Characterization of spectral and spatial distributions of penetration, scatter and lead X-rays in Ga-67 SPECT

Ga-67, a tumor-seeking imaging agent, has proven useful in the management of Hodgkin´s disease and non-Hodgkin´s lymphoma. However, Ga-67 presents challenges to accurate lesion detection and activity quantitation due to the range of energies of the gamma rays emitted. Medium- to high-energy photons produce collimator penetration, scatter and lead X-rays, which degrade contrast, resolution and quantitation in images of the more abundant, lower-energy photons. To study the effects of these interactions and to provide a tool for detection and estimation studies, the authors developed and tested a Monte Carlo (MC) simulation program. Simulated characteristic lead X-rays were compared with the difference of measurements made with and without the camera collimator, using Tc-99m (140.5 keV) in a tin source-collimator. Simulated energy spectra and images were compared with measurements of Cr-51 (320 keV) to evaluate collimator penetration and scatter, and backscatter from components behind the NaI crystal. Finally, spectra and images were compared for several emission peaks of Ga-67. The simulation provided good estimates of both spectral and spatial distributions