Silicon Photomultipliers (SiPM) as novel photodetectors for PET

Next generation PET scanners should fulfill very high requirements in terms of spatial, energy and timing resolution. Modern scanner performances are inherently limited by the use of standard photomultiplier tubes. The use of Silicon Photomultipliers (SiPMs) is proposed for the construction of a 4D-PET module of 4.8×4.8 cm2 aimed to replace the standard PMT based PET block detector. The module will be based on a LYSO continuous crystal read on two faces by Silicon Photomultipliers. A high granularity detection surface made by SiPM matrices of 1.5 mm pitch will be used for the x–y photon hit position determination with submillimetric accuracy, while a low granularity surface constituted by 16 mm2 SiPM pixels will provide the fast timing information (t) that will be used to implement the Time of Flight technique (TOF). The spatial information collected by the two detector layers will be combined in order to measure the Depth of Interaction (DOI) of each event (z). The use of large area multi-pixel Silicon Photomultiplier (SiPM) detectors requires the development of a multichannel Data Acquisition system (DAQ) as well as of a dedicated front-end in order not to degrade the intrinsic detector capabilities and to manage many channels. The paper describes the progress made on the development of the proof of principle module under construction at the University of Pisa

Depth identification accuracy of a three layer phoswich PET detector module

We describe a PET detector module that provides three levels of depth-of-interaction (DOI) information. The detector is a 9 x 9 array of 2 mm x 2 mm x 12 mm deep phoswich crystal elements, each consisting of 4 mm long LSO (entrance layer), GSO (middle layer) and BGO (exit layer) crystals joined optically together end-to-end. The BGO exit layer is directly coupled to a miniature position-sensitive photomultiplier tube (PSPMT). Delayed charge integration, a method that exploits differences in the light decay times of these scintillators, is used to determine the layer-of-interaction. DO1 accuracy, measured by scanning a slit source of 5 1 1 keV radiation along the length of the module was 86% for the LSO layer, 80% for the GSO layer and 84% for the BGO layer. Energy resolution at 511 keV was 19% for LSO, 21% for GSO and 40% for BGO. Apparent gain differed between layers in the ratios 2.7: 1.9: 1 .O (LS0:GSO:BGO). Crystal separation was good between crystals in the LSO layer, acceptable between crystals in the GSO layer and poor between crystals in the BGO layer due, primarily, to the pronounced spatial non-linearity of the PSPMT. The delayed charge integration method, however, does appear suitable for obtaining multi-level depth information when DO1 effects are particularly significant, e.g. in very small ring diameter PET scanners for small animal imaging.Was supported, in part, by a grant from CICYT (Spanish Government). S. S. was supported by a grant from the National Research Council.Publicad

Development and Testing of a High Resolution PET Detector for Prostate Imaging

According to the American Cancer Society one in six men will be diagnosed with prostate cancer in their lifetime. Current methods for screening of prostate cancer including various PSA blood tests, as well as the digital rectal exam, are unreliability while current imaging modalities clinically employed (US, CT, MRI) are unable to localize intraprostatic cancer(s). Consequently, diagnosis via core needle biopsy is problematic and a game of chance at best. Therefore, in response to new radiopharmaceuticals applicable to both internal and external prostate cancer visualization and localization, novel prostate specific nuclear medical imagers are being developed.;The first prototype of a compact prostate specific PET detector utilizing silicon photomultiplier (SiPM) technology has been developed and tested at West Virginia University. The compact detector is proposed as an endorectal probe placed proximally to the rectal wall/prostate interface and operating in coincidence with one or more externally mounted large area gamma detectors or in tandem with a clinical whole body PET scanner. To ensure high reconstruction resolution, the scintillation array of the compact detector will be coupled to SiPMs on both axial ends in a dual ended readout approach. Such an approach allows for the extraction of continuous depth of interaction (DOI) information thus minimizing the effects of parallax error and providing nearly isotropic and uniform spatial resolution throughout the entire detector field of view (FOV).;Two compact DOI based prototype detectors were developed and tested. While both utilize pixelated LYSO scintillation crystal arrays, the first has a crystal pitch of 1.0 mm and is coupled to SensL SiPMs, while the second has a crystal pitch of 0.7mm and is coupled to Hamamatsu SiPMs. Initial proof of concept studies were preformed using the SensL based detector while more extensive and systematic studies were preformed using the Hamamatsu based detector. Ultimately, when averaged over all crystals and all depths the Hamamatsu based detector achieved a depth of interaction resolution of 0.78+/-0.09 mm FWHM and an energy resolution of 13.2+/-0.7 % FWHM. Validation studies with regards to the efficacy of incorporating DOI information extracted from a small compact DOI based PET detector module into image reconstruction algorithms were also preformed

Atténuation des interactions électromagnétiques entre le module de détection LabPET II et l’IRM

Les scanners TEP/IRM simultanés offrent une occassion unique d'examiner en même temps les propriétés anatomiques et fonctionnelles des tissus malins, tout en évitant l'incertitude des systèmes séquentiels de TEP/IRM. Cependant, le couplage électromagnétique entre les deux modalités constitue un défi important à relever. Ces interférences électromagnétiques entravent les performances du scanner et altèrent la qualité d'image de chaque modalité. Bien que les métaux possèdent d'excellentes propriétés de blindage contre les fréquences radioélectriques, ils ne constituent pas nécessairement une option de blindage appropriée pour modifier les champs magnétiques induisant des courants de Foucault dans les couches métalliques. En conséquence, il existe une demande considérable pour un nouveau matériau de protection et une approche originale pour retirer les pièces métalliques du champ de vision IRM. L’objectif de ce projet était d’initier les études en vue de la réalisation d’un scanner TEP/IRM simultané basé sur des modules de détection LabPET II hautement pixélisés afin d’obtenir une résolution spatiale millimétrique pour le cerveau humain et le chien. L'électronique LabPET II comprend des circuits intégrés à application spécifique dans lesquels le signal est numérisé à proximité de la photodiode à avalanche et offre un environnement moins sensible aux interférences électromagnétiques. Pour atteindre l'objectif principal, premièrement, l'effet du matériau métallique des modules de détection LabPET II sur les performances de la TEP et de l'IRM est examiné théoriquement. Les résultats confirment que les composants métalliques du module de détection LabPET II altèrent le champ magnétique, génèrent des courants de Foucault ce qui augmente leur température. Ensuite, les performances électroniques des modules de détection LabPET II sous l’influence de bobines d’IRM faites sur mesure sont examinées. La résolution en énergie et la résolution temporelle se détériorent en présence de bobines RF et de bobines à gradient en raison des perturbations électromagnétiques. Subséquemment, un module de détection LabPET II blindé par une fine couche de composite cuivre-argent est étudié, prouvant que le blindage contre les interférences électromagnétiques avec le composite rétablit les performances en TEP, fournissant moins d'induction par courants de Foucault. En outre, une nouvelle configuration de blindage basée sur un composite de couche flexible de nanotubes de carbone a été fabriquée pour limiter les interférences électromagnétiques. Les composites de nanotubes de carbone créent une couche hautement conductrice avec des chemins conducteurs minimaux, ce qui permet de réduire les courants de Foucault. Le principal résultat scientifique de ce projet est que le blindage composite empêche les interférences de basses et hautes fréquences et réduit l'induction de courants de Foucault, offrant ainsi la flexibilité nécessaire pour acquérir une séquence rapide de commutation de gradients. D'un point de vue technique, le module de détection LabPET II ainsi blindé présente une excellente performance dans un environnement de type IRM, ce qui permet de concevoir un insert TEP basé sur la technologie LabPET II.Abstract: Simultaneous PET/ MRI scanners provide a unique opportunity to investigate anatomical and functional properties of malignant tissues at the same time while avoiding the uncertainty of a sequential PET/MRI systems. However, electromagnetic coupling between the two modalities is a significant challenge that needs to be addressed. These electromagnetic interferences (EMI) hinder the performance of both scanners and distort the image quality of each modality. Although metals have excellent radio-frequency shielding properties, they are not necessarily an appropriate shielding option for altering magnetic fields that induce eddy currents in any metallic layer. Thus, there is a considerable demand for a new shielding material and an original approach to remove metallic parts from the MRI field of view. The objective of this project was to initiate the realization of a simultaneous PET/MRI scanner based on highly pixelated LabPET II detection modules to achieve millimeter spatial resolution for the human brain and dogs. The LabPET II electronics include application specific integrated circuits where the signal is digitized near the avalanche photodiode and offers an environment less susceptible to EMI. To fulfill the main aim, for the first time, the effect of the metallic material of LabPET II on PET and MRI performance was theoretically examined. Results confirm that metallic components of the LabPET II detection modules distort the magnetic field, generate eddy currents, and increase temperature. Then, the LabPET II electronics performance under the influence of custom-made MRI coils was investigated. Its energy and timing resolutions deteriorate in the presence of both RF and gradient signals because of EMIs. Thus, a LabPET II detection module shielded by a thin layer of the copper-silver composite was investigated, proving that shielding EMIs with the composite restores the PET performance, with less eddy current induction. Besides, a new shielding configuration based on a flexible layer of carbon nanotube (CNT) composite was fabricated to limit the EMIs. The CNT composite creates a highly conductive layer with minimal conductive paths that allows eddy currents to be decreased. The primary scientific outcome of this project is that the novel composite shielding rejects both low and high-frequency interferences and reduces eddy current induction, offering the flexibility to acquire a fast gradient switching sequence. From a technical point of view, the shielded LabPET II detection module demonstrates an excellent performance in an MRI-like environment supporting the feasibility of designing a PET-insert based on LabPET II technology

Time of flight simulation and reconstruction in Hybrid MR-PET Systems

Tese de mestrado integrado, Engenharia Biomédica e Biofísica (Engenharia Clínica e Instrumentação Médica) Universidade de Lisboa, Faculdade de Ciências, 2018In traditional PET, coincidence electronics are used to determine the line of response along which an an- nihilation has occurred. With time-of-flight(TOF), the approximate position of the annihilation along the line of annihilation is calculated by measuring the difference between the arrival time of the photons in the detectors. In the literature, TOF images show (in general) a lower level of noise and better resolution compared to non-TOF images. The lower noise and amplified sensitivity of TOF reconstruction could favour a better use of the full resolution potential of PET scanners. The first part of this thesis focuses on the possibility of using faster simulating methods, more specifi- cally, the possibility of replacing the time consuming GATE simulations by a script (from Paola Solevi from Otto-von-Guericke-Universität Magdeburg) was studied. The results show that the values obtained in the simulations with the Hoffman Brain Phantom are very similar between the two methods, showing the viability of this script with this phantom. Then, the same procedure was performed using a Voxelized Brain Phantom. This time the results were different from the ones obtained before because the values obtained with the two methods are very different. Therefore, it is important to know if there is some kind of problem with the phantom used that origins those results or if the problem comes from the script. The second part of this thesis focuses on the development of reconstruction procedures for simulations done with the GE Signa PET-MR scanner. The methods includes the simulation of three phantoms (of- fcenter cylinder and Hoffman Brain Phantom to reconstruct and a large cylinder for the normalisation), a coordinates algorithm developed in MATLAB that can calculate the correct coordinates, for the sino- grams, from the GATE coordinate output and a method that, from an uncorrected sinogram, obtains an arc corrected sinogram that can be used in reconstructions. The results show that the reconstructions were successful, without any artifacts. The reconstructions done without each one of the corrections, show artifacts in both phantoms. These results show the importance of doing corrections before recon- structing the data