Silicon photomultipliers for gamma radiation detectors in nuclear medicine applications

Chil Pérez, Rigoberto

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Silicon photomultipliers for gamma radiation detectors in nuclear medicine applications

Autor(es): Chil Pérez, Rigoberto

Director(es): Vaquero López, Juan José

Titulación: Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de Madrid

Fecha de edición: 2019-09

Fecha de defensa: 2019-09-20

Tribunal: Presidente: Stefaan Vandenberghe.- Secretario: Susana Patón Álvarez.- Vocal: Pedro Fernando Rato Mendes

Agradecimientos: Nuclear medicine is the medical field that studies the different applications of radiation for the treatment and diagnosis of disease. In particular, nuclear medicine imaging is the imaging modality with the highest specificity. The small amounts of radiotracer injected to the patients in these studies are enough to identify lesions that would not be detected with other procedures. Preclinical imaging plays a decisive role in the development of new drugs and treatments. Animal studies, although controversial, are a necessary evil to ensure the safety and effectiveness of new therapies. The main difficulty of preclinical scanners is that they require higher resolution and better sensitivity than human systems, due to the size of the animal. In recent years silicon photomultipliers(SiPM) have become the detector of choice over photomultiplier tubes(PMT) on new scanners. SiPMs have proven to be more sensitive than PMTs, smaller in size and are not affected by magnetic fields in the same way that PMTs are. SiPMs are easier to manufacture than PMTs, and can be made into different shapes and sizes, making it possible consider new detector geometries that can potentially improve sensitivity and resolution of current designs. Whole body scanners, able to cover the entire the human body, that seemed impossible some years ago are now a reality and so are the benefits that come with them, an improvement in resolution and sensitivity that directly reduces the dose and the time required for these studies. Dosimetric studies are mandatory on all radioactive facilities, hospitals, nuclear power plants, etc. Dosimeter use is required for all radiation exposed personnel. An electronic dosimeter, that provides real time measurements and localization, can generate alerts as soon as an increase in radioactivity is detected, improving the safety of the workers exposed to radiation. Although a dosimeter may seem not a very complex device, it is complex enough to serve as a good testing platform for radiation detectors. In this work we developed an electronic dosimeter that has proven able to provide clean energy spectra in real time. These spectra can be used not only to measure the dose, but also to identify the radioactive isotopes generating it. This dosimeter allowed us to prove not only that SiPMs are a good candidate for PMT replacement, but also that most of the electronic designs used for PMTs work for SiPMs as well. A radiation detector for positron emission tomography (PET) should provide good energy and position information, such that these data can be used to reconstruct an image. The requirements for preclinical systems are more strict than those for human scanners. In this thesis we developed SiPM based detectors that can clearly distinguish the scintillator pixels on a scintillator array of 1.38mm pixel pitch. The energy resolution of these detectors is 13.37, facilitating a proper detection of the 511 keV photons used in PET imaging. These detectors were used in the construction of a ring to acquire tomographic images. With this ring configuration we were able to reconstruct a point source of 1.3mm placed in the center of the field of view. To overcome the limitations of current PET scanner designs, we proposed a new geometric configuration, based on an icosahedron, with the aim of improving the solid angle coverage of the system. This improvement in coverage has a direct impact in sensitivity and resolution, resulting in a more sensitive and accurate scanner. We introduced as well the use of new detector and scintillator geometries, that helps us achieve a more sensitive and precise scanner. A new SiPM based detector with an hexagonal shape was developed in this work. Thanks to the use of an application specific integrated circuit (ASIC) we were able to create a detector that can distinguish pixels of 1.28mm pixel pitch, the results were good enough to even consider the use of smaller pixels.