Investigation of the "block effect" on spatial resolution in PET detectors

Previous studies suggest that when block detectors are used in positron emission tomography (PET) scanners, a "block effect" of about 2 mm should be added in quadrature to the other effects blurring spatial resolution (SR). However, no explanation of the cause of the effect has ever been published. If the effect could be identified, isolated, and reduced then the SR of PET scanners should improve. We studied the origin of the "block effect," using experiments in which all other blurring effects on SR were minimized and precisely determined. Thin, 1 mm wide bismuth germanate (BGO) crystals and a small (1 mm) (68)Ge source were used to probe the SR of a HR+ block detector. Two precise translation stages were used to move detectors. We compared coincidence aperture functions (CAFs) for crystals in the block with CAFs of single crystals of various widths. From those measurements, done with one block detector, we concluded that the central crystals in the block showed an additional blurring of 0.8 mm whereas the edge ones showed no additional blurring. When the detectors were separated by 21 cm, the full width at half maximum (FWHM) for central crystals was found to be 2.20 +/- 0.04 mm and for edge crystals 1.98 +/- 0.04 mm. The FWHM for 4.4 mm width crystals (as used in the HR + block) was estimated to be 2.1 +/- 0.2 mm. Results from the crystal identification matrix show that the apparent centroids of the crystals are not located at the geometric centers, which would give errors in the reconstruction algorithm assumed uniform sampling. Our results suggest that the additional blurring previously reported in PET scanners with block detectors is not entirely due to the use of block detectors.