Quantitative SPECT Brain Imaging: Effects of Attenuation and Detector Response

Two major factors which degrade quantitative accuracy in SPECT imaging of the brain are attenuation and detector response. The purpose of this work was to implement two reconstruction methods which compensate for attenuation and detector response (a filtered backprojection method with Metz filter and Chang attenuation compensation and a 3D maximum likelihood-EM method) and compare the methods in terms of quantitative accuracy. The methods were tested on simulated data of the 3D Hoffman brain phantom. The simulation incorporated attenuation and distance-dependent detector response. The RMS error was measured in the reconstructed images in the gray matter, white matter, and ventricle regions. The results in the white matter showed the 3D maximum likelihood-EM (ML-EM) method over a range of iteration stopping points had a smaller error than the filtered backprojection method over a similar range of power factors. In the gray matter the reverse was found, and this was attributed to an edge artifact in the 3D ML-EM images. Reconstruction times for the 3D ML-EM method have been greatly reduced through efficient coding, limited source support, and computing attenuation factors only along rays perpendicular to the detector.