Abstract: Background: Organ doses in spiral CT scans depend on the tube start angle.
Purpose: 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 start angle.
Methods: Using Monte Carlo simulations, dose values for different tube positions with an angular increment of 10 ∘ $10^\circ$ and a longitudinal increment of 4.5 m m $4.5 \,\mathrm{m}\mathrm{m}$ were simulated over a range of 31.5 c m $31.5 \,\mathrm{c}\mathrm{m}$ with collimations of 40 mm $40\, \mathrm{mm}$ , 60 mm $60\, \mathrm{mm}$ , and 80 m m $80 \,\mathrm{m}\mathrm{m}$ . 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 start angles.
Results: With increasing pitch and collimation, the dose variations from the effective dose averaged over all start angles increase. While for a collimation of 40 m m $40 \,\mathrm{m}\mathrm{m}$ , the variations from the mean dose value stay below 5 % $5 \%$ for SSCT, we find that for a spiral scan with a pitch of 3.0 for DSCT with TCM and collimation of 80 m m $80 \,\mathrm{m}\mathrm{m}$ , the dose for the best starting angle is on average 16 % $16 \%$ lower than the mean value and 28 % $28 \%$ lower than the maximum value.
Conclusions: 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 patient risk.
(© 2024 The Author(s). Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.)
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