Lead and tungsten pinhole inserts for I-131 SPECT tumor imaging: experimental measurements and photon transport simulations

The potential use of lead and tungsten pinhole inserts for high-resolution SPECT imaging of intratumor activity in I-131 radioimmunotherapy was investigated using experimental point source measurements and photon transport 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 (PSRF's) for lead (Pb) and tungsten (W) pinhole inserts were measured experimentally. These response functions were simulated using a photon transport computer code that modeled the primary emission at 364 keV and secondary emissions at 284, 637, and 723 keV. Scatter within the pinhole insert, camera shielding, and scintillation crystal was modeled. There was good agreement between the experimental and simulated PSRF's. Simulated point source response functions for geometrically identical Pb and W pinhole inserts were narrower for the W insert due to reduced penetration. SPECT pinhole imaging with these inserts was simulated for 3-cm-diameter tumors with a central core and 3-5-mm-thick shells. For one set of simulations there was no core activity, and for a second set the shell:core activity concentration ratio was 5:1. In both cases, the tumor shells were better resolved with the W insect. As a result, shell:core activity ratios were more accurate and contrast was improved with the use of the W pinhole insert. This study suggests that W inserts have potential advantages over Pb inserts for high-resolution I-131 pinhole imaging.