Fast methods for including attenuation in the EM algorithm

A fast approach to including attenuation in iterative maximum-likelihood and least-squares algorithms for single-photon-emission computed tomography (SPECT) is presented. Ray-tracing and summing of attenuation coefficients are replaced by the use of two lookup tables, one to compute attenuated ray path integrals based on a set of polar grid points and one to perform polar-to-rectangular transformations. The resulting algorithm implements a spatial average which is comparable in accuracy to ray-tracing with rectangular pixels, yet requires less than one sixteenth the CPU time. >