Variability in patient CT radiation dose and image quality: the impact of positioning and body habitus via a virtual imaging trial study
Computed tomography (CT) dose and image quality data exhibit a large variability even for the same patient size. That is so despite the tube current modulation (TCM) systems adjusting radiation exposure according to patient body habitus to achieve consistent image quality. We conducted a Virtual Imaging Trial (VIT) to identify possible causes of this variability by repeated scanning of patients in different positions, a prospect that is ethically infeasible in real practice. Three anthropomorphic computational (XCAT) phantoms with different BMIs (24.4, 28.5, and 32.2 kg/m2) were imaged with an established VIT platform (DukeSim), using a clinical chest-abdomen protocol. XCAT phantoms were imaged at the isocenter with arms above the head and alongside the trunk. In addition, each phantom with arms up was imaged following vertical shifts (-80, -40, -20 mm; +40 mm). CTDIvol, tissue global noise index (GNI), organ noise (aorta and liver), and water equivalent diameter (WED) were evaluated and compared to the standard position values. Across all the positions with respect to the isocenter, the maximum percentage differences for patients 1, 2, and 3 were respectively 0.8%, 2.0%, and 2.3% for CTDIvol; 13.1%, 8.4%, and 6.0% for GNI; 24.2%, 11.5%, -5.7% for aorta noise, and 12.1%, 11.0%, 7.4% for liver noise. Maximum WED percentage differences were less than 0.2% in magnitude. The percentage differences between arms up and side for patients 1, 2, and 3 were respectively 65.6%, 44.7%, and 37.6% for CTDIvol; -9.6%, -5.4%, and - 2.1% for GNI; -7.7%, -6.4%, and -1.4% for aorta noise; -15.6%, -9.2%, and -6.0% for liver noise; and 6.1%, 5.2%, and 5.3% for WED. Radiation dose and image quality variability in clinical CT is influenced by patient positioning. VITs allow the assessment of scanner performance on the same patient under different conditions thus allowing a systematic characterization of factors assessing imaging performance.
