Compton scatter and X-ray crosstalk and the use of very thin inter-crystal septa in high resolution PET detectors

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. The authors investigated the use of extremely thin (⩽300 μm) 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) μm thick lead strip in between the crystals and a 300-700 keV energy window in both crystals. The authors 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 1- and 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