COMPTON SCATTER AND X-RAY CROSSTALK AND THE USE OF VERY THIN INTER-CRYSTAL SEPTA IN HIGH RESOLUTION - Nuclear Science Symposium and Medical Imaging Conference Record, 1995., 1995 IEEE

To improve spatial resolution, PET systems are being developed with finer detector elements. Unfortunately, using smaller crystal sizes increases inter-crystal Compton scatter and bismuth x-ray and (to a lesser degree) electron escape crosstalk, causing positioning errors that lead to degradation of image contrast. We investigated the use of extremely thin (I 300 ym) lead strips for passive shielding of this inter-crystal crosstalk. Using annihilation photons and small (2 and 3 mm wide) BGO crystals in coincidence, crosstalk studies were performed with either two small adjacent crystals (1-D) or one crystal inside a volume of BGO (2-D). The fraction of Compton scattered events 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, with a 300 (150) pm thick lead strip in between the crystals and a 300-700 keV energy window in both crystals. We could not measure a reduction in bismuth x-ray crosstalk with the use of lead septa because of the production of lead x-rays. The width of the coincident point spread function was not significantly different for the 1and 2-D studies, with or without the different thickness septa in place. These results indicate that intercrystal crosstalk does not affect the positioning resolution. A simple insertion of very thin lead strips may significantly reduce the inter-crystal scattering crosstalk of a high resolution PET system, thereby ultimately improving image contrast, without introducing a dead area.