Publication: Archimedean PET: new optimal tessellation
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2022-06
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2022-06-24
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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.
Description
Mención Internacional en el título de doctor.
Keywords
Positron Emission Tomography (PET), Computerized Tomography (CT), Medical image processing, Image reconstruction