System design considerations for collimation in a small-animal PET scanner

We propose using collimation in PET scanners to achieve spatial resolution beyond the limits of detector resolution. In this study, we use simulations to characterize and optimize the collimator for a small-animal PET scanner. We design trapezoidal tungsten collimator septa to mask half of each crystal in a PET scanner to detect collimated lines of response (LORs) within the unmasked portion of the crystals. Since each crystal can be conceptually split into 2 sub-crystals, each pair can measure 4 finer LORs with 4 collimator configurations. The advantages of collimation are: 1) reduced effective crystal width and increased spatial resolution, 2) increased linear and angular-sampling, 3) cost-effective integration-no requirement to modify existing PET systems, 4) reduced inter-crystal scatter and random events. Collimation reduces sensitivity; however, it can improve overall image quality and quantification even with reduced efficiency. We devised a collimator configuration to achieve complete LOR sampling by just rotating the collimator. We have developed a 3-D ray-tracing model for the collimator incorporating collimator penetration. We optimized the collimator by maximizing the average sensitivity within a 15 mm FOV subject to fixed LOR resolution inside the feasible geometric parameter space. We developed LabVIEW-controlled linear/rotation stages and electronics to synchronize collimator motion and PET acquisition. We also developed an LOR-interleaving image reconstruction for PET with collimation. In conclusion, using collimation can dramatically increase sampling and improve spatial resolution, which may have significant implications in PET imaging applications where high resolution is demanded.