A modular data acquisition system for high resolution clinical PET scanners

En las últimas dos décadas, la Tomografía por Emisión de Positrones (PET) ha demostrado ser una modalidad clave para el estudio de la biología del cúncer y trastornos cardíacos, y para la realizaciún imágenes moleculares, una tecnica que permite la terapia individualizada de la enfermedad [Weissleder01]. La mejor característica de la PET es su sensibilidad: es la tecnica que proporciona imúagenes moleculares con la mayor sensibilidad, y las imúagenes de cuerpo entero que produce no pueden ser igualadas por otras modalidades [Hoh97, Chae07]. Por otra parte, la PET no proporciona referencias anatómicas, lo cual es un problema facilmente resoluble a traves de su integraciún o coregistro con la tomografía computarizada de rayos X (CT) [Lu07]. Ademas, en com- paraciún con otras modalidades de imagen, la PET se caracteriza por bajas estadísticas de conteo (es decir, desintegraciones por unidad de tiempo), lo cual generalmente limita la resoluciún de la imagen. Sin embargo, detectores con geometrías dedicadas permiten solventar ester problema, y producir exce¬lentes resultados [Humm03]. Dos ejemplos destacables de especializacion, que tomaremos como aplicaciones de referencia, son la Mamografía por Emision de Positrones (PEM) y la monitorizacion en línea de dosis en hadroterapia (in-beam PET o, brevemente, ibPET). Las amplias posibilidades de especializacion tienen, sin embargo, una con¬trapartida: es necesario el desarrollo de sistemas de adquisicion igualmente especializados, cuyo coste y prestaciones puede impedir de hecho conseguir las ventajas teúricas proporcionadas por una geometría dedicada. El objetivo de esta tesis doctoral es proponer una nueva arquitectura tecnologica flexible, capaz de obtener prestaciones similares al estado del arte en distintas aplicaciones, a traves de una plataforma de adquisiciún compacta y eficiente en coste, adecuada para PEM e ibPET. En primer lugar, se exploraran el estado del arte y los problemas que han evitado la amplia difusion de equipos PET dedicados en entornos clínicos. Es¬pecial atenciún se dedicara a las soluciones tecnologicas y las características de los escaneres PEM anteriores. Se revisara tambien la situation de losequipos ibPET, especialmente las prestaciones requeridas para soportar efi¬cazmente la planificación de tratamiento en hadroterapia. En segundo lugar, se tratara un diseño conceptual propuesto como solucion al problema. Se propondrán y justificaran diferentes alternativas, con el fin de maximizar la eficiencia de detection y minimizar el coste. A traves de una serie de prototipos intermedios, se implementaran y caracterizarán las opciones de disenño elegidas. Por ultimo, se propondráa el disenño e implementacioán de un prototipo final de sistema de adquisicioán. El equipo en cuestiáon integraráa y extenderaá la soluciones validadas con los prototipos anteriores. x La investigation llevada a cabo durante esta tesis ha permitido realizar un sistema de adquisicion, con prestaciones al estado del arte, apto para el uso de PET dedicado en el entorno clínico del paciente, y que apoyara la investigation en PEM e ibPET. Summary In the last two decades, Positron Emission Tomography (PET) showed to be a key modality to interrogate biology for cancer and cardiac disorders, and to perform molecular imaging, a technology that permits individualized therapy of disease [Weissleder01]. PET's best characteristic is sensitivity: it is the most sensitive tech¬nique for medical molecular imaging, and the whole-body images it produces are unequalled by any other modality [Hoh97, Chae07]. Of course, it lacks anatomical reference, but this is a problem that can be easily overcome with X-ray (CT) (Computed Tomography) integration or coregistration [Lu07]. Moreover, even if PET uses relatively low statistics with respect to other modalities, which generally limits image resolution, it can produce excellent results by using dedicated detector geometries [Humm03]. Two remarkable examples for this kind of system specialization, that we will take as refer¬ence applications, are Positron Emission Mammography (PEM) and in-beam PET (ibPET) for dose delivery monitoring in hadrontherapy. However, the wide range of design possibilities has a counter effect: it re¬quires the development of specifically tailored acquisition systems, whose cost and performances could actually prevent the achievement of the theoretical advantages obtainable with a specialized detector assembly. This doctoral thesis aims at proposing an alternative technological archi¬tecture, able to achieve state of the art PET imaging performances by means of a compact, cost efficient acquisition platform, suitable for its adoption in both PEM and ibPET. Firstly, the state of the art and the controversies that prevent the broad use of dedicated PET in clinics will be explored. Special attention will be paid to the technological solutions and characteristics of previous PEM scanners. It will be also reviewed the current status of ibPET, with particular focus on the performances required to effectively support treatment planning in hadrontherapy. Secondly, a conceptual design solution will be discussed. Various alter-natives will be proposed and justified, with the aim of maximizing detection efficiency and minimize system cost. Through a series of intermediate pro¬totypes the various design choices are implemented and characterized. Thirdly, a final prototype of the acquisition system is designed and im¬plemented. This piece of hardware integrates and extends the solutions that have been validated through the previous systems. The research carried out during this thesis has allowed realizing a state of the art acquisition system that is suitable for specialized PET imaging in the clinical environment of the patient, and that will be used for further research in PEM and ibPET imaging

Low-resource synchronous coincidence processor for positron emission tomography

We developed a new FPGA-based method for coincidence detection in positronemissiontomography. The method requires low device resources and no specific peripherals in order to resolve coincident digital pulses within a time window of a few nanoseconds. This method has been validated with a low-end Xilinx Spartan-3E and provided coincidence resolutions lower than 6 ns. This resolution depends directly on the signal propagation properties of the target device and the maximum available clock frequency, therefore it is expected to improve considerably on higher-end FPGAs

Immagini dall'antimateria: la tomografia ad emissione di positroni.

L'antimateria è una parte dell'universo suggestiva e sfuggente. Oggi abbiamo imparato a usarne piccole quantità per osservare e studiare le funzioni biologiche invivo nell'uomo e negli animali

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

Evaluation of algorithms for photon depth of interaction estimation for the TRIMAGE PET component

The TRIMAGE consortium aims to develop a multimodal PET/MR/EEG brain scanner dedicated to the early diagnosis of schizophrenia and other mental health disorders. The PET component features a full ring made of 18 detectors, each one consisting of twelve 8x8 Silicon PhotoMultipliers (SiPMs) tiles coupled to two segmented LYSO crystal matrices with staggered layers. In each module, the crystals belonging to the bottom layer are coupled one to one to the SiPMs, while each crystal of the top layer is coupled to four crystals of the bottom layer. This configuration allows to increase the crystal thickness while reducing the depth of interaction uncertainty, as photons interacting in different layers are expected to produce different light patterns on the SiPMs. The PET scanner will implement the pixel/layer identification on a front-end FPGA. This will allow increasing the effective bandwidth, setting at the same time restrictions on the complexity of the algorithms to be implemented. In this work two algorithms whose implementation is feasible directly on an FPGA are presented and evaluated. The first algorithm implements a method based on adaptive thresholding, while the other uses a linear Support Vector Machine (SVM) trained to distinguish the light pattern coming from two different layers. The validation of the algorithm performance is carried out by using simulated data generated with the GAMOS Monte Carlo. The obtained results show that the achieved accuracy in layer and pixel identification is above the 90% for both the proposed approaches

Projector Model for Efficient List-Mode Reconstruction in PET Scanners with Parallel Planar Detectors

We have developed a new projector model specifically tailored for fast list-mode tomographic reconstructions in Positron emission tomography (PET) scanners with parallel planar detectors. The model provides an accurate estimation of the probability distribution of coincidence events defined by pairs of scintillating crystals. This distribution is parameterized with 2D elliptical Gaussian functions defined in planes perpendicular to the main axis of the tube of response (TOR). The parameters of these Gaussian functions have been obtained by fitting Monte Carlo simulations that include positron range, acolinearity of gamma rays, as well as detector attenuation and scatter effects. The proposed model has been applied efficiently to list-mode reconstruction algorithms. Evaluation with Monte Carlo simulations over a rotating high resolution PET scanner indicates that this model allows to obtain better recovery to noise ratio in OSEM (ordered-subsets, expectation-maximization) reconstruction, if compared to list-mode reconstruction with symmetric circular Gaussian TOR model, and histogram-based OSEM with precalculated system matrix using Monte Carlo simulated models and symmetries

Characterization and Test of a Data Acquisition System for PET

A small Positron Emission Tomography demonstrator based on LYSO slabs and Silicon Photomultiplier matrices is under construction at the University and INFN of Pisa. In this paper we present the characterization results of the read-out electronics and of the detection system. Two SiPM matrices, composed by 8 × 8 SiPM pixels, 1.5 mm pitch, have been coupled one to one to a LYSO crystals array. Custom Front-End ASICs were used to read the 64 channels of each matrix. Data from each Front-End were multiplexed and sent to a DAQ board for the digital conversion; a motherboard collects the data and communicates with a host computer through a USB port. Specific tests were carried out on the system in order to assess its performance. Futhermore we have measured some of the most important parameters of the system for PET application

A novel random counts estimation method for PET using a symmetrical delayed window technique and random single event acquisition

We have developed a novel logic scheme for the estimation of the random count distribution based on a dual symmetrical delayed window technique. The solution has been applied to a dual head PET case. We have also implemented a new method for noise variance reduction in the random count distribution

Online monitoring for proton therapy: A real-time procedure using a planar PET system

In this study a procedure for range verification in proton therapy by means of a planar in-beam PET system is presented. The procedure consists of two steps: the measurement of the β+-activity induced in the irradiated body by the proton beam and the comparison of these distributions with simulations. The experimental data taking was performed at the CNAO center in Pavia, Italy, irradiating plastic phantoms. For two different cases we demonstrate how a real-time feedback of the delivered treatment plan can be obtained with in-beam PET imaging