S. Rodrigue

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.

Detector response models for statistical iterative image reconstruction in high resolution PET

One limitation in a practical implementation of statistical iterative image reconstruction is to compute a transition matrix accurately modeling the relationship between projection and image spaces. Detector response function (DRF) in positron emission tomography (PET) is broad and spatially-variant, leading to large transition matrices taking too much space to store. In this work, the authors investigate the effect of simpler DRF models on image quality in maximum likelihood expectation maximization reconstruction. The authors studied 6 cases of modeling projection/image relationship: tube/pixel geometric overlap with tubes centered on detector face; same as previous with tubes centered on DRF maximum; two different fixed-width Gaussian functions centered on DRF maximum weighing tube/pixel overlap; same as previous with a Gaussian of the same spectral resolution as DRF; analytic DRF based on linear attenuation of /spl gamma/-rays in detector arrays weighing tube/pixel overlap. The authors found that DRF oversimplification may affect visual image quality and image quantification dramatically, including artefact generation. They showed that analytic DRF yielded images of excellent quality for a small animal PET system with long, narrow detectors and generated a transition matrix for 2-D reconstruction that could be easily fitted into the memory of current stand-alone computers.

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. >

Production of 64Cu on the Sherbrooke TR-PET cyclotron

At energies £15 MeV, 64Cu is most readily produced using the 64Ni(p,n)64Cu reaction. Space restrictions in the TR-PET target selector required a target system of compact design that would still allow the quick and easy removal of the irradiated target. The target consists of a 64Ni-plated rhodium disc held in place by a threaded cooling assembly. For removal, the cooling block is rotated slightly, allowing the disc to fall into a shielding container. The nickel is dissolved and the 64Cu separated by anion exchange or electrochemical deposition. Yields of up to 330 mCi of radiochemically pure 64Cu have been obtained.

Tuning of avalanche photodiode PET camera

The ability of a PET (positron emission tomography) system to reproduce the source distribution accurately depends, in the first place, on proper calibration and efficient quality control of the detectors and processing electronics. These procedures are simplified with individual detectors such as BGO (bismuth germanate)/avalanche photodiode detectors, but the large number of channels and the several interdependent parameters that must be adjusted make the process delicate and lengthy. The authors describe the procedures and algorithms being developed to adjust and verify the detector bias, the noise and energy thresholds, the signal references, and the timing delays for the Sherbrooke PET tomograph. All settings are adjusted using a sequence of iterative tuning algorithms whose convergence is optimized for each parameter. Preliminary results show that the complete calibration can be carried out automatically and reliably overnight and that specific checks can be performed in a few tens of seconds as needed. >

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.

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>>