Author: Craig K Abbey, John M. Boone, Richard E. Colbeth, Andrew M. Hernandez, Vance Robinson, Paul Schwoebel, Jeffrey H. Siewerdsen, Alejandro Sisniega, Wojciech B. Zbijewski, Huanyi Zhou 👨🔬
Affiliation: UC Santa Barbara, University of California, UT MD Anderson Cancer Center, Johns Hopkins University, University of New Mexico Albuquerque, UC Davis Health, Varex Imaging Corporation 🌍
Purpose: A Multiple-X-ray Source Array (MXA) for CT (MXA-CT) is under development to mitigate cone beam artifacts in axial CT scans using wide collimation. The purpose of this study is to assess the impact of different volumetric sampling schemes on radiation dose distributions using Monte Carlo (MC) simulations.
Methods: A validated MC engine (MCNP6) was used to simulate dose distributions in a voxelized 20cm-diameter phantom. The simulation geometry included a six source MXA arranged parallel to the axis of rotation (4.3cm source spacing), a source-to-detector distance of 100cm, and an irradiated volume sufficient to cover a 41x31cm detector FOV. Simulations were executed for three source-to-axis distances (SADs): (1) 55cm - regions on the interface between adjacent sources are covered by only partial rotations of these sources, (2) 65cm - entire volume is covered by a full rotation of at least one source, and (3) 60cm. The ratio of the dose to the skin (5mm outer annulus) to the dose in the center of the phantom (1cm diameter cylinder), termed skin-to-central dose, was used to quantify the effects of variations in the SAD.
Results: The geometry with an SAD of 60cm results in the most homogenous distribution of dose throughout the entire irradiated volume of the phantom. This observation was further quantified by analysis of line profiles within the central and skin regions as a function of z (long axis of the phantom) which revealed a lower magnitude of dose variations for the SAD of 60cm configuration. The skin-to-central dose was 0.44, 0.49, and 0.54 for an SAD of 55, 60, and 65cm, respectively.
Conclusion: The simulations presented in this study reveal that moving the SAD closer to the source in MXA-CT reduces skin dose significantly although it results in a more heterogenous distribution of dose in a uniform cylindrical phantom.