Archimedean PET: new optimal tessellation

Abstract

Since the early studies in cerebral Positron Emission Tomography (PET) imaging in the mid-seventies, the development of radiotracers, computer-based algorithms, photodetectors, scintillators and technology in a broader sense, has made of PET a prominent imaging technology in research, diagnoses and treatment in fields such as neurology, oncology, cardiology and drug development. Detectors are commonly dense inorganic scintillators capable to halt isotropic ejected gamma photons resulting from the decay of radioactive tracers. By similarity with Computerized Tomography (CT), technology earlier unveiled in the seventies, detectors were arranged in rings with the specimen lying along the axial direction. This way, it was possible to adapt the existing image reconstruction algorithms to accurately estimate the distribution of radiotracers. Despite of the fact that 3D algorithms and technology to promote to fully 3D scanners were available already in the 90’s, the evolution of PET scanners has not pursued the optimal spatial arrangement. It has headed, however, towards compact, affordable and competitive scanners or total body systems with large axial extend that significantly increases the sensitivity. Only few research groups have proposed brain scanners with shape of helmet, rings of decreasing diameter in the zenithal direction, dodecahedron or small but movable device. The ideal design, the one with the highest solid angle, is a spherical distribution of detectors as close as possible to the imaging subject, but the realization up to the present day is not doable with the state of the art of rigid scintillators and photodetectors. In chapter 2, it is presented a design for preclinical imaging, shaped as a truncated icosahedron with hexagonal scintillators and photodetectors covering its twenty faces. The proposal has some favourable geometrical characteristics, namely: equal morphology of facets, symmetrical and equidistant arrangement of detectors, lack of sharp angles between adjacent facets and overlap of 6/9 of diametrical opposite faces through the center. The figures of merit of sensitivity profiles, Noise Equivalent Count Rate (NECR) and spatial resolution are presented. A closer design to the sphere with the shape of buckysphere is also introduced. The hexagonal scintillators and photodetectors rise a question on the role of the geometry into the light yield and energy resolution. High symmetry bodies have larger Total Internal Reflection (TIR), thus lower light output. However, optical coupling media with the photodetector, smoothness of crystal’s surface and reflectors greatly break the symmetry. In chapter 3, four geometries are compared as feasible tessellation solutions: triangular, square, hexagon and cylinder. Simulations are conducted with specular and Lambertian reflectors and rough and polished finishes. Results are experimentally validated with four LYSO pieces cut with water jet. This chapter also explores different readout systems to optimize the scintillation position estimation with the minimum number of output channels. Pixel resolution and planar image are presented with 181 hexagonal scintillator crystals read by the 61 channels hexagonal photodetector. The thickness of the reflective material in segmented detectors is in the range of dozens of micrometers. This finite space between crystals reduces unavoidably the detection capabilities since gamma photons can freely go in the intervening reflective material. The application of material laser processing to imprint scintillation blocks can circumvent this issue as well as provide flexibility in the engraving pattern, size and optical barriers permeability. Chapter 4 devises the application of Sub-surface Laser Engraving (SSLE) with a cost-effective nanosecond laser in LYSO scintillation blocks up to 15 mm thick. Field flood images and pixel resolvability in crystals of different thicknesses are presented. Furthermore, two shifted layers detector with Depth of Interaction (DOI) capabilities in a crystal of 10 mm of height is also presented. Finally, triangular crystals of the side of a photodetector channel were created, engraving that would serve the hexagonal photodetector with 366 crystals. To sum up, this work covers the design of a mechanically and technologically feasible scanner, presents by means of Monte Carlo simulations the performance of the scanner, characterizes the custom-made hexagonal detectors and explores the versatility of a laser technique to create optical barriers from a monolithic block.