Dose conversion coefficients for CT examinations of adults with automatic tube current modulation.

Automatic tube current modulation (TCM) is used in modern CT devices. This is implemented in the numerical calculation of dose conversion coefficients for CT examinations. For four models of adults, the female and male reference models of ICRP and ICRU and a lighter and a heavier female model, dose conversion coefficients normalized to CTDI(vol) (DCC(CT)) have been computed with a Monte Carlo transport code for CT scans with and without TCM. It could be shown for both cases that reliable values for spiral CT scans are obtained when combining the results from an appropriate set of axial scans. The largest organ DCC(CT) are presented for typical CT examinations for all four models. The impact of TCM is greatest for chest, pelvis and whole-trunk CT examinations, where with TCM the effective DCC(CT) can be 20-25% lower than without TCM. Typical organs with strong dependence on TCM are thyroid, urinary bladder, lungs and oesophagus. While the DCC(CT) of thyroid and urinary bladder are mainly sensitive to angular TCM, the DCC(CT) of lungs and oesophagus are influenced primarily by longitudinal TCM. The impact of the body stature on the effective DCC(CT) is of the same order as the effect of TCM. Thus, for CT scans in the trunk region, accurate dose values can only be obtained when different sets of DCC(CT) are employed that are appropriate for the patient's sex and stature and the actual TCM settings.