Under-sampling in PET scanners as a source of image blurring

Abstract

Abstract The spatial resolution of PET scanners is degraded by a number of causes, both fundamental to the nature of positron decay, and to detectors. The crystal dimensions, their placement in the block and readout all contribute to the loss of resolution. We investigated the relative effects of sampling of the image space by the detectors, and the use of block detectors in a whole body PET scanner on resolution degradation. Three sources were mounted on a linear translation stage which moved them trans-axially through the central field of a Siemens CTI HR+ PET scanner. A 140 frame study was acquired as the sources moved horizontally 0.5 mm between frames. The vertical projection from each frame was summed over all slices. The FWHM of each source's representation was estimated in each frame in the summed projections. The response functions were much sharper from crystals at the edge of the detector blocks, than from those from the central crystals. They were consistent with the effects of source size, non-collinearity, and crystal dimensions. The response from the central crystals were degraded by an additional term of 1.2 mm added in quadrature to the other blurring effects. The intensity of the image of a small source depended on the source location, as did the FWHM of the response of the central crystals in the block detectors. The response functions of edge crystals were found to be sharper than those of the central crystals in the block detectors. However, a more interesting finding is that when very small sources are imaged in PET scanners their apparent intensity and their associated response functions depend on their location along any projection. This under-sampling results in resolution loss equivalent to about 1 2 of the crystal width.