Optimizing gold and platinum pinhole collimators for imaging of small volumes at ultra-high resolution

Monte Carlo simulation is used to study the spatial resolution and sensitivity of pinhole collimators for 140.5 keV Tc-99m gamma rays. Collimators made out of gold, platinum, and tungsten, with pinhole diameters of 0.1 mm to 0.5 mm are studied. Each collimator has a 60 degree acceptance angle. Gold has the same density as pure tungsten (s.g.=19.3) but a higher atomic number (79 for gold vs. 74 for tungsten). Platinum (Z=78, s.g.=21.4) is also more attenuating than tungsten. The potential improvement in resolution from using gold or platinum rather than tungsten is negligible for clinical human imaging, but is potentially useful for small animal imaging for biomedical research. Gold and platinum collimators may be produced at reasonable expense if a small insert is used, given that the loss of resolution due to gamma-ray penetration is only a concern at the pinhole edge. Monte Carlo simulations indicate that very high resolution is possible. For a gold or tungsten pinhole of 0.25 mm diameter, the model indicates a FWHM of 0.55 mm in the object plane. For pinholes of 0.1 mm diameter, the model predicts a FWHM of 0.28 mm for platinum, 0.30 mm for gold, and 0.34 mm for tungsten. These simulations were performed for pure tungsten; tungsten alloys would be slightly less attenuating. Sensitivity also diminishes as pinhole size is reduced, and the smallest pinholes would be most useful for very short distances, e.g., 1 or 2 cm. The sensitivity at such distances would be similar to that of larger pinholes for human imaging, working at larger distances.