Abstract
BACKGROUND: The rapid advancement of computed tomography (CT) has greatly improved clinical diagnosis but has also introduced new challenges in radiation protection. This study aimed to evaluate the relationship between organ doses from Monte Carlo (MC) simulations and CT dose metrics for head CT exams with organ dose modulation (ODM), and to develop a simplified method for estimating individual organ doses. METHODS: A CT source model including the X-ray energy spectrum, bowtie filter, fan beam shape, and rotational motion of the tube was constructed and validated. The modeling was divided into two different exposure regions based on the ODM technical principles: the 100° range on the anterior side of the skull (tube current reduction region) and the remaining 260° (tube current constant region). The source model was validated by comparing the error between the MC-simulated weighted CT dose index (CTDI(w)) and the measured CTDI(w). A total of 40 patients were retrospectively collected, and each patient's voxelized head models were constructed and used for MC simulation to calculate organ doses. The global volume CTDI (CTDI(vol)), regional CTDI(vol), size-specific dose estimate (SSDE), and organ-specific SSDE were derived based on the exposure (mAs) and water-equivalent diameters of each slice image. Linear regression fitting was used to explore the correlation between organ doses (including the brain, the eyeballs, the eye lens, and the salivary glands) and the four CT dose metrics mentioned above. RESULTS: Comparison results for CTDI(w) showed that the simulated source model error was within 5%, and the ODM model's error was below 0.05%. Organ doses correlated strongly with organ-specific SSDE (The R(2) between each organ dose and corresponding organ-specific SSDE were 0.92 for the brain, 0.91 for eyeballs, 0.90 for the eye lens, and 0.90 for the salivary gland). Estimation coefficients for estimating organ doses of the brain, eyeballs, eye lens, and salivary glands from organ-specific SSDE were 0.34, 0.59, 0.48, and 0.26, respectively, as a mean across all patients. CONCLUSIONS: There is a strong correlation between organ dose and organ-specific SSDE in ODM head CT examinations. However, activating the ODM results in significant differences in estimation coefficients for head CT exams with a fixed tube current, which provides a practical way to determine organ doses for individual patients undergoing head CT scans.