Peak Skin Dose Estimation in Adult Examinations for Selected Multi Detector Computed Tomography An Approach for Patient Dose Optimization.
Abstract
Computed Tomography (CT) is a valuable tool among diagnostic imaging
modalities. However, the increase in its utilization has resulted to a considerable increase
in its contribution to the collective dose from medical exposure. The main aim of this
study was to propose a patient dose optimization method from exposure parameters that
would estimate the peak skin doses (PSDs) to patients undergoing chest CT and abdomen
CT examinations. PSDs were estimated on the surface of a Perspex CT body phantom
using TLDs for chest and abdomen protocols at three fixed tube voltage and varying
current-time product (mAs) on two CT units at different facilities. Computed tomography
dose index (CTDI) in the phantom (weighted, w; volume, vol) and free-in-air were
estimated using dose measurements from ionization chamber at iso-centre and scanner
specific k-factors when helical mode is employed for all the considered exposure factor
scenarios. The console displayed CTDIvol was also recorded during each exposure factor scenario. CTDIvol values were also estimated using the ImPACT CT Dosimetry using the
same considered scan parameters. CT scanners, the CT body phantom and ionization
chamber were modeled using SimpleGeo for Monte Carlo N-Particle 6 (MCNP6)
simulation studies of the considered scan parameters. Considerable variations in both
PSDs and CTDIvol between the protocols on specific CT unit were observed. The range of
estimated PSDs at 120 kVp for the chest and abdomen protocols were (10.65 - 45.29)
mGy, (9.69 - 43.68) mGy and (5.67-37.87) mGy, (7.47-51.69) mGy for the 16 slice and
128 slice CT scanners respectively. The maximum percentage deviation between the
measured and the displayed CTDIvol across the scanners was 20.24% while ImPACT
values were lower.The measured and simulated CTDIvol values showed good agreement with maximum difference of 7.70% and compared well with international DRLs. A good
correlation was observed between the measured PSD and CTDIvol with R² >0.97 and
between PSD and kVp, mAs with R² between 0.86-0.92 with P values much less than the
significance level of 0.05. Exam and scanner-specific models to estimate PSDs were then
obtained. The mean calculated PSDs obtained from the patients‘ exposure factors were
19.88 ±1.93 mGy and 15.80 ± 1.49 mGy for abdomen and chest examinations
respectively. The established relationships between exposure/console display parameters
and PSD when well effected will go a long way of enhancing the protection of patients
during CT examinations.
Description
MPhil.
Keywords
Peak Skin Dose, Computed Tomography, Patient Dose Optimization