A transmission-map-based scatter correction technique for SPECT in inhomogeneous media

In this paper a method of modeling the distribution of scattered events in emission projection data is developed and applied. This method is based on the use of a transmission map to define the inhomogeneous scattering object. The key point is the use of the set of line integrals calculated as part of the attenuation correction technique, as the basis of a model of the distribution of scattered events. The probability of a photon being scattered through a given angle and being detected in the emission energy window is approximated using a Gaussian function. The parameters of this Gaussian are determined using Monte Carlo generated parallel-beam scatter line spread functions from a nonuniformly attenuating phantom. The model is incorporated into a two-dimensional projector-backprojector and used with the Expectation-Maximization-Maximum-Likelihood algorithm for the reconstruction of fan-beam phantom data. The correction is shown to perform well for a phantom that varies slowly in the axial direction. For the more clinically realistic situation of a torso phantom, the method produces improvements in terms of blood pool to myocardium contrast, but does not restore the contrast to the level exhibited in a reconstruction from "scatter free" data.