D. Rouleau

Initial results from the Sherbrooke avalanche photodiode positron tomograph

The design features and engineering constraints of a PET system based on avalanche photodiode (APD) detectors have been described in a previous report. Here, the authors present the initial results obtained with the Sherbrooke APD-PET scanner, a very high spatial resolution device designed for dynamic imaging of small and medium-sized laboratory animals such as rats, cats, rabbits and small monkeys. Its physical performance has been evaluated in terms of resolution, sensitivity, count rate, random and scatter fractions, contrast and relative activity recovery as a function of object size. The capabilities of the scanner for biomedical research applications have been demonstrated using phantom and animal studies.

A microvolumetric blood counter/sampler for metabolic PET studies in small animals

Quantitative metabolic imaging in small animals with positron emission tomography (PET) requires the determination of the tracer concentration in whole blood, arterial plasma and metabolites as a function of time. A blood counting and sampling system was designed to simultaneously measure the time-activity curve as microvolumes of blood are collected. The system consists of a flow-through counter made of a plastic scintillator to detect positrons and of a computer-controlled blood sampler based on the concept of bubble segmentation. The number and size of samples, the withdrawal speed and the sampling time are programmable and can be modified on-line. Samples as small as 10 /spl mu/l can be repetitively obtained from an implanted arterial catheter in the femoral vein or artery of small rats (150 g) or the jugular vein of mice (20 g). For medium sampling speed (100 /spl mu/l/min) at a constant rate, the standard deviation of the sample activity is typically less than 1%. By cutting the tubing at the bubbles at the end of the experiment, samples are made available for further processing and biochemical analysis. This apparatus has become an essential tool for quantitative animal PET studies, allowing easy, reliable sampling at a low cost.

A PET camera simulator with multispectral data acquisition capabilities

The Sherbrooke positron emission tomography (PET) simulator was designed and built to investigate parameters which influence the performance of a high-resolution PET camera based on avalanche photodiode detectors. The simulator consists of a computer controlled scanning table with 32 detection channels shared between front-end cassettes and FASTBUS boards, and of a PC-based multichannel analyzer (MCA) used as a histogramming memory for multiparametric data acquisition. Tomographic data are collected by scanning one of two opposite arrays of detectors and by rotating the object in a predetermined sequence to simulate a complete ring of detectors with various sampling schemes. All acquisition parameters are programmable through digital-to-analog converters or onboard registers. Data can be acquired in several modes: calibration, where direct or coincident energy spectra from all detectors can be registered simultaneously; standard, where only energy-validated coincident events are histogrammed as lines-of-response addresses; and multispectral, where the LOR address is encoded with the energy information to provide a multiparameter histogram. Data samples obtained in these modes are presented. >

Time discrimination techniques using artificial neural networks for positron emission tomography

Relevant information in positron emission tomography is currently being obtained mostly by analog signal-processing methods. New digital PET scanner architectures are now becoming available, which offer greater flexibility and easier reconfiguration capability as compared to previous PET designs. Moreover, new strategies can be devised to extract more information with better accuracy from the digitized detector signals. Trained artificial neural networks (ANN) have been investigated to improve coincidence timing resolution with different types of Avalanche PhotoDiode (APD)-based detectors. The signal at the output of a charge-sensitive preamplifier was digitized with an off-the-shelf, free-running 100-MHz, 8-bit analog-to-digital converter and time discrimination was performed with ANNs implemented in field-programmable gate array (FPGA). Results show that ANNs can be particularly efficient with slow and low light output scintillators, such as BGO, but less so with faster luminous crystals, such as LSO. In reference to a fast PMT-plastic detector, a time resolution of 6.5 ns was achieved with a BGO-APD detector. With LSO, the ANN was found to be competitive with other digital techniques developed in previous works. ANNs implemented in FPGAs provide a fast and flexible circuit that can be easily reconfigured to accommodate various detectors under different signal/noise conditions.

Initial results from the Sherbrooke avalanche photodiode PET scanner

The design features and engineering constraints of a PET system based on avalanche photodiode (APD) detectors have been described in a previous report. Here, the authors present the initial results obtained with the Sherbrooke APD-PET scanner, a very high spatial resolution device designed for dynamic imaging of small and medium-sized laboratory animals such as rats, cats, rabbits and small monkeys. Its physical performance have been evaluated in terms of resolution, sensitivity, count rate, random and scatter fractions, and activity recovery as a function of object size. The capabilities of the scanner for biomedical research applications have been demonstrated using phantom and animal studies.

A stationary sampling scheme for multilayer positron tomographs

A stationary sampling scheme applicable to tomographic instruments incorporating two or more detector layers is described and tested. In this concept, the detectors in adjacent layers are angularly offset by half the interdetector distance. By reconstructing in one single slice all lines of response defined by two adjacent rings of detectors, a fourfold increase in the number of coincidence lines is obtained and a uniform sampling distance equal to one-quarter the interdetector spacing is achieved. Whereas this is obtained at the expense of a 100% degradation of the resolution in the axial direction, with the recent breed of PET (positron emission tomography) scanners using nearly square cross section detectors, the resolution loss may be tolerable in many situations. In addition, normal reconstruction of the individual coincidence planes is always possible. The sampling concept was investigated experimentally with the help of the Universite de Sherbrooke PET camera simulator.

Design And Engineering Aspects Of Avalanche Photodiode PET Tomograph

Design and Engineering Aspects of Avalanche Photodiode

Front-end processing electronics for a PET tomograph based on BGO-avalanche photodiode detectors

Summary form only given, as follows. The front-end analog and digital signal processing electronics for the Sherbrooke animal PET (positron emission tomography) tomograph, based on BGO-avalanche photodiode detectors, is discussed. The system is implemented with high-density dual-side surface mount printed circuit boards mounted directly behind the detector arrays in the tomograph, and FASTBUS format boards housed in external cabinets. The signals from each detector are processed using fast/slow channels for timing/energy validation. Constant fraction discrimination is used to generate the timing pulses. Gated integration and analog-to-digital conversion of the slow signals allow energy discrimination to be performed digitally. The control parameters for delays and discriminator thresholds are all software-programmable through digital-to-analog converters or on-board memory registers, and their adjustment can be performed automatically by a computer.<<ETX>>

A new stationary sampling scheme for multi-layer positron tomographs

A stationary sampling scheme applicable to tomographic instruments incorporating two or more detector layers is described. In this concept, the detectors in adjacent layers are angularly offset by half the interdetector distance. By reconstructing in one single slice all lines of response defined by two adjacent rings of detectors, a fourfold increase in the number of coincidence lines is obtained and a uniform sampling distance equal to one quarter the interdetector spacing is achieved. Whereas this is obtained at the expense of a 100% degradation of the resolution in the axial direction, with the recent breed of PET (positron emission tomography) scanners using nearly square cross section detectors the resolution loss is tolerable; in addition, normal reconstruction of the individual coincidence planes is always possible. The new sampling concept was investigated experimentally with a PET camera simulator. Results obtained by simulating the proposed scheme confirmed the improvement in transaxial imaging performance.<<ETX>>