Improved modeling of system response in list mode EM reconstruction of compton scatter camera images

An improved List Mode EM method for reconstructing Compton scattering camer a images has been developed. First, an approximate method for computation o f the spatial variation in the detector sensitivity has been derived and va lidated by Monte Carlo computation. A technique for estimating the relative weight of system matrix coefficients for each gamma in the list has also b een employed, as has a method for determining the relative probabilities of emission having come from pixels tallied in each list-mode back-projection . Finally, a technique has been developed for modeling the effects of Doppl er broadening and finite detector energy resolution on the relative weights for pixels neighbor to those intersected by the back-projection, based on values for the FWHM of the spread in the cone angle computed by Monte Carlo . Memory issues typically associated with list mode reconstruction are circ umvented by storing only a list of the pixels intersected by the back-proje ctions, and computing the weights of the neighboring pixels at each iterati on step. Reconstructions have been performed on experimental data for both point and distributed sources.