Small animal Positron Emission Tomography (PET) studies require high-resolution detectors. Traditionally, inorganic scintillators coupled to Position Sensitive PMTs are used. Such PSPMTs are somewhat costly, operate at high voltage and have a relatively low packing fraction. However, the advantage, compared to standard solid state photodetectors, is their high signal-to-noise ratio. The Silicon Photomultiplier (SiPM) is a novel silicon diode detector that shows great promise as a photodetector for scintillators and hence for application in Nuclear Medicine imaging. The SiPM is a densely packed matrix of small, Geiger-mode avalanche photodiode (GAPD) cells (typically ~ 40 micron x 40 micron), with individual quenching resistors. The SiPMs developed within this project will have of the order of 1000 microcells and a quantum efficiency (QE) maximized in the wavelength range 420 - 470 nm. The Geiger-mode operation of each cell produces a large gain (of the order of 10^6) at low bias voltage (about 50 V). All the cell outputs are then connected in parallel to have the summed signal. This microcell MRS (Metal-Resistor-Semiconductor) structure of the SiPM gives an output proportional to the number of incident photons for moderate photon flux. The characterization of individual SiPMs showed their suitability for radiation detection, but their small dimensions are not adequate for the development of big active area detector. Hence, the goal of this project is to realize a large area 2D array of SiPMS on the same silicon substrate. As a first step, up to 1.2x1.2 cm^2 matrices of 8 x 8 SiPMs will be developed with read-out pads on the front side located at the edge of the die. The matrices will be composed of 1x1 mm2 active area SiPM pixels, on a 1.5 mm pitch. Then, 2 x 4 matrices will be arranged to form a 2.5 x 5.0 cm^2 total surface with minimum dead area. A dedicated readout system consisting of an integrated CMOS front-end electronics and a data acquisition system based on Field Programmable Gate Array (FPGA), will be also designed and developed. Such a compact silicon detector, with a performance similar to a PMT, is obviously well disposed to being developed into a close-packed array in order to have a position-sensitive detection surface. Current systems tend to rely on finely pixilated matrices of scintillators (1.5x1.5 - 2x2 mm^2 pixels) to maximize the spatial resolution. In order to achieve sub-millimeter resolution, the pixels should be made increasingly small, whilst retaining a reasonable length to maintain efficiency. This cross-section reduction has two important consequences: the light yield is decreased and the cost of the matrix production is greatly increased. Therefore, a method that eliminates the use of pixels and returns to a continuous crystal would be desirable. Moreover, the continuous crystal approach would make viable the use in PET of advanced scintillators like LuI3, that can not be made in form of pixels. With these considerations in mind, we propose a novel, miniature, high-resolution camera for a small-animal PET imaging system that is based on a combination of SiPM with a continuous scintillation crystal. The design is based upon the classic Anger camera principle; a detector module consists of a continuous slab of scintillator crystal (i.e. LYSO), viewed by 8 matrices of SiPM. The interaction position is measured by calculating the center of mass of the measured signals. Two of such SiPM gamma cameras will be mounted on a rotating gantry, being the opposite heads in time coincidence to implement the PET concept. The system as a whole is expected to provide a potentially sub-millimeter spatial resolution after image reconstruction. The spatial resolution achievable with the proposed system will be close to the intrinsic limit of the PET technique and it will represent a significant step forward with respect to the state of art of the present small animal PET scanners. The SiPM base...

A very high spatial resolution small animal PET scanner based on high granularity silicon photomultipliers

LANCONELLI, NICO;
2007

Abstract

Small animal Positron Emission Tomography (PET) studies require high-resolution detectors. Traditionally, inorganic scintillators coupled to Position Sensitive PMTs are used. Such PSPMTs are somewhat costly, operate at high voltage and have a relatively low packing fraction. However, the advantage, compared to standard solid state photodetectors, is their high signal-to-noise ratio. The Silicon Photomultiplier (SiPM) is a novel silicon diode detector that shows great promise as a photodetector for scintillators and hence for application in Nuclear Medicine imaging. The SiPM is a densely packed matrix of small, Geiger-mode avalanche photodiode (GAPD) cells (typically ~ 40 micron x 40 micron), with individual quenching resistors. The SiPMs developed within this project will have of the order of 1000 microcells and a quantum efficiency (QE) maximized in the wavelength range 420 - 470 nm. The Geiger-mode operation of each cell produces a large gain (of the order of 10^6) at low bias voltage (about 50 V). All the cell outputs are then connected in parallel to have the summed signal. This microcell MRS (Metal-Resistor-Semiconductor) structure of the SiPM gives an output proportional to the number of incident photons for moderate photon flux. The characterization of individual SiPMs showed their suitability for radiation detection, but their small dimensions are not adequate for the development of big active area detector. Hence, the goal of this project is to realize a large area 2D array of SiPMS on the same silicon substrate. As a first step, up to 1.2x1.2 cm^2 matrices of 8 x 8 SiPMs will be developed with read-out pads on the front side located at the edge of the die. The matrices will be composed of 1x1 mm2 active area SiPM pixels, on a 1.5 mm pitch. Then, 2 x 4 matrices will be arranged to form a 2.5 x 5.0 cm^2 total surface with minimum dead area. A dedicated readout system consisting of an integrated CMOS front-end electronics and a data acquisition system based on Field Programmable Gate Array (FPGA), will be also designed and developed. Such a compact silicon detector, with a performance similar to a PMT, is obviously well disposed to being developed into a close-packed array in order to have a position-sensitive detection surface. Current systems tend to rely on finely pixilated matrices of scintillators (1.5x1.5 - 2x2 mm^2 pixels) to maximize the spatial resolution. In order to achieve sub-millimeter resolution, the pixels should be made increasingly small, whilst retaining a reasonable length to maintain efficiency. This cross-section reduction has two important consequences: the light yield is decreased and the cost of the matrix production is greatly increased. Therefore, a method that eliminates the use of pixels and returns to a continuous crystal would be desirable. Moreover, the continuous crystal approach would make viable the use in PET of advanced scintillators like LuI3, that can not be made in form of pixels. With these considerations in mind, we propose a novel, miniature, high-resolution camera for a small-animal PET imaging system that is based on a combination of SiPM with a continuous scintillation crystal. The design is based upon the classic Anger camera principle; a detector module consists of a continuous slab of scintillator crystal (i.e. LYSO), viewed by 8 matrices of SiPM. The interaction position is measured by calculating the center of mass of the measured signals. Two of such SiPM gamma cameras will be mounted on a rotating gantry, being the opposite heads in time coincidence to implement the PET concept. The system as a whole is expected to provide a potentially sub-millimeter spatial resolution after image reconstruction. The spatial resolution achievable with the proposed system will be close to the intrinsic limit of the PET technique and it will represent a significant step forward with respect to the state of art of the present small animal PET scanners. The SiPM base...
A. Del Guerra; F. Corsi; N. Lanconelli; G.U. Pignatel
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/76514
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