Directional resolution of limited-angle multi-pinhole SPECT cameras

Small animal and clinical SPECT cameras based on multi-pinhole detector arrangements typically boast spatial resolutions of 1-2mm and 4mm respectively. However, directional variability of the local imaging capability within the field-of-view (FOV) can impact our ability to detect abnormalities. We aimed to demonstrate the dependence of resolution on object position and orientation within the FOV in a multi-pinhole configuration using a resolution model validated by maximum likelihood expectation maximization (MLEM) reconstructions. The local directional resolution model was based on the smallest angle of view to all pinholes with respect to the normal to the resolution direction. The test configuration was two arcs of 16 pinholes covering 150° in 10° intervals connected by 2 pinholes at each end totaling 36 pinholes. Pinholes were 45mm from the Z-axis, the focal length was 132mm, and detectors were 215mm×215mm leading to a ~28.5mm radius FOV. For validation, a simulated phantom of 3-disks (radius of 5.75mm, 0.75mm thick, and 1.5mm center-to-center spacing) was shifted to different positions in both the X- and Y-directions where differences in resolution were predicted by the theory. At each position the disks were oriented to align their normals with the X and Y-axes. Simulations using idealized pinholes were performed with noise-free forward projections without attenuation and scatter effects (256×256 pixels of 0.84mm×0.84mm in size) and forty iterations of MLEM reconstructions (128-cubed with 0.375mm-cubed voxel size). As predicted by the model, disks with the normal in the Y-direction were reconstructed with nearly uniform resolution at all positions along the X-axis of the FOV. The same was true for disks with the normal in the X-direction at various Y-positions. However, at many of those locations, the resolution behaved poorly when the disk phantom was rotated 90°. Overall, the analytical model and simulations reconstructed using MLEM demonstrated a strong dependence of resolution on position and orientation of an object within the FOV. The greatest resolution losses were observed for positions outside of the hemisphere enclosed by the pinhole array.