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 iv 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.