VALIDATION OF THE CENTRAL-RAY APPROXIMATION FOR ATTENUATED DEPTH-DEPENDENT CONVOLUTION IN QUANTITATIVE SPECT RECONSTRUCTION

In order to model photon attenuation and detector resolution variation as a depth-dependent convolution for efficient reconstruction of quan titative SPECT, a central-ray approximation is necessary. This work in vestigates the impact of the approximation upon reconstruction accurac y and computational efficiency. A patient chest CT image was acquired and converted into an object-specific attenuation map. From a segmenta tion of the map, an emission thorax phantom was constructed with a car diac insert. To generate a system-specific resolution-variant kernel, a point source was measured at several depths from the surface of a lo w-energy, high-resolution, parallel-hole collimator of a SPECT system. Projections of parallel-beam geometry were simulated from the phantom , the map, and the kernel on an elliptical orbit. Reconstruction was p erformed by the ML-EM algorithm with and without the central-ray appro ximation. The approximation cuts down dramatically (more than 100 fold ) the computing time with a negligible loss (less than 1%) of reconstr uction accuracy.