MODELING THE SCATTER RESPONSE FUNCTION IN INHOMOGENEOUS SCATTERING MEDIA FOR SPECT

We have previously developed a method for accurately and rapidly model ing the scatter response function in uniform media. In this work we st udied the extension of this method to nonuniform attenuators. Our appr oach was to use the water-equivalent source depth, i.e., the integral of the attenuation coefficient through the nonuniform object from the source to the surface divided by the attenuation coefficient of water. We have investigated the accuracy of three different methods based on this approximation using Monte Carlo (MC) simulation methods. Line so urces were placed at the same water equivalent depth in slabs composed of water, bone, and lung. We observed that, for perfect collimation, the scatter response functions (SRFs) obtained for these slabs are com parable. To determine whether attenuation coefficients alone are suffi cient to estimate the SRF or whether one must know the elemental compo sition, we simulated SRFs for sources in bone and lungs as well as bon e- and lung-equivalent materials. The bone- and lung-equivalent materi als have the same attenuation coefficient as bone and lung, respective ly, but the same elemental composition as water. For the lung-equivale nt material, the SRFs were essentially equivalent to those from lung; for the bone-equivalent material, the SRFs were closer to those from w ater. We have also placed voids of various sizes in slab phantoms whil e keeping the same water-equivalent source depth. For these simple geo metries two of the methods based on the effective depth were adequate for predicting the SRF. Finally, we have simulated the SRF for a line source in a realistic thorax phantom using several methods based on ge ometric and water-equivalent distances. The results indicate that one of the methods gives reasonably good agreement with direct MC simulati ons in terms of predicting the magnitude and shape of the SRFs and pro jection data for a complex distribution simulating the heart. This met hod was not the same as the one that gave good agreement in the case o f the simpler phantoms.