The signal-to-noise ratio in Positron Emission Tomography (PET) improves with precise timing resolution. PET systems enabling the capability of Time-of-Flight (ToF) are nowadays available. This study assesses various data configurations, comparing the obtained timing performances applicable to ToF-PET systems. Different readout configurations were evaluated together with Silicon Photomultipliers (SiPMs) photosensors from the Fondazione Bruno Kessler (FBK), with and without the so-called metal trench (MT) technology. The tests were carried out with scintillation crystals of 3×3×5 mm3 (LYSO:Ce,Ca) from SIPAT. Two onboard FPGA-based systems, namely the Felix Time-to-Digital Converter (TDC) from Tediel S.r.l. and the ASIC-based FastIC from the University of Barcelona, along with custom-made high-frequency electronics (CM-HF), were compared. Considering only photopeak events, the best coincidence timing resolution (CTR) results obtained were 71 ps with the MT SiPMs. This result worsened to 88 ps with the old version of the same device that does not include the MT technology (called HD). The results demonstrate substantial CTR improvements when MT SiPMs were used across the different scenarios, resulting in a timing improvement in the 10 to 45 ps range compared to HD SiPMs. Notably, the Felix TDC achieved sub-100 ps timing results, emphasizing the potential of FPGA technology in ToFPET applications. Moreover, the fully passive version of the CMHF connected to the MT SiPMs shows only a degradation of 8 ps difference compared to the version using amplifiers. The novel MT-type SiPMs promise superior timing performance, enhancing accuracy and efficiency in PET imaging systems.

Positron emission tomography scanners are commonly characterized by their gamma 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, costly and time consuming. 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 algorithm based on an analytical approach 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 approach 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.

The working principle of metascintillators is based on sharing the energy of an impinging gamma ray between their composing materials. Such can be a dense crystal such as LYSO or BGO to maximize the gamma stopping potential and a fast organic or inorganic compound such as BC-422, EJ232 or BaF2 for its light production kinetics. In this work we look into the details of metascintillator pulses as modelled through a double bi-exponential model. We analyze the extent of energy sharing, as understood through analysis, simulation and experiment in a coincidence timing resolution (CTR) measurement setup, using 3x3x15 mm3 metascintillators, against a reference detector. Features of individual pulses allow choosing the photoelectric interactions and provide insight on the energy sharing extent of each gamma interaction. We evaluate the quality of energy sharing surrogates for different metascintillator designs. Different populations of photoelectric interactions depending on the extent of energy sharing are defined, that have different contribution of fast photons in the first picoseconds and hence different timing. We benchmark this selection through using these features to apply a timewalk correction on an event-to-event basis. A significant improvement is demonstrated in all cases, while for a 3:1 volume ratio BGO:EJ232 metascintillator this improvement rises up to ~25% for the whole photopeak (204.7 ps), while the 10% events with higher production in the fast emitter show a ~50% improvement to 54.7 ps. This shows that while metascintillators with comparable light yield components still provide the best alternative, it is possible through simple pulse analysis to measure and isolate the photoelectric interactions in every metascintillator with two components

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.

Presentation of simulation results on a new metascintillator concept (semi-monolithic) combining state-of-the art timing resolution and depth-of-interaction, with the application of neural networks to provide accurate estimations of energy sharing and energy of interaction.

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

A document by Georgios Konstantinou . Click on the document to view its contents.