The impact of ASiR-V on abdominal CT radiation dose and image quality – A phantom study

Abstract

IntroductionTo investigate how ASiR-V and kVp changes in Computed tomography (CT) affect radiation dose and image quality, when using automatic tube current modulation (ATCM) for different sized phantoms. MethodsA liver-phantom with two different liver inserts (QRM, Moehrendorf, Germany), with extension rings, representing fat, were additionally applied to the phantom to simulate patients of different sizes (small: 30cm diameter, medium: 35cm and large: 40cm). Abdominal scans were performed on a 256 slice CT scanner (GE Healthcare, Milwaukee, WI, USA), with consistent pitch (0.992), rotation time (0.5s), slice thickness (0.625mm) and collimation (80mm), while other parameters were varied (kVp: 80/100/120/140; Noise Index: 13/22; mA interval 80-720, ASiR-V: 30/60/100%). CTDI and DLP was recorded for each scan and image quality was assessed using objective metrics in predefined anatomic areas (HU and noise). Radiation dose and image quality metrics were compared between protocols. ResultsCTDI decreased by 80% from ASIR-V 30% to ASiR-V 100% for prescribed NI 13, and by 79% for the prescribed NI of 22. For 100% ASiR-V and a prescribed NI of 22 the CTDI remained the same regardless of phantom size for the different kVp settings. Pairwise comparison revealed significant differences in CTDI (p < 0.0001) for all combinations of prescribed NI and ASIR-V levels, except the difference between ASIR-V levels of 30 and 60%, with a prescribed NI of 13 (p = 0.124).When data from the three phantom sizes were combined, increasing ASIR-V from 30-100%, resulted in noise decreases of 22% for NI of 13 and by 8% for NI of 22. Notably, image quality in the low contrast area of the liver insert was impaired when the large phantom was scanned with 100% ASiR-V and either 80/100kVp (NI 22), because of the large reduction in tube current applied (down to 80 mA). ConclusionSubstantial radiation dose reductions (up to 80%) resulted from increasing ASiR-V levels. However, image quality deteriorates when 100% ASiR-V is applied due to low applied tube current by the ATCM.