David Lapointe

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.

Cross-validation stopping rule for ML-EM reconstruction of dynamic PET series: effect on image quality and quantitative accuracy

A major shortcoming of the Maximum Likelihood Expectation Maximization (ML-EM) method for reconstruction of dynamic PET images is to decide when to stop the iterative process for image frames with largely different statistics and activity distributions. A widespread practice to overcome this problem involves over-iteration of an image estimate followed by smoothing. In this work, the authors investigate the qualitative and quantitative accuracy of the cross-validation procedure (CV) as a stopping rule, in comparison to over-iteration and post-filtering. For the reconstruction of phantom and small animal dynamic FDG-PET data acquired in 2-D mode. The CV stopping rule ensured visually acceptable image estimates with balanced resolution and noise characteristics. However, quantitative accuracy required more than 10/sup 5/ events per image. The effect of the number of ML-EM iterations on time-activity curves and metabolic rates of glucose extracted from image series is discussed. A dependence of the CV defined number of iterations on projection counts was found which simplifies reconstruction and reduces computation time.

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.

Determination of blood curve and tissue uptake from left ventricle using FADS in rat FDG-PET studies

Blood input function is necessary for quantitative pharmacokinetic studies in Positron Emission Tomography (PET). However, external blood sampling from small animals presents many limitations. In this work, we show that the blood input function in rats can be extracted from the left ventricular blood pool using Factor Analysis of Dynamic Structures (FADS). Images of the heart from eight rats were acquired with the Sherbrooke animal PET scanner. A region of interest (ROI) was drawn around the left ventricle (LV) and decomposed into the blood pool and the myocardium tissue using FADS. The input curve (IC) obtained with FADS is comparable to that obtained from measured images for the first frames, while it significantly decreases with increasing time. The myocardium tissue segments present lower amplitude at early times and approximately no change at later times in comparison to the same ROIs obtained from measured images. On average, the variation of the total counts were about 18%, 19% and 43% in the IC, anterior and septal ROIs obtained from images reconstructed with maximum likelihood algorithm. IC and myocardium tissue uptake can be safely obtained from rat heart scans and corrected for spillover using FADS for images obtained with either iterative reconstruction or with the usual filtered backprojection.

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.