A Fast Timing Layer Concept for a Compton-TOF-PET Module

The possibility to improve depth of interaction (DOI) resolution and coincidence time resolution (CTR) of a pixellated PET module by enabling light re-circulation inside it with a light guide is well known. Typically the light guide consists of a non-scintillating material of about 1 mm thickness. In this work, we propose to further extend the concept by replacing the passive light guide with a fast scintillating material, in order to combine the benefits of light re-circulation with a fraction of very fast events, where the DOI is precisely known. Several configurations with such an active layer are proposed and studied in this work by means of Monte Carlo simulations with experimental verification. First, the possibility of replacing the glass light guide with a layer of LYSO is investigated. This configuration allows to reach DOI resolutions beyond the possibilities of a simple glass guide, while retaining comparable performances in terms of energy and timing resolutions. Then, the performance of two fast scintillators (BaF$_2$ and BC422) used as light guides, in combinations with crystal arrays made of both LYSO and BGO, is investigated. The fraction of shared events (i.e. those events where the 511 keV gamma ray scatters in the light guide and deposits the rest of its energy in the crystal array) in a 3 mm light guide is found to be around 1% for BC422, and 12.1% for BaF$_2$. Therefore, the configuration using the latter material is investigated in depth, and two alternative readout schemes are proposed, to maximize the collection of light produced by BaF$_2$. The results show that ∼ 100 ps FWHM CTR can be reached for shared events using a BaF$_2$ light guide. Finally, the possibility to use such a detector design as a Compton camera is discussed.