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

Abstract

In positron emission mammography (PEM) sampling artifacts can be introduced 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 backprojecting lines of response (LORs) onto seven, equally spaced, virtual image planes. LORs are assumed to start and end on a unique point which is the centroid of interaction of gamma rays within each crystal. This effect limits the sampling of the image space and introduces gridlike 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 in PEM, the regions associated with each crystal in the crystal identification matrix may overlap. We postulate that by allowing the LORs to shift away from the centroid, toward the next most probable crystal element, sampling artifacts should be reduced. The solid angle function is an image uniformity correction function that 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 by reordering the solid angle function and the smoothing algorithm, image quality should improve. Reformatting data from contrast resolution and point source phantom studies with the new strategies showed no or very few sampling artifacts. Contrast resolution measurements showed an improvement in both contrast and signal-to-noise ratio. In most cases point source measurements showed an improvement in spatial resolution and a reduction in noise. The clinical PEM images were reformatted with the new strategies where they showed no sampling artifacts and were smoother. The reformatted images were easier to interpret.