Implementation of Monolithic Crystals in Stand- Alone Brain PET, and PET-MR Insert, Developments

This contribution shows the feasibility of using thick monolithic scintillation crystals, in particular of the type LYSO, for the development of brain PET systems. We described the characterization of two main designs. The first is used in a PET alone system where the main field of application is Alzheimer Disease. In this case a 15 mm thick block was used with 50 × 50 mm² size. The second design described in this work is a thicker 20 mm LYSO crystal also with 50 × 50 mm² size. Both designs make use of an optical layer at the crystal entrance, so-called retroreflector, which bounces back the light to the source of emission. This allows us to preserve the light distribution while transferring a larger number of scintillation photons to the photosensor. Both approaches make use of a SiPM array with 12 × 12 elements (3 × 3 mm² dimensions) with about 4.2 mm pitch. SiPMs with low dark count rates of about 30 kcps/mm2 at the breakdown voltage have been selected. We have characterized both main detector blocks namely 20 mm and 15 mm thick. We characterized their measured spatial and energy resolution as a function of the photon depth of interaction (DOI). The DOI resolution was also evaluated. Average values for the whole scintillation volume of 1.9 mm spatial resolution (FWHM) was obtained, together with about 13% energy resolution. DOI performance showed FWHM values nearing 4 mm. In the case of the 15 mm block, studies showing the system capability to return accurate results at impact positions as close as 0.3 mm from the crystal edge were also carried out. We have also, within the MINDView collaboration for the brain PET insert, and together with Oncovision team, acquired and reconstructed pilot data of a mini Derenzo-like phantom showing for both cases the possibility to visualize the 1.6 mm in diameter rods.