Noise characteristics of SPECT iterative reconstruction with a mis-matched projector-backprojector pair

SPECT reconstructed images are degraded by photon attenuation, the limited spatial resolution of the gamma camera, and scatter. Iterative reconstruction with the expectation-maximization (EM) algorithm provides an attractive solution for SPECT in that the physics of photon transport can be modeled into the projector and backprojector. Unfortunately, the implementation of this algorithm (including modelling of the physics) requires a large computational load which makes it unsuitable for routine clinical use. One acceleration method that has been suggested is to use a mis-matched projector and backprojector pair. By doing this, the computational time can be reduced. Barrett et al. (1994) have recently presented a theory which describes how noise in the projection data is propagated into the reconstructed image when using the EM algorithm. The authors have used this theory to evaluate and compare the differences in pixel mean and variance, as well as performance for the task of estimating activity within a region-of-interest, when reconstructing using the EM algorithm with a matched and mismatched projector-backprojector pair. Results indicate that EM reconstruction can be accelerated by not modeling attenuation into the backprojection process, with little penalty in terms of estimating activity in regions-of-interest.