Effects of nonuniform collimator sensitivity on variance of attenuation-corrected SPECT images

The first step in all intrinsic attenuation-correction algorithms is multiplication of each measured projection by a function which compensates for photon attenuation between a line through the center of rotation, parallel to the detector, and the (convex) external object contour nearest the detector. A nonuniform collimator sensitivity profile which accomplishes this compensation physically rather than computationally would reduce the noise in the projections and, consequently, in the reconstructed images. The authors have compared the variance of reconstructed images of cylindrical phantoms of homogeneous activity concentration and attenuation coefficient for three collimator sensitivity profiles using both the original intrinsic attenuation-correction technique of Tretiak-Metz (1980) and Gullberg-Budinger (1981) and the variant developed by Tanaka (1984). The collimator sensitivity profiles the authors considered were the standard profile, with uniform sensitivity along the projection, and two nonuniform sensitivity profiles which were peaked at the center of the projection. The nonuniform collimator sensitivity profiles led to reduced variances throughout most of the image for both attenuation correction algorithms. These reductions in variance would be expected to lead to improved performance in quantitative imaging tasks