The design of optimal multipinhole collimators for a seamless SPECT detector ring

The purpose of the study was to systematically design two multipinhole (MPH) collimators for a MR compatible seamless small animal SPECT detector ring (DR) that, for a given system resolution (SR), maximized detector area usage and achieved highest possible geometric efficiency (GE) within the system´s design constraints. The SPECT DR consists of 5 rings of 19 2.56×2.56cm² individual CZT detectors connected seamlessly with a total area of 62,259mm². A set of multipinhole apertures was arranged on a cylindrical collimator sleeve (CS) and had a common-volume-of-view (CVOV) of 50mm diameter. From theoretical calculations, we determined 3 rows of 7, 8, 9, and 10 pinholes would provide the target SR and achieve GE that are among the highest possible while maximizing detector usage and maintaining at most 20% overlap between the pinhole projections. The projections of the pinholes onto the surface of the RD were arranged to resemble a hexagonal close packing pattern and then shifted so they were equally spaced along the longitudinal direction to improve axis sampling. To evaluate the performance of the MPH collimators, projections from a hot-spot phantom were simulated using an analytical projector that includes the response function of the pinholes. They were reconstructed using a 3D MPH iterative ML-EM reconstruction method. This MPH collimator design resulted in a discontinuity of the profile of the total response function with an elevated central region (ECR) and a sharp drop of ~20% at the edge. As the CS diameter decreased, the ECR diameter decreased concurrently. By increasing the CS diameter while fixing the number of PH, we broadened the ECR diameter by 28% while decreasing the GE at the center of the CVOV by less than 10%. Using this process, two MPH collimators were designed with targeted SR of 2 and 3mm with21 and 27 pinholes, CS diameters of 72.2 and 76mm, and GE of 0.05% and 0.16% at the center of the CVOV, respectively. The t- o MPH collimators were designed achieve the highest possible GE by maximizing the use of the detector area of the SPECT RD. Performance of the designs was confirmed with simulations.