Comparison of two small animal PET scanners: Pinhole collimation vs. electronic collimation

PET imaging of rodents is increasingly used in preclinical research, but its utility is limited by the spatial resolution and statistical quality of the images. In a new approach, a specially designed pinhole collimator enables high-resolution, simultaneous imaging of PET and SPECT tracers. Such a physical collimation technique strongly departs from traditional electronic collimation achieved via coincidence detection in PET. This work compares two small animal PET scanners, one with electronic collimation (Focus120) and one with physical collimation using clustered pinholes (VECTor). Data were compared from Jaszczak (hot-rod) and uniform phantoms and point source measurements. Mouse brain images from [¹⁸F]FDG PET were acquired on both systems, and compared with quantitative ex-vivo autoradiography as a gold standard. Using typical reconstruction settings, the pinhole system resolved the smallest rods (0.85 mm diameter) in the Jaszczak phantom while the coincidence system resolved 1.3 mm diameter rods. This was in agreement with the recovered contrast. The contrast-to-noise ratio was better for the pinhole system when imaging small rods (<;1.1 mm) for a wide range of activity levels, but this reversed for larger rods. Image uniformity on the coincidence system (<;3%) was superior to that on the pinhole system (10%), as was the peak sensitivity (4% compared to 0.4%). The high [¹⁸F]FDG uptake in the striatum of the mouse brain was fully resolved using the pinhole system with contrast to nearby regions equaling that from autoradiography; a lower contrast was found using the coincidence PET system. To conclude, in cases where small regions need to be resolved in scans with reasonably high activity or reasonably long scan times, a lower sensitivity clustered pinhole system with higher reconstructed spatial resolution (in this case the first generation VECTor with sub-optimal reconstruction software) can provide superior image quality in terms of contrast and the contrast-to-noise ratio as compared to a traditional system.