Patient dose in HR-pQCT bone scans can be reduced by suitably setting the tube start angle: A proof of concept

Background High-resolution peripheral quantitative computed tomography (HR-pQCT) assesses the three-dimensional microarchitecture of distal tibia and radius for bone health evaluation. The effective dose (E) for an HR-pQCT examination is less than 2 μSv. In the standard protocol for patient scans, a default start angular position of the X-ray tube is adopted, in a half-scan geometry. The total half-scan angle (180° + fan angle 22°) determines an uneven distribution of the absorbed dose in the tissues, given the specific anatomy of the bones in the ankle and the wrist. Purpose We carried out a Monte Carlo dosimetry study to investigate whether a different choice of the tube start angle could produce scans with lower organ doses and lower E, so reducing the patient radiation risk. Methods We performed Monte Carlo simulations at varying tube start angles of the Scanco XtremeCT II scanner, with an angular step of 11°, using a digital twin of the scanner and voxelized phantoms (left and right tibia and radius) derived from a previously available scan of a volunteer subject. Results We showed that the tissue-averaged organ dose (for bone, bone marrow, skin, muscle, and fat) depends on the tube start angle, with a pronounced minimum for bone and yellow bone marrow occurring at a specific start angle. At the dose-optimized angle, E shows a minimum, about 10 % lower than for the standard start angle, for the left tibia; for the radius, the dose-optimized protocol allows for a maximum dose sparing of 6.6 %, possibly increasing to over 16 % (tibia scan) if the presence of the limb holder is accounted for. No significant morphologic difference emerged in the CT reconstructions obtained for the standard and dose-optimized protocol. Conclusions For patients undergoing HR-pQCT scans the radiation dose, and the related patient risk, could be reduced by selecting the tube start angle.