Penetration and scatter in channel micro-pinholes for SPECT: a Monte Carlo investigation

Pinhole SPECT is rapidly gaining popularity for imaging laboratory animals. High resolution images can be achieved by using small pinhole diameters, but micro-pinhole images have higher contributions of penetrated and scattered photons. A characterization of penetration and scatter in the pinhole aperture metal can be used in the design of small animal SPECT hardware or for corrections during micro-SPECT image reconstruction. This study investigates the penetration and scatter fractions for gold knife edge and channel edge pinholes under varying angles of incidence of photons from a Tc-99m point source. The study uses Geant 4.5 for Monte Carlo simulation of 0.3 mm diameter gold pinhole apertures with opening angles up to 60 degrees. At perpendicular incidence, a channel pinhole has lower penetration and scatter fractions than a knife edge pinhole. This advantage disappears at higher angles of incidence: the scatter fraction is even larger when the angle of incidence approaches the pinhole half cone angle. In addition, the total sensitivity decreases substantially with increasing channel height. In conclusion, advantages of channel pinholes are not obvious and further research involving full SPECT system simulation is required to determine whether and how channel pinholes lead to better reconstructed images.