Gold pinhole collimators for ultra-high resolution Tc-99m small volume SPECT

The use of gold pinhole collimators is investigated with a view towards small-animal Tc-99m SPECT imaging. Functional imaging of laboratory animals offers increased return of data for a given investment of resources compared to conventional research techniques, but small research animals require high resolution imaging. At short distances, pinhole collimators show a more favorable resolution-sensitivity tradeoff than other collimator types. Penetration at pinhole edges degrades image quality, especially for small pinhole diameters. Tungsten pinhole collimators are favored for Tc-99m imaging over lead collimators due to the higher density of tungsten alloys (Z=74, s.g.=18.5) than of lead (Z=82, s.g.=11.35), even though lead, with its higher atomic number, has more stopping power per atom. Pinholes made of depleted uranium (Z=92, s.g.=18.95) have been investigated for ultra-high resolution I-131 and F-18 imaging. Residual radioactivity gives some background counts, and chemical and radiological toxicity imposes handling and bookkeeping requirements for uranium components. Gold (Z=79, s.g.=19.3) has stopping power between that of tungsten and uranium, but without the complications of the latter. Only the pinhole edges are vulnerable to penetration, so for Tc-99m imaging, a small, reasonably inexpensive gold insert may be used.