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

Abstract

Objective: The purpose of this study is to compare dose-length product (DLP)-based calculation of effective dose (ED_DLP) with Monte Carlo simulation organ-based calculation of effective dose (ED_MCO) in 16- and 64-MDCT examinations, with the use of clinical examinations with automatic tube current modulation.

Materials and methods: 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_MCO and the mean patient effective diameter were calculated using commercially available software. The ED_DLP was also calculated. Both the mean difference and percentage difference between ED_DLP and ED_MCO were calculated, and they were statistically compared according to patient sex, type of examination performed, and type of scanner used.

Results: ED_DLP significantly underestimated the ED_MCO 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_DLP overestimated the ED_MCO 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).

Conclusion: ED_DLP underestimates ED_MCO, the reference standard for dose calculation, by 19-39% in unenhanced head CT examinations and, among women, in chest CT examinations. ED_DLP deviates from ED_MCO 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.

Keywords: CT; Monte Carlo; automatic tube current modulation; dose-length product; effective dose.