Characterisation of photon penetration and scatter in Pb, W, Au, and Pt micropinholes

In pinhole gamma-camera imaging, both penetration and scattering in the pinhole edge material can account for a significant fraction of the total number of photons detected, particularly if the pinholes are tiny. This study characterises the effects of penetration and scatter with micropinholes made in lead, tungsten, gold and platinum. Monte Carlo simulations are performed for I-125 (27-35 keV) and Tc-99m (140 keV) point sources. The simulations account for the effects of photoelectric interaction, Rayleigh scattering, Compton scattering, ionisation, bremsstrahlung and electron multiple scattering. As a typical example, in the case of a Tc-99m point source and pinholes with a diameter of 300 microns in gold or platinum, approx. 55% of the photons detected resulted from penetration and approx. 3% from scatter. For pinhole diameters ranging from 100 to 500 microns the penetration fraction for tungsten and lead was approx. a factor of 1.0 to 1.6 higher and the scatter fraction was 1.0 to 1.8 times higher than in case of gold or platinum. Using I-125 instead of Tc-99m decreases the penetration fraction by a factor of 3 to 11 and the scatter fraction by a factor of 12 to 35. In addition, for all materials studied, the total amounts of penetrated and scattered photons changed linearly with respect to the pinhole diameter.