Recent trends in Positron Emission Tomography (PET) use the time-of-flight (ToF) information in the image reconstruction process to improve the signal-to-noise ratio and the positioning of the annihilation event. One of the components that most contributes to the accuracy of the ToF-PET is the scintillation crystal. The metascintillator approach has been proposed to overcome the time resolution limits of commonly used scintillators. The metascintillator is an engineered composition of small units that combines and optimizes several features in a single scintillator heterostructure. In this work, metascintillator-based brain PET systems were modeled using the GATE Monte Carlo toolkit and compared with designs based on bulk LYSO or BGO. Sensitivity, noise equivalent count rate and scatter fraction were evaluated following the NEMA guidelines. Only data in the list mode format was used for comparison purposes to avoid dependence on the image reconstruction algorithm. To achieve the same peak sensitivity of a system based on a 15 mm thick bulk BGO, the metascintillator-based scanners using BGO/BaF2 , BGO/EJ232, LYSO/BaF2 and LYSO/EJ232 must have thicknesses of 23.2 mm, 22.5 mm, 29.7 mm and 31.1 mm, respectively. The objective of this work is to determine the clinical value of using metascintillator-based detectors in brain PET.

Positron emission tomography scanners are commonly characterized by their photon sensitivity. Scanner design often requires Monte Carlo simulations to probe different geometries and materials. However, the computational load of such simulations can be significant and costly. Furthermore, the applicability of the Monte Carlo approach in optimization loops is limited as each instance, such as source position or scanner dimensions, has to be simulated independently. In this work, Monte Carlo results have been accurately replicated by an analytical model that uses characteristics of the foreseen cylindrical scanner and returns the sensitivity profile following NEMA guidelines. BGO and LYSO bulk materials and several metascintillator scenarios have been used. The mean absolute error (MAE), mean absolute percentage error (MAPE) and standard deviation of the error (SDE) are as low as 0.49%, 2.22% and 0.26% when no energy window is used, respectively. With an energy window applied, the analytical model presents the lowest values of MAE and SDE, with MAPE value being 8.19%. A normalization factor has been used to compensate for the scattered events included in the 350-650 keV window. This work facilitates significantly the development of cylindrical scanners, allowing direct probing of their axial sensitivity profiles.

In time-of-flight positron emission tomography (TOF-PET), the timing capabilities of the scintillation-based detector play an important role. An approach for fast timing is using the so-called metascintillators, which combine two materials leading to the synergistic blending of their favourable characteristics. An added effect for BGO-based metascintillators is taking advantage of better transportation of Cherenkov photons through UV-transparent materials such as plastic (type EJ232). To prove this, we use an optimized Coincidence Time Resolution (CTR) setup based on electronic boards with two output signals (timing and energy) and near-ultraviolet (NUV) and vacuum-ultraviolet (VUV) silicon photomultipliers (SiPMs) from Fondazione Bruno Kessler (FBK), along with different coupling materials. As a reference detector, we employed a 3×3×5 mm3 LYSO:Ce,Ca crystal pixel coupled with optical grease to a NUVHD SiPM. The evaluation is based on low threshold rise-time, energy and time of arrival of event datasets. Timing results of a BGO and EJ232 metapixel show Detector Time Resolutions (DTRs) of 159 ps for the full photopeak. We demonstrate the possibility of event discrimination using subsets with different DTR from the rising time distributions (RTD). Finally, we present the synergistic capability of metascintillators to enhance Cherenkov photons detection when used along with VUV-sensitive SiPMs.Â