Evaluation of coincidence plane definitions for improved volume resolution uniformity in PET

A new scheme for defining coincidence planes in positron emission tomography (PET) with the aim of improving the overall uniformity in volumetric spatial resolution has been investigated. The axial resolution in high-resolution PET systems rapidly degrades at off-center positions, in particular in tangential direction. This resolution loss is caused by the addition of coincidence planes with relatively large ring differences to form a single detection plane. Here, a different way of defining coincidence planes is evaluated to improve the overall spatial uniformity. This method only assigns coincidence combinations that fall within a specified slice thickness to contribute to a specific plane. The method produces overlapping axial planes, which allows a particular coincidence plane combination to contribute to several axial planes. Measurements of a point source at the center and at 20 cm off-center position showed a degradation in resolution from 4.6 to 12.4 mm full width at half-maximum (FWHM), tangentially using the conventional plane definition (max ring difference of eight). Using the new plane definition scheme (max ring difference of four), the resolution only degraded from 8.6 to 9.7 mm FWHM. This method should provide an improved resolution uniformity and image quality in particular for whole body PET.