Purpose: To evaluate the impact of tube potential (kV) on water-equivalent-diameter (Dw) and size-specific-dose-estimate (SSDE).
Methods: 250 non-contrast head, chest and abdomen CT exams were obtained for pediatric and adult patients. CT numbers of bone were simulated for 10 kVs (70 to 150, Sn100, Sn150) using scaling factors obtained from phantom scans. Size-dependent conversion factors (f-factors) were obtained for all kVs using methods similar to those described in AAPM Report 204. With scaled patient CT images (bone attenuation scaled to simulate that at alternative kVs) and kV-dependent f-factors, Dw and SSDE were calculated for each patient at each kV and compared to reference 120 kV values.
Results: For chest and abdomen exams, Dw was independent of kV for both adult and pediatric patients (RÂ²=1), where bone was only 4.5 to 10.1% by volume. However, kV dependence was observed for SSDE, mainly due to changes in the f-factor as kV varied. Compared to 120 kV, the average SSDE difference ranged from -11.4% (70 kV) to +12.3% (Sn150 kV) and -12.4% to +9.93% for adult chest and abdomen exams, respectively. Less kV dependence was observed for pediatric patients: -1.1% to 8.43% and -0.74% to +6.19% for chest and abdomen exams, respectively. Head exams showed stronger kV dependence for both Dw and SSDE compared to chest and abdomen exams due to the larger bone volume fraction (20.7%). Approximately 1 cm (~6%) difference in Dw and -12.0% (70 kV) to +18.4% (Sn150 kV) difference in SSDE were observed compared with 120 kV. These differences are all below 20%, which is the accuracy limit for SSDE suggested in AAPM Report 204.
Conclusion: Tube potential has negligible impact on Dw for chest and abdomen CT, and a small (<10%) impact for head CT. Tube potential alters SSDE for head, chest and abdomen CT by <20%.