Penetration, Scatter and Sensitivity in Channel Micro-Pinholes for SPECT: A Monte Carlo Investigation

Channel-edge pinhole designs have been proposed in order to reduce the penetration of gamma rays through the edge of the pinhole aperture. A characterization of penetration and scatter in the pinhole aperture metal can be used in the design of small animal SPECT collimators or for model based corrections during micro-SPECT image reconstruction. In this study the penetration and scatter contributions of micro-pinholes were compared for Tc-99m, I-123, and I-125 for knife-edges and channel-edges. To this end, Geant 4 Monte Carlo simulations of 0.3 mm and 0.5 mm diameter apertures with acceptance angles ranging from 20 to 60 degrees were performed. At perpendicular incidence of the photons, channel pinholes had lower penetration and scatter fractions than did knife-edge pinholes. This advantage disappeared at higher angles of incidence. In addition, the total sensitivity decreased substantially with increasing channel height. Planar projection images of a grid of spheres showed that channel-edge pinholes resulted in a slightly higher spatial resolution than knife-edge pinholes with an equal diameter, when combined with a high-resolution detector. However, the channel-edge pinhole´s sensitivity and Contrast Inverse Coefficient of Variation were lower than the knife edge pinhole´s at the edges of the detector. We conclude that channel pinholes can result in lower imaging performance when used with non-perpendicular incidence photons because of loss of sensitivity