor, for the chest-bucky and for all barrier are lower than those from the NCRP-147. 4.1.2. Unshielded primary air-kerma at 1 meter of the focal spot Table 4.6 shows unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the floor for facility A. University of Ghana http://ugspace.ug.edu.gh 79 Table 4.6: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the floor for facility A. kVp Workload kVp (W(kVp)) Unshielded primary air-kerma per unit workload at 1 m per kVp ( ) Unshielded primary air-kerma 1 m per kVp ( ) 40 0.00000 0.71889 0.00000 45 0.00878 0.87210 0.00766 50 0.08035 1.06550 0.08562 55 0.02055 1.29672 0.02665 60 0.03195 1.56342 0.04995 65 0.02652 1.86325 0.04941 70 0.34095 2.19385 0.74801 75 0.08171 2.55287 0.20859 80 0.13947 2.93796 0.40976 85 0.02560 3.34677 0.08569 90 0.06858 3.77695 0.25905 95 0.07087 4.22614 0.29952 100 0.01280 4.69200 0.06007 105 0.05212 5.17216 0.26960 Unshielded primary air-kerma per patient at 1 m ( mGy patient -1 ) 2.55962 Table 4.7 shows unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the chest-Bucky for facility A. University of Ghana http://ugspace.ug.edu.gh 80 Table 4.7: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the chest-Bucky for facility A. kVp Workload kVp (W(kVp)) Unshielded primary air- kerma per unit workload at 1 m per kVp ( ) Unshielded primary air-kerma 1 m per kVp ( ) 50 0.00000 1.06550 0.00000 55 0.00557 1.29672 0.00723 60 0.11950 1.56342 0.18683 65 0.12362 1.86325 0.23033 70 0.31837 2.19385 0.69845 75 0.23557 2.55287 0.60139 80 0.21463 2.93797 0.63058 85 0.07749 3.34678 0.25934 90 0.02805 3.77696 0.10595 95 0.02802 4.22615 0.11843 100 0.03456 4.69200 0.16219 105 0.00195 5.17216 0.01008 110 0.01044 5.66428 0.05915 115 0.00788 6.16601 0.04858 120 0.02667 6.67499 0.17802 125 0.01237 7.18887 0.08893 130 0.00179 7.70530 0.01385 135 0.00000 8.22193 0.00000 Unshielded primary air-kerma per patient at 1 m ( mGy patient -1 ) 3.39937 Table 4.8 shows unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the floor for facility B. University of Ghana http://ugspace.ug.edu.gh 81 Table 4.8: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the floor for facility B. kVp Workload kVp (W(kVp)) Unshielded primary air Kerma per unit workload at 1 m per kVp ( ) Unshielded primary air Kerma 1 m per kVp ( ) 40 0.00000 0.71889 0.00000 45 0.00030 0.87210 0.00026 50 0.00829 1.06550 0.00883 55 0.06135 1.29672 0.07956 60 0.03886 1.56342 0.06075 65 0.32459 1.86325 0.60481 70 0.51168 2.19385 1.12256 75 0.02497 2.55287 0.06376 80 0.00906 2.93796 0.02663 85 0.00000 3.34678 0.00000 90 0.00406 3.77695 0.01534 95 0.00000 4.22614 0.00000 100 0.00000 4.69200 0.00000 105 0.00047 5.17216 0.00244 110 0.00000 5.66428 0.00000 115 0.00000 6.16601 0.00000 120 0.00000 6.67499 0.00000 125 0.15911 7.18887 1.14384 130 0.00000 7.70531 0.00000 Unshielded primary air-kerma per patient at 1 m ( mGy patient -1 ) 3.12878 Table 4.9 shows unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the chest-Bucky for facility B. University of Ghana http://ugspace.ug.edu.gh 82 Table 4.9: Unshielded primary air-kerma per patient at 1 meter ( mGy patient -1 ) for the chest-Bucky for facility B. kVp Workload kVp (W(kVp)) Unshielded primary air- kerma per unit workload at 1 m per kVp ( ) Unshielded primary air- kerma 1 m per kVp ( ) 50 0.00000 1.06550 0.00000 55 0.00000 1.29672 0.00000 60 0.00000 1.56342 0.00000 65 0.00666 1.86325 0.01240 70 0.28291 2.19385 0.62066 75 0.00259 2.55287 0.00663 80 0.01480 2.93797 0.04349 85 0.01473 3.34678 0.04930 90 0.03666 3.77696 0.13847 95 0.03753 4.22615 0.15863 100 0.04424 4.69200 0.20757 105 0.00156 5.17216 0.00806 110 0.00106 5.66428 0.00602 115 0.00095 6.16601 0.00582 120 0.00000 6.67499 0.00000 125 0.08923 7.18887 0.64150 130 0.00000 7.70531 0.00000 Unshielded primary air-kerma per patient at 1 m ( mGy patient -1 ) 1.89855 Table 4.10 presents the unshielded primary air-kerma at 1 meter ( mGy patient -1 ) for each of the radiographic installation for facility A and facility B compared with standard values from NCRP-147. University of Ghana http://ugspace.ug.edu.gh 83 Table 4.10: Unshielded primary air-kerma at 1 meter for each of the radiography installation for facility A and facility B compared with standard values from NCRP-147. Workload distribution (mGy patient -1 ) Facility A (mGy patient -1 ) Facility B (mGy patient -1 ) NCRP-147 Floor 2.56 3.13 5.20 Chest Bucky 3.40 1.90 2.30 From Table 4.10, the unshielded primary air-kerma at 1 meter from the tube for the chest-bucky for facility A is greater than the standard value but the one for the floor is less than the standard value. For facility B, the unshielded primary air-kerma at 1 meter from the tube for the floor and for the chest-bucky are lower than those from the standard. From Table 4.5 and Table 4.10 we can see that the higher the normalized workload, the higher the unshielded primary air-kerma at 1 meter from the tube. 4.1.3 Scattered air-kerma at 1 meter of the source of scatter For the conservative safe assumption the scattered air-kerma have been calculated for 90 degree scatter and for 135 degree scatter. Table 4.11 presents the scatter air-kerma (mGy) at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the floor of the facility A. University of Ghana http://ugspace.ug.edu.gh 84 Table 4.11: Scatter air-kerma (mGy) at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the floor of the facility A. kVp Unshielded primary air Kerma 1 m per kVp ( ) a1(90, kVp) a1(135, kVp) Ks (90, kVp) Ks(135, kVp) 40 0.00000 3.77384 5.71912 0.00000 0.00000 45 0.00766 3.85384 5.79912 0.00002 0.00003 50 0.08562 3.93384 5.87912 0.00028 0.00043 55 0.02665 4.01384 5.95912 0.00009 0.00013 60 0.04995 4.09384 6.03912 0.00017 0.00026 65 0.04941 4.17384 6.11912 0.00017 0.00026 70 0.74801 4.25384 6.19912 0.00273 0.00397 75 0.20859 4.33384 6.27912 0.00077 0.00112 80 0.40976 4.41384 6.35912 0.00155 0.00223 85 0.08569 4.49384 6.43912 0.00033 0.00047 90 0.25905 4.57384 6.51912 0.00102 0.00145 95 0.29952 4.65384 6.59912 0.00119 0.00169 100 0.06007 4.73384 6.67912 0.00024 0.00034 105 0.26960 4.81384 6.75912 0.00111 0.00156 By summing the scatter air-kerma for each kVp interval we obtain the scatter air- kerma for the floor of the facility A. Table 4.12: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the floor of the facility A. Side-scatter (90) (mGy patient -1) Forward/backward-scatter (135) (mGy patient -1) 0.00970 0.01400 Table 4.13 shows the scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the chest-Bucky of the facility A. University of Ghana http://ugspace.ug.edu.gh 85 Table 4.13: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each interval for the chest-Bucky of the facility A. kVp Unshielded primary air- kerma 1 m per kVp ( ) a1(90, kVp) a1(135, kVp) Ks (90, kVp) Ks(135, kVp) 50 0.00000 3.93384 5.87912 0.00000 0.00000 55 0.00723 4.01384 5.95912 0.00001 0.00001 60 0.18683 4.09384 6.03912 0.00034 0.00050 65 0.23032 4.17384 6.11912 0.00043 0.00063 70 0.69845 4.25384 6.19912 0.00133 0.00195 75 0.60139 4.33384 6.27912 0.00117 0.00169 80 0.63058 4.41384 6.35912 0.00125 0.00180 85 0.25934 4.49384 6.43912 0.00052 0.00075 90 0.10595 4.57384 6.51912 0.00021 0.00031 95 0.11843 4.65384 6.59912 0.00024 0.00035 100 0.16219 4.73384 6.67912 0.00034 0.00049 105 0.01008 4.81384 6.75912 0.00002 0.00003 110 0.05915 4.89384 6.83912 0.00013 0.00018 115 0.04857 4.97384 6.91912 0.00011 0.00015 120 0.17802 5.05384 6.99912 0.00040 0.00056 125 0.08893 5.13384 7.07912 0.00021 0.00028 130 0.01385 5.21384 7.15912 0.00001 0.00004 135 0.00000 5.29384 7.23912 0.00000 0.00000 By summing the scatter air-kerma for each kVp interval we obtain the scatter air- kerma for the chest-bucky of the facility A. Table 4.14: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the chest-bucky of the facility A. Side-scatter (90) (mGy patient -1) Forward/backward-scatter (135) (mGy patient -1) 0.00680 0.00980 University of Ghana http://ugspace.ug.edu.gh 86 Table 4.15 shows the scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the floor of the facility B. Table 4.15: scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each interval for the floor of the facility B. kVp Unshielded primary air- kerma 1 m per kVp ( ) a1(90, kVp) a1(135, kVp) Ks (90, kVp) Ks(135, kVp) 40 0.00000 3.77384 5.71912 0.00000 0.00000 45 0.00026 3.85384 5.79912 0.00000 0.00000 50 0.00883 3.93384 5.87912 0.00001 0.00002 55 0.07956 4.01384 5.95912 0.00015 0.00022 60 0.06075 4.09384 6.03912 0.00011 0.00017 65 0.60481 4.17384 6.11912 0.00121 0.00178 70 1.12256 4.25384 6.19912 0.00229 0.00335 75 0.06376 4.33384 6.27912 0.00013 0.00019 80 0.02663 4.41384 6.35912 0.00005 0.00008 85 0.00000 4.49384 6.43912 0.00000 0.00000 90 0.01534 4.57384 6.51912 0.00003 0.00004 95 0.00000 4.65384 6.59912 0.00000 0.00000 100 0.00000 4.73384 6.67912 0.00000 0.00000 105 0.00244 4.81384 6.75912 0.00000 0.00000 110 0.00000 4.89384 6.83912 0.00000 0.00000 115 0.00000 4.97384 6.91912 0.00000 0.00000 120 0.00000 5.05384 6.99912 0.00000 0.00000 125 1.14383 5.13384 7.07912 0.00283 0.00389 130 0.00000 5.21384 7.15912 0.00000 0.00000 By summing the scatter air-kerma for each kVp interval we obtain the scatter air- kerma for the floor of the facility B. University of Ghana http://ugspace.ug.edu.gh 87 Table 4.16: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the floor of the facility B. Side-scatter (90) (mGy patient -1) Forward/backward-scatter (135) (mGy patient -1) 0.00690 0.00980 Table 4.17 shows the scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the chest-Bucky of the facility B. Table 4.17: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each kVp interval for the chest-Bucky of the facility B. kVp Unshielded primary air Kerma 1 m per kVp ( ) a1(90, kVp) a1(135, kVp) Ks (90, kVp) Ks(135, kVp) 50 0.00000 3.93384 5.87912 0.00000 0.00000 55 0.00000 4.01384 5.95912 0.00000 0.00000 60 0.00000 4.09384 6.03912 0.00000 0.00000 65 0.01240 4.17384 6.11912 0.00003 0.00005 70 0.62066 4.25384 6.19912 0.00187 0.00273 75 0.00663 4.33384 6.27912 0.00002 0.00002 80 0.04349 4.41384 6.35912 0.00014 0.00019 85 0.04930 4.49384 6.43912 0.00016 0.00022 90 0.13847 4.57384 6.51912 0.00045 0.00064 95 0.15863 4.65384 6.59912 0.00052 0.00074 100 0.20757 4.73384 6.67912 0.00069 0.00098 105 0.00806 4.81384 6.75912 0.00002 0.00003 110 0.00602 4.89384 6.83912 0.00002 0.00002 115 0.00582 4.97384 6.91912 0.00002 0.00002 120 0.00000 5.05384 6.99912 0.00000 0.00000 125 0.64150 5.13384 7.07912 0.00234 0.00322 130 0.00000 5.21384 7.15912 0.00000 0.00000 University of Ghana http://ugspace.ug.edu.gh 88 By summing the scatter air-kerma for each kVp interval we obtain the scatter air- kerma for the chest-bucky of the facility B. Table 4.18: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for the chest-bucky of the facility B. Side-scatter (90) (mGy patient -1) Forward/backward-scatter (135) (mGy patient -1) 0.00630 0.00890 Table 4.19 shows scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each radiography installation within facility A and facility B Table 4.19: Scatter air-kerma at 1 meter from the source of scatter for 90 and 135 degree scatter for each radiography installation within facility A and facility B. Scatter angle KS (mGy patient -1 ) Facility A KS (mGy patient -1 ) Facility B KS (mGy patient -1 ) NCRP-147 90 degree Floor 9.7 6.9 23.0 Chest Bucky 6.8 6.3 4.9 All barriers 33.0 31.0 34.0 135 degree Floor 14.0 9.8 33.0 Chest Bucky 9.8 8.9 6.9 All barriers 47.0 44.0 48.0 For the floor of facility A and facility B Table 4.15 shows that side-scatters and forward/backscatters are lower than the standard values and for the chest Bucky, side- scatter and forward/backscatter is bigger than the standard value. These result show University of Ghana http://ugspace.ug.edu.gh 89 that for facility A and facility B, the major part of patient examinations are done at the chest Bucky. 4.1.4 Leakage air-kerma at 1 meter of the source of tube The Tables 4.20, 4.21, 4.22, and 4.23 give the unshielded leakage air-kerma at 1 meter of the tube for each radiography installation for facility A and facility B. Table 4.20: Leakage air-kerma at 1 meter of the tube for the floor of the facility A. kVp Unshielded primary air- kerma 1 m per kVp ( ) Leakage air-kerma KL(kVp) 40 0.00000 0.00000 45 0.00766 0.00001 50 0.08562 0.00010 55 0.02665 0.00003 60 0.04995 0.00006 65 0.04941 0.00006 70 0.74801 0.00091 75 0.20859 0.00025 80 0.40976 0.00049 85 0.08569 0.00010 90 0.25905 0.00031 95 0.29952 0.00036 100 0.06007 0.00007 105 0.26960 0.00033 Total leakage air-kerma (mGy patient -1) 0.00300 University of Ghana http://ugspace.ug.edu.gh 90 Table 4.21: Leakage air-kerma at 1 meter of the tube for the chest-Bucky of the facility A. kVp Unshielded primary air- erma 1 m per kVp ( ) Leakage air-kerma KL(kVp) 50 0.00000 0.00000 55 0.00723 0.00001 60 0.18683 0.00042 65 0.23033 0.00052 70 0.69846 0.00157 75 0.60139 0.00135 80 0.63058 0.00142 85 0.25935 0.00058 90 0.10595 0.00024 95 0.11844 0.00026 100 0.16219 0.00036 105 0.01008 0.00002 110 0.05915 0.00013 115 0.04857 0.00011 120 0.17802 0.00040 125 0.08893 0.00020 130 0.01385 0.00001 135 0.00000 0.00001 Total leakage air-kerma (mGy patient -1) 0.00770 University of Ghana http://ugspace.ug.edu.gh 91 Table 4.22: Leakage air-kerma at 1 meter of the tube for the floor of the facility B. kVp Unshielded primary air Kerma 1 m per kVp ( ) Leakage air Kerma KL(kVp) 40 0.00000 0.00000 45 0.00026 0.00000 50 0.00883 0.00001 55 0.07956 0.00005 60 0.06075 0.00004 65 0.60481 0.00041 70 1.12256 0.00076 75 0.06376 0.00004 80 0.02663 0.00001 85 0.00000 0.00000 90 0.01534 0.00001 95 0.00000 0.00000 100 0.00000 0.00000 105 0.00244 0.00000 110 0.00000 0.00000 115 0.00000 0.00000 120 0.00000 0.00000 125 1.14384 0.00078 130 0.00000 0.00000 Total leakage air Kerma (mGy patient -1) 0.00210 University of Ghana http://ugspace.ug.edu.gh 92 Table 4.23: Leakage air-kerma at 1 meter of the tube for the chest-Bucky of the facility B. kVp Unshielded primary air- kerma 1 m per kVp ( ) Leakage air-kerma KL(kVp) 50 0.00000 0.00000 55 0.00000 0.00000 60 0.00000 0.00000 65 0.01240 0.00004 70 0.62065 0.00220 75 0.00663 0.00002 80 0.04348 0.00015 85 0.04930 0.00017 90 0.13847 0.00049 95 0.15863 0.00056 100 0.20757 0.00074 105 0.00805 0.00002 110 0.00602 0.00002 115 0.00582 0.00002 120 0.00000 0.00000 125 0.64150 0.00228 130 0.00000 0.00000 Total leakage air-kerma (mGy patient -1) 0.00680 Table 4.24 presents the leakage air-kerma at 1 meter of the focal spot of the tube for each radiography installation for facility A and for facility B compared with standard values from NCRP-147. University of Ghana http://ugspace.ug.edu.gh 93 Table 4.24: Leakage air-kerma at 1 meter of the focal spot of the tube for each radiography installation for facility A and for facility B compared with standard values from NCRP-147. Radiographic room Leakage air-kerma at 1 meter (mGy patient -1) Facility A Leakage air-kerma at 1 meter (mGy patient -1) Facility B Leakage air- kerma at 1 meter (mGy patient -1) NCRP-147 Floor 3 2.1 1.4 Chest-Bucky 7.7 6.8 3.9 All barriers 1.1 8.9 5.3 4.1.5. Unshielded secondary air-kerma at dS = dL = 1 meter The unshielded secondary air-kerma at dS = dL = 1 meter has two components:  The unshielded secondary air-kerma at dS = dL = 1 meter composed of Leakage and side-Scatter radiation.  The unshielded secondary air-kerma at dS = dL = 1 meter composed of Leakage and Forward/Backward-Scatter radiation. Table 4.25 provides the unshielded secondary air-kerma at dS = dL = 1 meter each of the two components. University of Ghana http://ugspace.ug.edu.gh 94 Table 4.25: Leakage and side-scatter and leakage and forward/backward-scatter for facility A and for facility B Facility A Facility B Radiographic room Leakage and side-scatter (mGy patient - 1) Leakage and forward/backw ard-scatter (mGy patient -1) Leakage and side-scatter (mGy patient -1) Leakage and forward/backwa rd-scatter (mGy patient -1) Floor 1.27 1.70 9.00 1.19 Chest-Bucky 1.45 1.75 1.31 1.57 All barriers 4.35 5.77 3.99 5.29 4.1.6. Primary barriers thicknesses for facility A and for facility B A total of 729 patients have been examined for three months in the facility A, which gives an average of 60 patients in a week. However for busy week the number of patients may reach 90. For facility B, a total of 706 patients have been examined for three months, then an average of 60 patients is assumed and for busy week it may reach 90. For the two facilities the shielding material used for the construction of primary barriers is the ordinary concrete of density 2.35 g/cm3 and the existing primary barriers thickness is 30 cm. Table 4.26: Calculated chest-Bucky wall thickness for facility A and for facility B Rad room Facility A Facility B number of patients per week number of patients per week Average Busy Average Busy 60 90 60 90 Calculated chest-bucky wall thickness (in mm) 41 46 29 33 University of Ghana http://ugspace.ug.edu.gh 95 For these two facilities, the radiography room is located at the first floor, then the calculation of the thickness of the floor is not useful. There is no workers, patient or public to be protected there. 4.1.7. Secondary barriers thicknesses for facility A and for facility B Table 4.27 provides the calculated thickness of each barrier for facility A and B. For both of these facility the shielding material used for the construction of secondary barriers is the ordinary concrete of density 2.35 g/cm3 except from the lead plate for the control windows which is in plate glass, and the existing secondary barriers thickness is 30 cm. The thicknesses are all measured in millimetre (mm). Table 4.27: Secondary barriers thicknesses for facility A and for facility B Rad room Facility A Facility B number of patient per week number of patient per week Average Busy Average Busy 60 90 60 90 Calculated chest-Bucky wall (wall1)thickness (in mm) 24 28 23 27 Calculated ceiling thickness (in mm) 18 21 17 20 Calculated room opposite to chest Bucky (wall2) (in mm) 14 16 9 12 Calculated cross-table lateral wall (wall4) (in mm) 2 4 9 12 Calculated secondary wall Calculated opposite to the cross-table lateral wall (wall3) (in mm) 9 7 10 13 Control wall (plate glass in mm) 10 12 8 11 University of Ghana http://ugspace.ug.edu.gh 96 4.1.8. Average DLP for head and for body scan in facility A and in facility B Table 4.28 shows the average DLP for head and for body scan in facility A and in facility B. Table 4.28: Average DLP for head and for body scan in facility A and in facility B. Facility A Facility B NCRP-147 Average DLP (mGy cm) (head) Average DLP (mGy cm) (body) Average DLP (mGy cm) (head) Average DLP (mGy cm) (body) Average DLP (mGy cm) (head) Average DLP (mGy cm) (body) 1830 ± 610 859 ± 438 806 ± 346 305 ± 154 1200 550 4.1.9. Weekly secondary air-kerma at 1 meter of the isocentre of the gantry for facility A and facility B For the facility A, the weekly number of patients undergoing body scans is 25 for the average, and 30 for busy weeks, and that of patients undergoing head scans is 15 for the average, and 20 for busy weeks. For facility B, the number of patients for body scans is 17 for the average, and 20 for busy weeks and that of head scans is 14 for the average and 20 for busy weeks. Table 4.29 shows weekly secondary air Kerma at 1 meter of the isocentre of the gantry for facilities A and B. University of Ghana http://ugspace.ug.edu.gh 97 Table 4.29: Weekly secondary air-kerma at 1 meter of the isocentre of the gantry for facilities A and B. Facility A Facility B Head Body Total Head Body Total Average busy Average busy average busy Aver age busy average busy average Busy 2.50 3.30 7.70 9.30 10.20 12.60 1.01 1.45 0.62 0.73 1.63 2.18 4.1.10. Secondary barriers thicknesses for facility A and for facility B Table 4.30 shows the secondary barriers thicknesses for facility A and for facility B. Table 4.30: Secondary barriers thicknesses for facility A and for facility B. Secondary barrier Thicknesses (in mm) Facility A Facility B number of patient per week number of patient per week Average Busy Average Busy Wall 1 (in mm) 14 16 6 9 Wall 2 (in mm) 27 29 3 5 Wall 3 (in mm) 26 28 8 11 Wall 4 (in mm) 21 23 7 10 4.1.11. Dose rate measurements 4.1.11.1 Dose rate measurement for the radiography room of facility A. Table 4.31 shows the dose rate at some given location for the radiography room of facility A. University of Ghana http://ugspace.ug.edu.gh 98 Table 4.31: Dose rate at some given location for the radiography room of facility A LOCATION Distance from the tube (m) DOSE RATE (µSv/ h -1 ) REMARKS Wall 1 2.1 0.06 Lower than the derived dose rate limit (7.5 µSv/ h -1 ) Wall 2 2.65 0.06 Lower than the derived dose rate limit (2.5 µSv/ h -1 ) Wall 3 1.55 0.06 Lower than the derived dose rate limit (2.5 µSv/ h -1 ) Wall 4 4.15 0.06 Lower than the derived dose rate limit (2.5 µSv/ h -1 ) Door 1.55 0.06 Lower than the derived dose rate limit (2.5 µSv/ h -1 ) Screen 2.4 1.41 Lower than the derived dose rate limit (7.5 µSv/ h -1 ) Ceiling 2.3 0.06 Lower than the derived dose rate limit (2.5 µSv/ h -1 ) 4.1.11.2 Dose rate measurement for the CT room of facility A Table 4.32 shows the dose rate at some given location for the CT room of facility A. University of Ghana http://ugspace.ug.edu.gh 99 Table 4.32: Dose rate at some given location for the CT room of facility A. LOCATION Distance from the isocentre (m) DOSE RATE (µSv h-1) REMARKS Wall 1 4.25 0.07 Lower than the derived dose rate limit (7.5 µSv/ h-1) Wall 2 2.30 1.02 Lower than the derived dose rate limit (7.5 µSv/ h-1) Wall 3 2.75 0.22 Lower than the derived dose rate limit (2.5 µSv/ h-1) Wall 4 3.40 0.06 Lower than the derived dose rate limit (2.5 µSv/ h-1) Door 3.60 0.65 Lower than the derived dose rate limit (2.5 µSv/ h-1) Screen (control room) 4.25 0.24 Lower than the derived dose rate limit (7.5 µSv/ h-1) Door of the control room 4.25 0.09 Lower than the derived dose rate limit (7.5 µSv/ h-1) Ceiling 3.10 0.06 Lower than the derived dose rate limit (2.5 µSv/ h-1) 4.1.11.3 Dose rate measurement for the radiography room of facility B Table 4.33 shows the dose rate at some given location for the radiography room of facility B. University of Ghana http://ugspace.ug.edu.gh 100 Table 4.33: Dose rate at some given location for the radiography room of facility B. LOCATION Distance from the tube (m) DOSE RATE (µSv h-1) REMARKS Wall 1 2.10 0.07 Lower than the derived dose rate limit (7.5 µSv/ h-1) Wall 2 3.50 2 Lower than the derived dose rate limit (2.5 µSv/ h-1) Wall 3 3.30 0.18 Lower than the derived dose rate limit (7.5 µSv/ h-1) Wall 4 1.18 1.96 Lower than the derived dose rate limit (2.5 µSv/ h-1) Door ------- The entrance is not protected Screen of the control room 2.00 1.5 Lower than the derived dose rate limit (7.5 µSv/ h-1) Ceiling 2.70 0.09 Lower than the derived dose rate limit (2.5 µSv/ h-1) 4.1.11.4 Dose rate measurement for the CT room of facility B Table 4.34 shows the dose rate at some given location for the CT room of facility B. University of Ghana http://ugspace.ug.edu.gh 101 Table 4.34: Dose rate at some given location for the CT room of facility B. LOCATION Distance from the isocentre (m) DOSE RATE (µSv h-1) REMARKS Wall 1 3.50 0.18 Lower than the derived dose rate limit (7.5 µSv/ h-1) Wall 2 5.60 0.49 Lower than the derived dose rate limit (2.5 µSv/ h-1) Wall 3 2.50 0.15 Lower than the derived dose rate limit (2.5 µSv/ h-1) Wall 4 2.80 0.25 Lower than the derived dose rate limit (2.5 µSv/ h-1) Door 3.80 0.09 Lower than the derived dose rate limit (2.5 µSv/ h-1) Screen of the control room 3.10 2.65 Lower than the derived dose rate limit (7.5 µSv/ h-1) Door of the control room 3.50 0.12 Lower than the derived dose rate limit (7.5 µSv/ h-1) Floor 2.90 0.08 Lower than the derived dose rate limit (2.5 µSv/ h-1) Remarks:  The derived dose rate limit is 7.5 µSv/ h-1 for controlled area and 2.5 µSv/ h-1 for non-control (uncontrolled area) [13].  The measurements was done three time and average to get the final values  The background radiation for the facility A is 0.06 µSv/ h-1;  The background radiation for the facility B is 0.07 µSv/ h-1  The measurements have been done at 0.3 m behind each barrier except from the ceiling which have been measured at 0.5 m. University of Ghana http://ugspace.ug.edu.gh 102 CHAPTER FIVE CONCLUSION AND RECOMMENDATION 5.1 Conclusion This study was designed to assess the structural shielding within two medical diagnostic imaging facilities and check whether these structural shielding in place provides adequate protection of workers and the public so that doses received are below the design dose limit established by the regulatory authority. The assessment for general radiography was based on 729 examinations for facility A and 706 examinations for facility B. For computed tomography (CT) rooms, a total of 293 body and 190 head procedures were used for facility A, and 195 body and 160 head procedures for facility B. For the general radiography, the workload distribution and the normalized workload per patient have been determined for each room type (floor and chest-Bucky). For facility A, the normalized workload per patient was 0.96 mA min patient-1 for the floor and 1.25 mA min patient-1 for the chest-bucky, the unshielded primary air-kerma per patient for the floor was 2.56 mGy patient-1 and 3.4 mGy patient-1 for the chest- bucky in the general radiography room. The average DLP were 1830 610 mGy cm and 859 438 mGy cm for head and body respectively in the CT room. For facility B, the normalized workload per patient was 1.14 mA min patient-1 for the floor and 0.53 mA min patient-1 for the chest-bucky, the unshielded primary air-kerma per patient was 3.13 for the floor and 1.9 for the chest-bucky in general radiography room. The average DLP were 806 346 mGy cm and 305 154 mGy cm for head University of Ghana http://ugspace.ug.edu.gh 103 and body respectively in the CT room. For each of the two facilities, the thicknesses of the walls that are recommended are smaller than the thicknesses of the walls in place. Dose rate measurements confirmed that the different barriers thicknesses are enough to maintain the dose receive by workers and the public below the required dose limits. 5.2 Recommendations The followings are recommendations from the study are as follows: Diagnostic X-ray Facilities  Re-evaluation of shielding adequacy should be done by the RPO or qualified experts when the factors that affect the shielding integrity changes;  A door with appropriate shielding thickness should be provided in facility B to protect people at the entrance of the radiography room;  Each medical imaging facility should have a radiation protection officer (RPO) to oversee the implementation of the facility Radiation Protection Programme;  The staff should be periodically trained;  A warning light should be placed at the entrance to any room;  A sign should also be posted to indicate that the X-ray room is a controlled area;  QC tests should be done periodically to ensure the x-ray equipment is functioning properly with time. University of Ghana http://ugspace.ug.edu.gh 104 Regulatory Authority The regulatory authority should provide applicable regulations and guidance documents under medical exposure control which should include requirements for safety assessment of Structural Shielding Design evaluation for Medical X-ray Imaging facilities. University of Ghana http://ugspace.ug.edu.gh 105 REFERENCES [1] IAEA, International Basic safety Standards for Radiation Protection against Ionizing radiation and for the Safety of radiation sources, IAEA Safety Series No. 115 (Vienna, IAEA) (1996). [2] Fletcher, J.J., Diagnostic x-ray shielding in some African countries, Int. J. Bio. Chem. Phys. Vol 2 pp. 93-96 (1993). [3] HPRH, The Health Physics and Radiological Health Handbook, Exposure and Shielding from external radiation, Scinta Inc. Publishers (USA) (1993). [4] A.Nkansah, C. Schandorf, M. Boadu1 and J. J. Fletcher, “Assessment of the integrity of structural shielding of four computed tomography facilities in the greater Accra region of Ghana”, Radiation Protection Dosimetry pp. 1-9 (2013). [5] National Council on Radiation Protection and Measurements. Structural shielding design for medical of X-rays imaging facilities, recommendations of the National Council on Radiation Protection and Measurements. NCRP Report No. 147 (2004). [6] Sutton, D. G. and Williams, J. R. Radiation shielding for diagnostic X-rays: report of a Joint British Institute of Radiology and Institute of Physicists in Medicine. BIR/IPEM Working Party pp. 1-78 (2000). [7] ICRP, 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60 annals ICRP 21 (1-3) (1991). [8] Structural Shielding design and evaluation for medical use of x-rays and gamma rays of energies up to 10 MeV. NCRP 49, (1976). University of Ghana http://ugspace.ug.edu.gh 106 [9] International Atomic Energy Agency (IAEA), Applying Radiation Safety Standards in Diagnostic Radiology and Interventional procedures Using X rays. IAEA Safety Report Series No.39 (2006). [10] Benjamin R. Archer, Thomas R. Fewell, Burton J. Conway and Philip W. Attenuation properties of diagnostic x-ray shielding materials pp. 1-17 (1994) [11] International Atomic Energy Agency Safety Standard, “Radiation Protection and Safety of Radiation Sources” IAEA BSS GSR Part3, 2011. [12] International Commission on Radiological Protection. ICRP Publication 103, 2007. [13] Melissa C. Martin, M.S., FACR, FACMP Therapy Physics Inc., Gardena, CA 90248. Shielding Design Methods for Radiation Oncology Departments. ACMP 25th Annual Meeting Seattle, WA May 4, 2008. [14] Simpkin D.J. and Dixon R.L., Secondary-shielding barriers for diagnostic x-ray facilities: scatter and leakage revisited, Health Phys. Vol.74 (1998) [15] The European Union Guidelines on radiation dose to patients. Retrieved in January 2010 from EU website http://www.drs.dk/guidelines/ct/quality/page02.htm (2009). [16] Mohammad Javad Keikhai Farzaneh, Sabihe Farsi, Fatemeh Ramroodi, Mahdi shirin shandiz and Mojtaba Vardian. The assessment of shielding status of conventional radiographic rooms according to the National Council on Radiation University of Ghana http://ugspace.ug.edu.gh 107 Protection reports No.49 and No.147 and recommendation to national and international authorities of radiation protection to prevent wasting shielding costs of conventional radiographic rooms (2011). [17] J. Valentin, Annals of the ICRP, Publication 103, the 2007 Recommendations of the International Commission on Radiological Protection (2007). [18] Douglas J.Simpkin, Ph.D. Aurora St. Luke’s medical Crt Milwaukee, WI. RSMI 2009 Session II-general concepts (2009). [19] International Atomic Energy Agency (IAEA), Radiological Protection for Medical Exposure to Ionizing Radiation, No. RS-G-1.5 (2002). [20] I. Pesianian, A. Mesbahi, and A. Shafaee “Shielding evaluation of a typical radiography department: a comparison between NCRP reports No.49 and 147” (2009). [21] AERB SAFETY CODE NO.AERB/MED-2(Rev1). “Safety code for medical diagnostic x-ray equipment and installation” (2010). University of Ghana http://ugspace.ug.edu.gh 108 APPENDIX A1: calibration certificate of the EBERLINE RO20 University of Ghana http://ugspace.ug.edu.gh 109 University of Ghana http://ugspace.ug.edu.gh 110 A2: Extract of CT data for body examinations in facility A University of Ghana http://ugspace.ug.edu.gh 111 A3: Extract of CT data for head examinations in facility A University of Ghana http://ugspace.ug.edu.gh 112 A4: Extract of CT data for body examinations in facility B University of Ghana http://ugspace.ug.edu.gh 113 A5: Extract CT data for head examinations in facility B University of Ghana http://ugspace.ug.edu.gh 114 A6: Extract of data for Rad Rom (chest-bucky) examination in facility A University of Ghana http://ugspace.ug.edu.gh 115 A7: Extract of data for Rad Rom (floor) examination in facility A University of Ghana http://ugspace.ug.edu.gh 116 A8: Extract of data for Rad Rom (chest-bucky) examination in facility B University of Ghana http://ugspace.ug.edu.gh 117 A9: Extract of data for Rad Rom (floor) examination in facility B University of Ghana http://ugspace.ug.edu.gh