Characterization of scatter and penetration using Monte Carlo simulation in I-131 imaging

In I-131 SPECT, image quality and quantification accuracy are degraded by o bject scatter as well as scatter and penetration in the collimator. The cha racterization of energy and spatial distributions of scatter and penetratio n performed in this study by Monte Carlo simulation will be useful for the development and evaluation of techniques that compensate for such events in I-131 imaging. Methods: First, to test the accuracy of the Monte Carlo mod el, simulated and measured data were compared for both a point source and a phantom. Next, simulations to investigate scatter and penetration were per formed for four geometries: point source in air, point source in a water-fi lled cylinder, hot sphere in a cylinder filled with nonradioactive water, a nd hot sphere in a cylinder filled with radioactive water. Energy spectra w ere separated according to order of scatter, type of interaction, and gamma -ray emission energy. A preliminary evaluation of the triple-energy window (TEW) scatter correction method was performed. Results: The accuracy of the Monte Carlo model was verified by the good agreement between measured and simulated energy spectra and radial point spread functions. For a point sou rce in air, simulations show that 73% of events in the photopeak window had either scattered in or penetrated the collimator, indicating the significa nce of collimator interactions. For a point source in a water-filled phanto m, the separated energy spectra showed that a 20% photopeak window can be u sed to eliminate events that scatter more than two times in the phantom. Fo r the hot sphere phantoms, it was shown that in the photopeak region the sp ectrum shape of penetration events is very similar to that of primary (no s catter and no penetration) events. For the hot sphere regions of interest, the percentage difference between true scatter counts and the TEW estimate of scatter counts was <12%. Conclusion: In I-131 SPECT, object scatter as w ell as collimator scatter and penetration are significant. The TEW method p rovides a reasonable correction for scatter, but the similarity between the 364-keV primary and penetration energy spectra makes it difficult to compe nsate for these penetration events using techniques that are based on spect ral analysis.