Uranium pinhole collimators for 511-keV photon SPECT imaging of small volumes

Depleted uranium (DU) pinhole collimators are evaluated for small-volume SPECT imaging of 511-keV gammas, with a view toward small-animal imaging. Tungsten (W) pinholes are examined for comparison. Line source measurements show planar full-width at half-maximum (FWHM) as low as 6.5 mm for a 1-mm DU pinhole, with SPECT system FWHM of 5.0 mm at 43-mm ROR and 6.7 mm at 69-mm ROR. Planar FWHM is smaller for DU than for W pinholes but is insensitive to aperture size for pinholes of 1-3 mm diameter at this energy for the collimators studied. These trends indicate penetration at pinhole edges, as confirmed by LSF shape and full-width at tenth-maximum trends. In SPECT scans of rod phantoms at 71-mm ROR, DU pinholes were only barely able to resolve cold rods as large as 4.7 mm (center-to-center spacing=2×diameter). For both F-18 alone and F-18 imaged along with Tc-99m, 3.9-mm hot rods were resolved. Tc-99m scans resolved 2.3- mm rods with F-18 present and 1.5-mm rods without F-18. DU pinhole SPECT at 511 keV does not currently match the spatial resolution of high-resolution positron emission tomography (PET) but is open to improvement and, unlike PET, is capable of simultaneous multiple-isotope acquisitions