Study on the effective dose of CT in PET/CT on automatic tube current modulation

Objective: To investigate the relationship between effective dose and tube current threshold, as well as noise index of CT in PET/CT, and to provide theoretical basis for optimal acquisitions through the mode of automatic tube current modulation (ATCM). Methods: Two sets of PET/CT (Discovery ST-16 and Discovery Elite) and an anthropomorphic phantom (RS-550) were used to acquire the effective dose caused by CT in PET/CT. CT acquisition conditions were as follows in both ST-16 and Elite PET/CT: tube voltage 120 kVp, pitch 1.375, rotation speed 0.8 s, noise index ranged from 8 to 30 with an interval of 2, the low limit of current was 30 mA, and the high limit ranged from 200 mA to 350 mA with an interval of 50 mA. Simulating the clinical head-neck and body conditions, the phantom was scanned, and the dose-length product (DLP) was recorded to estimate the effective dose (EDCT) of CT. Results: With the same acquisition conditions, the effective dose decreased along with the increasing noise index, and the curve became steeper with increasing high limit of tube current. The whole body effective dose caused by CT was lower in Elite PET/CT than that in ST-16 with the same acquisition conditions. Conclusion: The effective dose caused by CT in PET/CT differs from variable scan conditions. It shows that the radiation dose can be effectively reduced by choosing appropriate acquisition conditions according to different clinical needs. (authors)