Application of lead and tungsten pinhole inserts to I-131 SPECT tumor imaging: a Monte Carlo investigation

The potential use of lead and tungsten pinhole inserts for high resolution SPECT imaging of intratumor activity in I-131 radioimmunotherapy is investigated using experimental point source measurements, and raytracing and Monte Carlo simulations. I-131 imaging is challenging because the primary photon emission is at 364 keV and penetration through the insert near the pinhole aperture is significant. Point source response functions (PSRFs) for Pb and W pinhole inserts are measured and are modeled using raytracing simulations. More accurate simulation of the PSRFs may require the modeling of scatter within the pinhole insert and of higher energy emissions at 637 and 723 keV. A numerical study of geometrically identical pinhole inserts made of Pb and W shows narrower point source response functions for the W insert due to reduced penetration. Monte Carlo modeling was used to compare these inserts for SPECT pinhole imaging of 3 cm diameter tumors with a central core and 3-5 mm thick shells. The shell:core activity concentration ratio was 5:1. The tumor shells are resolved for the W insert but not for the Pb insert. As a result, shell:core activity ratios are more accurate with the use of the W pinhole insert. Experimental SPECT acquisitions are needed to confirm the potential advantages of a W insert over a Pb insert suggested by this study.