For a long time the discovery of new scintillators has been more serendipitous than driven by a deep understanding of the mechanisms at the origin of the scintillation process This situation has dramatically changed since the 1990's with an increased demand for scintillators of better performance for large particle physics experiments as well as for medical imaging It is now possible to design a scintillator for a specific purpose The bandgap can be adjusted, the traps energy levels and their concentration can be finely tuned and their influence can be damped or on the contrary enhanced by specific doping for an optimization of the performance of the scintillator Several examples are given in this paper of such crystal engineering attempts to improve the performance of crystal scintillators used in medical imaging devices An attention is also given to spectacular progress in crystal production technologies, which open new perspectives for large scale and cost effective crystal production with consistent quality

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Development of new scintillators for medical applications

Similar to analog silicon photomultipliers (SiPMs), digital SiPMs (dSiPMs) essentially consist of an array of single-photon avalanche photodiodes (SPADs). Instead of a passive quench resistor, however, an active quenching circuit is locally integrated with each SPAD, making the sensor response faster and less sensitive to the gains of the individual SPADs. Moreover, additional circuits for the fully digital acquisition, processing, and readout of optical signals are integrated within the sensor. As a result, dSiPMs offer high photo-detection efficiency, high single-photon time resolution (SPTR), and high uniformity, as well as many practical advantages, such as a very compact form factor, low voltage operation, magnetic field compatibility, high stability of operation, low gain drift, and a high degree of scalability. At the same time, dSiPMs represent a new paradigm in low-level light sensing technology. That is, their fully digital operation makes them true photon counting devices, preserving at least partly the discrete spatio-temporal structure of the information embedded in the optical signal. This means that the operation of dSiPMs can be fully understood only in statistical terms, but also opens up novel possibilities for extracting information from the measured data. So far, the main driver behind the development of dSiPMs has been the detection of scintillation pulses in detectors for time-of-flight (TOF) positron emission tomography (PET). Several types of dSiPM have been developed in recent years. Moreover, first imaging devices based on dSiPMs have been realized by various groups. This review summarizes the main dSiPM concepts and technologies currently under development, provides an overview of the results obtained recently with dSiPMs-based PET and SPECT devices, and presents a critical outlook on the challenges and chances for dSiPMs in future radiomolecular imaging systems.

Advances in digital SiPMs and their application in biomedical imaging

This paper presents a comprehensive statistical analysis of timing resolution in a digital silicon photomultiplier (DSiPM) when multi timestamp is available. We look at the effect of dart count rate (DCR) on timing resolution in these devices as compared to analog silicon photomultipliers (A-SiPMs). The analysis includes photon detection efficiency (PDE), DCR, electrical jitter of the detector with a LYSO crystal. Timing resolution is analyzed utilizing a single timestamp or multiple timestamps. Simulation results show that D-SiPMs utilizing multiple timestamps (Multi-channel D-SiPM) can be more tolerant to DCR than those utilizing a single timestamp. The timing resolution is 13%, 20% and 40% better at 1000 detected photons, 200 ps rise time and 40 ns decay time of a LYSO scintillator, without DCR, with 1 MHz DCR and with 10 MHz DCR, respectively. Based on these findings, we propose new architectures to acquire multiple timestamps without sacrificing fill-factor. This paper also includes the implementation and characterization of 4 × 4 Multichannel D-SiPMs coupled with an array of 44ps LSB TDCs for coincidence detection of gamma rays. The pitch of the SiPMs is 800 μm and the number of pixels in one SiPM is 416. The pixel has a 57% fill factor with 121 ps timing resolution for a single photon. The SiPM timing resolution for single photon detection is 179 ps FWHM.

Multi-channel digital SiPMs: Concept, analysis and implementation

The combination of positron emission tomography (PET) and MRI has attracted the attention of researchers in the past approximately 20 years in small-animal imaging and more recently in clinical research. The combination of PET/MRI allows researchers to explore clinical and research questions in a wide number of fields, some of which are briefly mentioned here. An important number of groups have developed different concepts to tackle the problems that PET instrumentation poses to the exposition of electromagnetic fields. We have described most of these research developments in preclinical and clinical experiments, including the few commercial scanners available. From the software perspective, an important number of algorithms have been developed to address the attenuation correction issue and to exploit the possibility that MRI provides for motion correction and quantitative image reconstruction, especially parametric modelling of radiopharmaceutical kinetics. In this work, we give an overview of some exem...

https://www.birpublications.org/doi/pdf/10.1259/bjr.20160363

Advances in PET/MR instrumentation and image reconstruction.

Positron emission tomography (PET) is a medical imaging technique for non-invasive detection of glucose metabolism in human organs, such as the brain, heart, and lungs. A positron-emitting molecule, generally Fluorodeoxyglucose-18 (18F-FDG), is injected and subsequently identified by coincident detection of two gamma photons released during the annihilation of an electron with the positron lost by 1BF-FDG. Conventionally, coincident gamma photons are detected through scintillation by two sensors placed at 180° in a ring of identical sensors (Fig. 11.4.1a). In EndoTOFPET-US [1], two sensors are used, one placed in a semicircular ring outside the patient and the other placed on an endoscope (Fig. 11.4.1b). The endoscopie sensor is a miniaturized array of silicon photomultipliers (SiPMs) coupled to a mini-scintillator. To exploit the Fishburn-Seifert lower bound [2] in timing resolution, a large number of photon timestamps must be generated, as is done in a multi-channel digital SiPM (MD-SiPM).

11.4 A 67,392-SPAD PVTB-compensated multi-channel digital SiPM with 432 column-parallel 48ps 17b TDCs for endoscopic time-of-flight PET

The EndoTOFPET-US project aims to develop a multimodal detector to foster the development of new biomarkers for prostate and pancreatic tumors. The detector will consist of two main components: an external plate, and a PET extension to an endoscopic ultrasound probe. The external plate is an array of LYSO crystals read out by silicon photomultipliers (SiPM) coupled to an Application Specific Integrated Circuit (ASIC). The internal probe will be an highly integrated and miniaturized detector made of LYSO crystals read out by a fully digital SiPM featuring photosensor elements and digital readout in the same chip. The position and orientation of the two detectors will be tracked with respect to the patient to allow the fusion of the metabolic image from the PET and the anatomic image from the ultrasound probe in the time frame of the medical procedure. The fused information can guide further interventions of the organ, such as biopsy or in vivo confocal microscopy.

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M Reinecke

Papers

EndoTOFPET-US: a novel multimodal tool for endoscopy and positron emission tomography