This doctoral thesis focuses on both the design and experimental validation of gamma-ray detectors suitable for Positron Emission Tomography (PET) scanners. The main objective is the design of high efficiency PET detector blocks. The PET technique constitutes one of the main diagnostic tools in Nuclear Medicine, which is a medical specialty that uses radioactive substances for diagnostic and therapeutic purposes. This Medical Imaging technique allows one to visualize physiological and biochemical processes of the human body in vivo, by means of the administration of a radiotracer element. Radiotracers are chemical compounds, similar to the body's natural substances, in which one or more atoms are replaced by radionuclides that emit photons, for use in gamma and SPECT cameras, or positrons (the antiparticle of the electron) for PET. The first chapter of the thesis introduces the main Medical Imaging techniques that are currently used, including functional and anatomical imaging as well as their possible merging generating multimodal images. In the second chapter, since PET imaging is the focus of this thesis, an extensive description of this technique is outlined. The chapter begins with a brief history of PET, showing the advances over the last 60 years until being established as an essential diagnostic tool in medicine. This chapter also describes the physical principles of PET, the reconstruction algorithms and the applied image corrections techniques. In addition to the basic concepts, the role of Time of Flight (TOF) and DOI in PET are described in this chapter. The third chapter describes in detail the materials and methods used in PET, making special emphasis on those used for the development of this work. Currently, most commercial PET systems consist of detector blocks based on pixelated scintillation crystals (matrices of small crystals). These crystals allow one for an easy estimation of the planar impact coordinates of the gamma-ray within the crystal (x, y). However, estimating the depth of interaction coordinate (z), which is essential to obtain a good spatial resolution especially at the edges of the field of view of the scanner, is a difficult task that requires the use of extra materials and therefore increases the price of the scanner. An alternative to that configuration is the use of monolithic crystals, which are constituted by a single piece of scintillating material that permits to characterize the complete, flashing light distribution. This information is used to obtain the 3D impact coordinates of the photon (x, y, z) within the crystal with high resolution and without the need for extra materials. For this reason, this doctoral thesis focusses on the design of PET detector blocks based on monolithic crystals coupled to solid state photosensors. These components are compatible with magnetic fields and therefore, suitable for their simultaneous use with Magnetic Resonance Imaging (MRI) systems. This chapter summarizes the results obtained in the characterization of different types of crystals, geometries and treatments applied to the crystal surface of the detector blocks. Finally, a brief description of the PET systems that have motivated the studies carried out in the thesis is presented. Since this thesis is based on a compendium of the most relevant articles published during the course of the PhD studies, Chapter 4 presents a copy of those research articles, as exactly shown in the different per-reviewed journals, including a brief introduction highlighting their main results. A total of 6 articles are presented, which contain the main results obtained during the doctoral studies. Given the quality of the overall obtained results, two of the designed detector blocks have been selected as the basis of two PET systems dedicated to the study of the human brain namely, i) the MINDView insert (European Union FP7 project) and, ii) the CareMiBrain stand-alone scanner (European Union project Horizon 2020). The MINDView system, which is a PET insert compatible with MRI scanners, has been installed at Technical University of Munich and is currently starting scanning patients with depression and schizophrenia. The thesis includes the validation tests carried out at the level of the detector block and of the final equipment. The CareMiBrain system, which is a PET scanner dedicated to the study of the Alzheimer disease and other causes of cognitive decline, has been installed in a hospital in Madrid and the first patients have already been successfully scanned. The thesis contains the design and results of characterization of the CareMiBrain detector block. In addition to the design and characterization of those detector blocks, other research studies have been carried out during the course of this thesis, such as the characterization of a large variety of photosensor geometries, the optimization of light extraction in BGO crystals, both in pixelated and monolithic geometries, and a hybrid detector approach that uses monolithic and pixellated layers in the same detector block. Special emphasis has been given to the characterization and estimation of the DOI coordinate within monolithic blocks, reducing the parallax error in the reconstructed final image. Chapter 5 contains a summary of the results and conclusions of this thesis. Chapters 6 and 7 summarize, in Spanish and Valencian, respectively, the objectives, motivation, materials, methods, results and conclusions of this doctoral thesis. A complete list of all the per-reviewed articles (including those selected for this compendium) and the conference proceedings published during the development of this thesis can be found in Appendix A.
