Characterization of a High-Aspect Ratio Detector With Lateral Sides Readout for Compton PET

Abstract

Although positron emission tomography is probably the most specific molecular imaging modality, it still lacks a high detection sensitivity. One way of improving this is by implementing larger axial scanners and, therefore, increasing the solid angle coverage. Alternatively, it is possible to increase the sensitivity gain by improving the timing capabilities of the detectors. However, from the most fundamental nature of particle interactions with matter, the 511-keV gamma rays suffer, in most of the cases, from scatter collisions either in the patient or within the detector block, before a photoelectric event eventually occurs. Recovering all scattered (Compton) events would improve scanner sensitivity. In this work, we show the performance of a detector block geometry suitable for the development of PET scanners based on several detector layers. A geometry using multiple layers favor the process of scattered events, at the time that allows one for their proper identification. The detector block consists of a LYSO crystal with 51.5 × 51.5 mm 2 surface and 3 mm thickness, resulting in a very high aspect ratio above 17. Four custom-made SiPM arrays of 1 × 16 elements with 3 × 3 mm 2 area each are coupled to the lateral sides of the crystal. Four different methods to estimate the gammaray interaction position using the information collected by the four SiPM arrays have been implemented and compared in order to assess the most suitable one for this detector configuration and aspect ratio. A novel calibration method based on Voronoi diagrams has been successfully implemented, allowing us to recover data for the entire detector block. We have reached an intrinsic spatial resolution for the whole block of less than 1.6 mm FWHM, combined with an energy resolution of 12.1%. We have also compared the performance results with detector blocks using the same crystal but employing the standard backreading approach. Similar results were obtained but making use of four times less SiPM active area in the case of the lateral reading compared to the backreading method, and with the possibility to minimize the undesirable scatter in the photosensor layers.

Metascintillators for Ultrafast Gamma Detectors: A Review of Current State and Future Perspectives

“…Metascintillators challenge these traditional approaches. At the moment, we are developing prototypes in two directions, high-aspect ratio [35] quasimonolithic designs and metapixels.…”

Tomographic imaging with Compton PET modules: ideal case and first implementation

“…To further study possible factors causing the discrepancy between simulation and experimental results, we conducted one more set of phantom simulations in the full ring scanner configuration. The phantom was the hot rods phantom embedded in a cylinder of 36.0 mm in diameter and 11 mm in thickness, which had the same dimensions and material as in the experiment (figure 4b).…”

“…For monolithic detector designs, the spatial resolution degrades with increasing scintillator thickness [5,6]. To mitigate these negative effects on detector performance when using thicker scintillator, a layer structure detector design with side readout has recently been proposed [7][8][9][10][11]. In this side readout design, even though scintillation light needs to be detected on all four sides of the module, it is possible to use fewer photodetectors than a conventional detector design by using a detector module with relatively 2021 JINST 16 T04007 large cross-sectional area compared to its depth, or using sparsely populated photodetectors [9].…”

Implementation of New Algorithms for an Accurate Gamma-Ray Impact Determination in Scintillation Monolithic Blocks for PET Applications

“…The Voronoi calibration was first implemented and validated at the detector level using different detector block configurations [1] [284]. In all cases, the calibration map was obtained acquiring data from an array of 11´11 22 Na sources that allows us to retrieve all the calibration positions without the use of complex hardware setups, thus facilitating the procedure and reducing calibration times.…”

“…The article [1], enclosed as part of this thesis, explains the details of the experimental procedure, implementation of the method, calibration process and most relevant experimental results obtained with the developed methodology. The Voronoi calibration was also validated at the detector level using LYSO monolithic crystals of 51.5´51.5´3 mm 3 (very high aspect ratio), read out using the lateral faces 51.5´3 mm 2 by means of 1´16 SiPM elements [284]. This detector block design was envisaged for its application in PET systems with multiple separated crystal layers, so each layer can work as a PET or as a Compton camera.…”