Scatter correction in 3-D PET.

Modern multiring positron emission tomographs allow the acquisition of 3-D data sets to increase their sensitivity. A substantial part of this data is due to scattered radiation. The authors describe the experimental dependence of point source scatter distributions on energy window setting, source location, and scatter volume in geometries relevant for brain studies. The point source scatter distribution was parametrized accurately by a broad, 2-D Gaussian, which included a shift parameter to account for asymmetry of the scatter medium relative to the source. This parametrization was used to formulate two fast scatter correction algorithms suitable for brain scans. In both algorithms, a 2-D subset of the measured projections was transformed into a scatter projection. An image of the 3-D scatter distribution was reconstructed using 2-D algorithms. It was then subtracted from the total (true+scattered) 3-D image. Both algorithms were implemented in different combinations with the additional attenuation correction and were tested on point source and phantom measurements. It was shown that, for the situation typical for brain scans, reconstructed scatter fractions could be reduced to 5% or less.