Quantitative SPECT imaging: compensation for nonuniform attenuation, scatter, and detector divergence

Reconstruction methods which compensate for nonuniform attenuation, scatter, and detector divergence in SPECT (single photon emission computed tomography) imaging have been developed. The attenuation map is reconstructed from transmission data collected on one of the cameras of a three-headed SPECT system, with a line source of activity positioned at the focal line of its fan beam collimator. The other two cameras simultaneously collect the emission data. To control the noise in the attenuation map, the reconstruction algorithm employs a robust estimation technique based on an iteratively reweighted least squares criterion. The position-dependent point spread functions (PSFs) modeling the spatially variant detector divergence and scatter response are obtained from acquisitions taken of a point source of activity inside an attenuating medium. The nonuniform attenuation map and the PSFs are incorporated in the projector used for implementing a modified Chang algorithm, which also utilizes the same robust estimation technique for noise control. Using measurements taken on a physical phantom of the human torso it is demonstrated that this modified iterative Chang algorithm substantially improves the quality and quantitative accuracy of the reconstructed SPECT images of the emission densities