Analysis of factors affecting positron emission mammography (PEM) image formation

Abstract

Image reconstruction for positron emission mammography (PEM) with two parallel, planar detectors is usually performed by backprojection to image planes. Three major factors affecting backprojection image reconstruction are investigated: 1) image uniformity (flood) corrections; 2) image sampling (pixel size); and 3) count allocation methods. An analytic expression for uniformity correction is developed that incorporates factors for spatial-dependent detector sensitivity and geometric effects from acceptance angle limits on coincidence events. There is good agreement between experimental uniformity correction images and numerical simulations. The analytic uniformity corrections are successfully applied to image reconstruction of breast phantoms and reduce the need for multiple flood scans. Experimental and simulated breast phantom studies show that lesion contrast is improved when the image pixel size is half of, rather than equal to, the detector pixel size, at the expense of some additional image noise. In PEM reconstruction counts usually are allocated to the pixel in the image plane intersected by the line of response (LOR) between the centers of the detection pixels. An alternate count allocation method is investigated that distributes counts to image pixels in proportion to the area of the tube of response (TOR) connecting the detection pixels that they overlay in the image plane. This TOR method eliminates some image artifacts and increases tumor signal-to-noise ratios (SNRs) at the expense of a slight decrease in tumor contrast. Image reconstruction by the iterative maximum likelihood expectation maximization (MLEM) method also is investigated. MLEM reconstruction improves the tumor contrast and SNR compared with the backprojection method. Further work is required to optimize reconstruction parameters for particular detection or quantitation tasks.