Comparing Dose-Length Product-Based and Monte Carlo Simulation Organ-Based Calculations of Effective Dose in 16- and 64-MDCT Examinations Using Automatic Tube Current Modulation

The purpose of this study is to compare dose-length product (DLP)-based calculation of effective dose (ED ) with Monte Carlo simulation organ-based calculation of effective dose (ED ) in 16- and 64-MDCT examinations, with the use of clinical examinations with automatic tube current modulation. Dose data were obtained from 50 consecutive unenhanced head CT examinations, unenhanced chest CT examinations, and unenhanced and contrast-enhanced abdominopelvic CT examinations performed using 16- and 64-MDCT scanners, as well as from 50 pulmonary CT angiography (CTA) examinations performed using a 64-MDCT scanner and 31 pulmonary CTA examinations performed using a 16-MDCT scanner. The ED and the mean patient effective diameter were calculated using commercially available software. The ED was also calculated. Both the mean difference and percentage difference between ED and ED were calculated, and they were statistically compared according to patient sex, type of examination performed, and type of scanner used. ED significantly underestimated the ED by 0.3 mSv (19%) for men who underwent unenhanced head CT, 0.5 mSv (29%) for women who underwent unenhanced head CT, 0.9-1.4 mSv (9-13%) for men who underwent chest CT, and 4.7-4.8 mSv (39%) for women who underwent chest CT (p < 0.001). The ED overestimated the ED by 1.9-2.0 mSv (12-14%) for men who underwent abdominopelvic CT (p < 0.001), with no significant difference noted for women who underwent abdominopelvic CT's. No significant difference was noted in the percentage difference in ED between the 16- and 64-MDCT scanners (p ≥ 0.13). ED underestimates ED , the reference standard for dose calculation, by 19-39% in unenhanced head CT examinations and, among women, in chest CT examinations. ED deviates from ED by less than 15% for chest CT examinations of men and for abdominopelvic CT. These differences can be attributed to variable patient body habitus, automatic tube current modulation, and sex-neutral k-coefficients, and they should be considered when calculating ED, particularly in women.