Patient radiation risk reduction by controlling the tube start angle in single and dual source spiral CT scans: A simulation study.

Abstract

Organ doses in spiral CT scans depend on the tube startangle. To determine the effective dose in single source CT (SSCT) and dual source CT (DSCT) scans as a function of tube start angle and spiral pitch value to identify the dose reduction potential by selecting the optimal startangle. Using Monte Carlo simulations, dose values for different tube positions with an angular increment of and a longitudinal increment of were simulated over a range of with collimations of , , and . The simulations were performed for the thorax region of six adult patients based on clinical CT data. From the resulting dose distributions, organ doses and effective dose were determined as a function of tube angle and longitudinal position. Using these per-view dose data, the individual organ doses, as well as the total effective dose, were determined for spiral scans with and without tube current modulation (TCM) with pitch values ranging from 0.5 to 1.5 for SSCT and up to 3.0 for DSCT. The dose of the best and worst tube start angle in terms of dose was determined and compared to the mean dose over all tube startangles. With increasing pitch and collimation, the dose variations from the effective dose averaged over all start angles increase. While for a collimation of , the variations from the mean dose value stay below for SSCT, we find that for a spiral scan with a pitch of 3.0 for DSCT with TCM and collimation of , the dose for the best starting angle is on average lower than the mean value and lower than the maximumvalue. Variation of the tube start angle in spiral scans exhibits substantial differences in radiation dose especially for high pitch values and for high collimations. Therefore, we suggest to control the tube start angle to minimize patientrisk.