Voxelized ray-tracing simulation dedicated to multi-pinhole molecular breast tomosynthesis

Accurate gamma photon transport simulations of emission tomography systems are important to optimise system geometries and for iterative image reconstruction. Monte Carlo simulation (MCS) is widely established for this purpose but has the disadvantage of being prohibitively slow. Voxelized ray tracing (VRT) can be used as an alternative but the accuracy of VRT needs to be assessed for each simulation task at hand. The aim of this work is to propose and validate a dedicated VRT code for a novel radionuclide-based multi-pinhole molecular breast tomosynthesis (MP-MBT) scanner. The MP-MBT system images radionuclide distributions in a mildly compressed breast using two opposing gamma cameras, each equipped with a focusing multi-pinhole collimator, that slide along opposite sides of the breast. VRT simulates gamma photon transport by tracing rays efficiently through the voxelized phantom, collimator, and detector volumes using Siddon's raytracing algorithm, accelerated by dual-grid methods. To assess its accuracy, we compare point spread functions (PSFs) calculated with VRT for different voxel sizes with those generated by the established MCS toolkit GATE. Furthermore, VRT and MCS-simulated projections of realistic anthropomorphic XCAT phantoms with different compressed breast sizes are compared, as well as reconstructed images obtained from these projections. With VRT, PSFs for MP-MBT can be simulated accurately when the fine voxel size of the VRT's dual-grid is 1/8 mm. Reaching a similar deviation from noiseless PSFs takes 29 300 times longer with full MCS than with VRT. Furthermore, XCAT phantom simulations show that VRT-generated projections are very close to MCS-generated low-noise projections when these are corrected for scatter by the triple energy window method. However, we also find that primary gamma photons from the torso may in some cases reach the detector, meaning that torso activity should not be neglected in VRT. Finally, reconstructed images obtained from projections generated by VRT and MCS are visually very similar and have no significant difference in contrast and noise characteristics. We conclude that VRT can accurately and efficiently simulate MP-MBT even though it neglects scattered photons originating from the torso.