COMPTON SCATTER AND X-RAY CROSSTALK AND THE USE OF VERY THIN INTERCRYSTAL SEPTA IN HIGH-RESOLUTION PET DETECTORS

To improve spatial resolution, positron emission tomography (PET) syst ems are being developed with finer detector elements. Unfortunately, u sing a smaller crystal size increases intercrystal Compton scatter and X-ray escape crosstalk, causing positioning errors that fan lead to d egradation of image contrast, We investigated the use of extremely thi n (less than or equal to 300 mu m) lead strips for passive shielding o f this intercrystal crosstalk. Using annihilation gamma rags and small (2- and 3-mm wide) Bismuth Germanate (EGO) crystal detectors in coinc idence, crosstalk studies were performed with either two small adjacen t crystals [(one-dimensional) (1-D)] or one crystal inside a volume of BGO [(two-dimensional) (2-D)]. The fraction of Compton scattered even ts from one crystal into an adjacent one was reduced, on average, by a factor of 3.2 (2.2) in the 1-D experiment and by a Factor of 3.0 (2.1 ) in 2-D one, with a 300 (150)-mu m-thick lead strip in between the cr ystals end a 300-700-keV energy window in bath crystals. We could not measure a reduction in bismuth X-ray crosstalk with the use of lead se pta due to the production of lead X-rays of similar energy, The full-w idth at half-maximum (FWHM) of the coincident point-spread function (C PSF) was not significantly different For the 1- and 2-D studies, with or without the different septa in place, However, the FWTM was roughly 20% smaller with the 300-mu m lead shielding in place, These results indicate that intercrystal crosstalk does not affect the positioning r esolution at FWHM, but does affect the tails of the CPSF. Thus, withou t introducing any additional dead area, an insertion of very thin lead strips can reduce the extent of positioning errors, Reducing the inte rcrystal crosstalk in a high-resolution BET detector array could poten tially improve tomographic image contrast in situations where intercry stal crosstalk plays a significant role in event mispositioning.