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.