Comparison of penetration and scatter effects on defect contrast for GE and Siemens LEHR collimators in myocardial perfusion SPECT-A simulation study

The goal of this study is to evaluate the effects of collimator penetration and scatter on myocardial SPECT image quality. We chose two designs: a LEHR collimator for GE Millennium VG with a longer bore and thicker septa, and a LEHR collimator for Siemens E.CAM with a shorter bore and thinner septa. These two collimators have similar resolution properties, but very different penetration fractions. In addition, the Siemens collimator has higher detection efficiency. We used Monte Carlo (MC) simulation to simulate projection data from the 3D NCAT phantom. For each collimator, we generated three sets of projection data: one includes only the geometric components of the detected photons, one includes both the geometric and penetration components, and the other one includes geometric, penetration and collimator scatter effects. The resulting projections were reconstructed with OSEM including attenuation and geometric response compensation. For each collimator and reconstruction, we computed the defect contrast in a short axis slice. We found very small differences in defect contrast between the two collimators with and without penetration and collimator-scattered photons. Since the collimator with higher penetration had greater detection efficiency and showed no loss in defect contrast, a collimator with higher penetration fraction may be acceptable for use in Tc-99m myocardial perfusion imaging.