Dual-layer collimator for improved spatial resolution in SPECT with CZT cameras: An analytical and Monte Carlo study

Current hole matching pixel detector (HMPD) collimators for SPECT imaging exist in two configurations: one hole per pixel (1HMPD) or four holes per pixel (4HMPD). The aim of this study was to assess the performance of a dual-layer collimator made by stacking up these two collimator types (1H/4HMDP) for low and medium-energy gamma emitters. Analytical equations describing 1H/4HMDP collimator geometrical efficiency and full width at half maximum (FWHM) were derived. In addition, a fast dedicated gamma ray-tracing Monte Carlo (MC) code was developed to assess the collimator’s point spread function (PSF) and to simulate planar and SPECT acquisitions. A relative agreement between analytical equations and MC simulations better than 3% was observed for the efficiency and better than 1% for the FWHM. The length of the two layers was optimized to get the best spatial resolution while keeping the geometrical efficiency equal to that of the 45mm-length 1HMPD collimator. An optimized combination of the 1H/4HMPD configuration with respective hole lengths of 20mm and 12.95mm has been derived. For source-collimator distances above 5 cm and equal collimator geometrical efficiency, the spatial resolution of this optimal 1H/4HMDP collimator supersedes that of the 45mm-length 1HMPD collimator, and that of the 19.1mm-length 4HMPD collimator. This improvement was observed in simulations of bar phantoms planar images and of hot rods phantom SPECT. Remarkably, the spatial resolution was preserved along the depth of the Jaszczak phantom slices. The 1H/4HMDP collimator is a promising solution for CZT SPECT imaging of low- and medium-energy emitters.