Tests and characterization of gamma cameras for medical applications
Test et caractérisation de caméras gamma pour le médical
Résumé
The application of nuclear and particle physics techniques in the field of medical diagnosis and pathology treatment is nowadays well-established in the clinical routine. In particular, elementary particles are the basis of several medical imaging techniques (Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT), Computed Tomography (CT) scans, etc.), as well as of treatment methods, mainly concerning cancer, which causes about 9 millions deaths per year all over the world. In this context, ion beam therapy is a promising technique in cancer treatment because of the ion defined range and favorable dose delivery features with respect to standard photon radiotherapy. Strict and precise treatment planning and monitoring are now key points for the method development and full exploitation. In particular, with the aim of optimizing the ion treatment effectiveness, the ion range monitoring is mandatory: different solutions have been explored, but an online treatment check is still a challenge. The ion beam treatment monitoring can be performed by means of secondary charged or neutral particles. In this context, the detection of the Prompt-Gammas (PGs) emitted during treatments has proven its potential in the ion range control in real time. Since the first evidence of the existing correlation between the emitted gamma profile fall-off and the Bragg peak position, several groups are involved in research activities in order to develop and optimize instruments and methods with the aim of improving this monitoring technique. Among the others, collimated and Compton cameras are being investigated for this application. The same detectors can also be employed in nuclear medicine for the detection of the radioactive elements decay products.
A collaboration of 4 laboratories in France, called Contrôle en Ligne de l’hadronthérapie par Rayonnements Secondaires (CLaRyS), is involved in the parallel development of two composite detectors for ion beam monitoring and nuclear medicine applications, and this thesis is carried out within this collaboration with the detectors clinical trial as final aim. The development project started a few years ago and is now at the final stage. The two cameras have been designed according to simulation studies, and the different components are now under tests. The collimated camera is composed of a multi-slit tungsten mechanical collimator, set in front of an absorber composed of 30 Bismuth Germanium Oxide - Bi12GeO20 (BGO) blocks, for a total size of 210x175x30 mm3; each block presents a streaked structure with a 8x8 pseudo-pixel matrix and the signal is read-out by 4 Photo-Multipliers (PMs). A 3 ns time resolution can be achieved on average for the prompt-gamma detection. The same absorber is part of the Compton camera, in addition to a scatterer section composed of 7 Double-sided Silicon Strip Detectors (DSSDs), 96x96x2 mm3 each.
With the collimated camera, the parallel emitted photons are selected by the collimator and a mono-dimensional emission profile can be reconstructed. The Compton camera has a more efficient detection technique, thanks to the absence of a mechanical collimation system, and could potentially lead to three-dimensional information via the reconstruction of the Compton cones. These features make it suitable for the application in nuclear medicine, in particular as an alternative to the present SPECT collimated cameras, allowing for accurate and efficient image reconstructions with the usage of high energy gamma sources, which should reduce image blurring effects due to attenuation in the patient and the total released dose with respect to the present clinical routine.
Concerning the monitoring of ion beam therapy treatments, an additional detector component is needed to temporally and spatially tag the incoming beam ions and help rejecting background events (mostly due to neutrons) which strongly affects the prompt-gamma yield. A scintillating fiber tagging hodoscope, which can be coupled to both collimated and Compton camera, is under development: it is composed of 2 perpendicular planes of 128 scintillating fibers, read-out from both sides by 8 multi-anode (64 channels) PMs by Hamamatsu.
This thesis work consists in the critical evaluation, characterization and tuning of the different components, together with the associated electronics, and of the complete detector systems on beam. In parallel, simulation studies can improve the detection techniques and optimize the detector structure, as well as pave the way for further applications.
This manuscript is divided into three parts. Part I introduces the thesis rationale. After a general introduction devoted to the thesis context in chapter 1, an overview of the state of the art of the gamma detection techniques applied in medicine and the employed gamma detection systems is given in chapter 2. Part II consists of four chapters and describes my personal work. Chapter 3 focuses on the two cameras developed by the CLaRyS collaboration; the camera components are described in details, and all the characterization measurements performed during the three years of my PhD thesis are explained. Chapters 4 and 5 present the simulation studies I performed with the aim of investigating the potential of the developed detectors for the application in ion beam therapy monitoring and nuclear medicine, respectively.
Chapter 6 is dedicated to the description of the tests performed on proton beams for the detector/electronics characterization measurements. In Part III I summarize and discuss all the results obtained in this thesis work; furthermore, the perspectives of the project are fixed on a time-line for the next future.
L'application des techniques de physique nucléaire et de la physique des particules dans le domaine du diagnostic médical et du traitement de la pathologie est aujourd'hui bien établie dans la routine clinique. Les particules élémentaires sont notamment à la base de plusieurs techniques d’imagerie médicale (tomographie par émission de positrons (TEP), tomographie par émission de photons uniques (SPECT), tomographie par ordinateur (CT), etc.), ainsi que de méthodes de traitement, concernant principalement: le cancer, qui cause environ 9 millions de décès par an dans le monde entier. Dans ce contexte, la thérapie par faisceaux d'ions est une technique prometteuse dans le traitement du cancer en raison de la plage définie par les ions et des caractéristiques de délivrance de dose favorables par rapport à la radiothérapie à photons standard. Une planification et un suivi stricts et précis du traitement sont désormais des points essentiels pour le développement et l’exploitation complète de la méthode. En particulier, dans le but d’optimiser l’efficacité du traitement par ions, le contrôle de la plage d’ions est obligatoire: différentes solutions ont été explorées, mais la vérification du traitement en ligne reste un défi. La surveillance du traitement par faisceau ionique peut être effectuée au moyen de particules chargées secondaires ou neutres. Dans ce contexte, la détection des Prompt-Gammas (PG) émises pendant les traitements a prouvé son potentiel dans le contrôle de la gamme d'ions en temps réel. Depuis la première preuve de la corrélation existante entre la chute du profil gamma émis et la position maximale de Bragg, plusieurs groupes sont impliqués dans des activités de recherche afin de développer et d’optimiser des instruments et des méthodes visant à améliorer cette technique de surveillance. Parmi les autres, des caméras collimatées et Compton sont à l'étude pour cette application. Les mêmes détecteurs peuvent également être utilisés en médecine nucléaire pour la détection des produits de désintégration des éléments radioactifs.
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