Improving the spatial resolution and image noise in densely pixilated detectors for positron emission mammography

Sampling artifacts are introduced in positron emission mammography (PEM) from the use of discrete crystals which form planar stationary detectors. With the breast positioned in between the two detectors, the images are reconstructed by back-projecting lines of response (LORs) onto seven, equally spaced, virtual image planes. LORs are assumed to start and end on a unique point, the centroid of interaction within each crystal. This limits the sampling of the image space and introduces grid like sampling artifacts due to the variable density of LORs that intersect each pixel in the image. When using crystals that are very densely pixilated, as it is the case in PEM, the regions associated with each crystal in the crystal identification matrix may overlap. We postulate that allowing the LORs to shift away from the centroid, according to their relative position in the region, should reduce sampling artifacts. The solid angle function is an image uniformity correction function. It corrects for the varying geometrical efficiencies of image pixels. The solid angle function is the last thing applied before the images are displayed. We postulate that re-ordering the solid angle function and the smoothing algorithm should improve image quality. Reformatting data from contrast resolution and point source phantom studies with the new strategies showed no or very few sampling artifacts. More than half of the 15 cases studied showed an improvement of 50% in signal-to-noise ratio, 2% in spatial resolution, and a reduction of 25% of the noise, despite a 3% loss in contrast resolution. When clinical PEM images were reformatted with the new strategies they showed no sampling artifacts and were smoother.