M'hamed Bentourkia

Brain fuel metabolism, aging, and Alzheimer’s disease

Lower brain glucose metabolism is present before the onset of clinically measurable cognitive decline in two groups of people at risk of Alzheimers disease--carriers of apolipoprotein E4, and in those with a maternal family history of AD. Supported by emerging evidence from in vitro and animal studies, these reports suggest that brain hypometabolism may precede and therefore contribute to the neuropathologic cascade leading to cognitive decline in AD. The reason brain hypometabolism develops is unclear but may include defects in brain glucose transport, disrupted glycolysis, and/or impaired mitochondrial function. Methodologic issues presently preclude knowing with certainty whether or not aging in the absence of cognitive impairment is necessarily associated with lower brain glucose metabolism. Nevertheless, aging appears to increase the risk of deteriorating systemic control of glucose utilization, which, in turn, may increase the risk of declining brain glucose uptake, at least in some brain regions. A contributing role of deteriorating glucose availability to or metabolism by the brain in AD does not exclude the opposite effect, i.e., that neurodegenerative processes in AD further decrease brain glucose metabolism because of reduced synaptic functionality and hence reduced energy needs, thereby completing a vicious cycle. Strategies to reduce the risk of AD by breaking this cycle should aim to (1) improve insulin sensitivity by improving systemic glucose utilization, or (2) bypass deteriorating brain glucose metabolism using approaches that safely induce mild, sustainable ketonemia.

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.

Abnormal in vivo myocardial energy substrate uptake in diet-induced type 2 diabetic cardiomyopathy in rats

The purpose of this study was to determine in vivo myocardial energy metabolism and function in a nutritional model of type 2 diabetes. Wistar rats rendered insulin-resistant and mildly hyperglycemic, hyperinsulinemic, and hypertriglyceridemic with a high-fructose/high-fat diet over a 6-wk period with injection of a small dose of streptozotocin (HFHFS) and control rats were studied using micro-PET (microPET) without or with a euglycemic hyperinsulinemic clamp. During glucose clamp, myocardial metabolic rate of glucose measured with [(18)F]fluorodeoxyglucose ([(18)F]FDG) was reduced by approximately 81% (P < 0.05), whereas myocardial plasma nonesterified fatty acid (NEFA) uptake as determined by [(18)F]fluorothia-6-heptadecanoic acid ([(18)F]FTHA) was not significantly changed in HFHFS vs. control rats. Myocardial oxidative metabolism as assessed by [(11)C]acetate and myocardial perfusion index as assessed by [(13)N]ammonia were similar in both groups, whereas left ventricular ejection fraction as assessed by microPET was reduced by 26% in HFHFS rats (P < 0.05). Without glucose clamp, NEFA uptake was approximately 40% lower in HFHFS rats (P < 0.05). However, myocardial uptake of [(18)F]FTHA administered by gastric gavage was significantly higher in HFHFS rats (P < 0.05). These abnormalities were associated with reduced Glut4 mRNA expression and increased Cd36 mRNA expression and mitochondrial carnitine palmitoyltransferase 1 activity (P < 0.05). HFHFS rats display type 2 diabetes complicated by left ventricular contractile dysfunction with profound reduction in myocardial glucose utilization, activation of fatty acid metabolic pathways, and preserved myocardial oxidative metabolism, suggesting reduced myocardial metabolic efficiency. In this model, increased myocardial fatty acid exposure likely occurs from circulating triglyceride, but not from circulating plasma NEFA.

A New Tool for Molecular Imaging: The Microvolumetric β Blood Counter

Radiotracer kinetic modeling in small animals with PET allows absolute quantification of physiologic and biochemical processes in vivo. It requires blood and tissue tracer concentrations as a function of time. Manual sampling, the reference method for blood tracer concentration measurements, requires fairly large amounts of blood besides being technically difficult and time-consuming. An automated microvolumetric β blood counter (μBC) was designed to circumvent these limitations by measuring the blood activity in real time with PET scanning. Methods: The μBC uses direct β-particle detection to reduce its footprint and is entirely remote controlled for sampling protocol selection and real-time monitoring of measured parameters. Sensitivity has been determined for the most popular PET radioisotopes (18F, 13N, 11C, 64Cu). Dispersion within the sampling catheter has been modeled to enable automatic correction. Blood curves obtained with the μBC were compared with manual samples and PET-derived data. The μBC was used to estimate the myocardial blood flow (MBF) of mice injected with 13N-ammonia and to compare the myocardial metabolic rate of glucose (MMRG) of rats injected with 18F-FDG for arterial and venous cannulation sites. Results: The sensitivity limit ranges from 3 to 104 Bq/μL, depending on the isotope and the catheter used, and was found to be adequate for most small-animal studies. Automatic dispersion correction appears to be a good approximation of dispersion-free reference curves. Blood curves sampled with the μBC are well correlated with curves obtained from manual samples and PET images. With correction for dispersion, the MBF of anesthetized mice at rest was found to be 4.84 ± 0.5 mL/g/min, which is comparable to values found in the literature for rats. MMRG values derived from the venous blood tracer concentration are underestimated by 60% as compared with those derived from arterial blood. Conclusion: The μBC is a compact automated counter allowing real-time measurement of blood radioactivity for pharmacokinetic studies in animals as small as mice. Reliable and reproducible, the device makes it possible to increase the throughput of pharmacokinetic studies with reduced blood sample handling and staff exposure, contributing to speed up new drug development and evaluation.

Endotoxin-induced heart dysfunction in rats: assessment of myocardial perfusion and permeability and the role of fluid resuscitation.

Objective:The pathophysiology of sepsis-induced myocardial dysfunction is still controversial. Whether microcirculatory hypoperfusion together with capillary leakage can occur in the heart wall also remains a matter of debate. The objective was to evaluate the impact of fluid resuscitation on endotoxin-induced myocardial dysfunction. Design:Adult rats were given intraperitoneal injection of endotoxin (lipopolysaccharide, Escherichia coli, 10 mg/kg) or phosphate-buffered solution, followed up by echocardiography and acetate micro-positron emission tomography scan imaging, together with final hemodynamic, biochemical, and pathologic evaluations up to 48 hrs. Setting:University laboratory. Subjects:Pathogen-free male Wistar rats (350 g). Interventions:Influence of isovolumic fluid infusion type (saline vs. pentastarch) on these variables was assessed in 11 groups of six animals including an unchallenged control one. Measurements and Main Results:Endotoxin injection induced a) myocardial dysfunction (decrease of ∼15–20% in left ventricular ejection fraction); b) ventricular enlargement (∼1.5- to 1.7-fold increase in left ventricular systolic volume); c) cardiac output increase (10–15%); d) myocardial hypoperfusion (∼1.5- to 2-fold decrease in acetate k1 constant rate); e) increased oxygen consumption (k2); and f) interstitial wall increase. Endotoxin injection also enhanced levels of arterial lactates and troponin I. Colloid (pentastarch) over crystalloid (saline) fluid resuscitation significantly reversed echocardiographic changes, some positron emission tomography imaging alterations, and lactate and troponin I levels without further enhancing interstitial spaces. Conclusion:Endotoxin can induce reversible myocardial alterations with evidence of coronary hypoperfusion and heart wall enlargement/damage, some of which can be prevented by fluid resuscitation. The use of crystalloids is less beneficial than pentastarch.

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.

Energy dependence of scatter components in multispectral PET imaging

High resolution images in PET based on small individual detectors are obtained at the cost of low sensitivity and increased detector scatter. These limitations can be partially overcome by enlarging discrimination windows to include more low-energy events and by developing more efficient energy-dependent methods to correct for scatter radiation from all sources. The feasibility of multispectral scatter correction was assessed by decomposing response functions acquired in multiple energy windows into four basic components: object, collimator and detector scatter, and trues. The shape and intensity of these components are different and energy-dependent. They are shown to contribute to image formation in three ways: useful (true), potentially useful (detector scatter), and undesirable (object and collimator scatter) information to the image over the entire energy range. With the Sherbrooke animal PET system, restoration of detector scatter in every energy window would allow nearly 90% of all detected events to participate in image formation. These observations suggest that multispectral acquisition is a promising solution for increasing sensitivity in high resolution PET. This can be achieved without loss of image quality if energy-dependent methods are made available to preserve useful events as potentially useful events are restored and undesirable events removed.

PET study of 11C-acetoacetate kinetics in rat brain during dietary treatments affecting ketosis

Normally, the brains fuel is glucose, but during fasting it increasingly relies on ketones (beta-hydroxybutyrate, acetoacetate, and acetone) produced in liver mitochondria from fatty acid beta-oxidation. Although moderately raised blood ketones produced on a very high fat ketogenic diet have important clinical effects on the brain, including reducing seizures, ketone metabolism by the brain is still poorly understood. The aim of the present work was to assess brain uptake of carbon-11-labeled acetoacetate (11C-acetoacetate) by positron emission tomography (PET) imaging in the intact, living rat. To vary plasma ketones, we used three dietary conditions: high carbohydrate control diet (low plasma ketones), fat-rich ketogenic diet (raised plasma ketones), and 48-h fasting (raised plasma ketones). 11C-acetoacetate metabolism was measured in the brain, heart, and tissue in the mouth area. Using 11C-acetoacetate and small animal PET imaging, we have noninvasively quantified an approximately seven- to eightfold enhanced brain uptake of ketones on a ketogenic diet or during fasting. This opens up an opportunity to study brain ketone metabolism in humans.

Tracer Kinetic Modeling in PET

Molecular imaging using PET provides a unique tool for noninvasively investigating the biochemistry of living organs. The wide range of radiolabeled molecules makes it possible to explore various biochemical, physiologic, and pharmacologic processes in vivo. Because each radiotracer is characterized by its particular kinetic behavior in the human body, the quantification of this behavior is a critical component for the improvement of imaging protocols and for rapid translation from research and development to the clinic. Suitable image reconstruction algorithms combined with tracer kinetic modeling techniques are needed to assess parametric or quantitative biologic images from the available fourdimensional images. An appropriate mathematical model is generally used to fit the time-activity curves of a region or volume of interest, thus allowing the assessment of biologic parameters.

Object and detector scatter-function dependence on energy and position in high resolution PET

The authors have shown in previous works that distinct non-stationary analytical scatter kernels can be extracted from line source measurements and used to independently subtract object scatter and subtract or restore detector scatter in high resolution PET. In this work, the dependence of the scatter components on energy threshold and source position was investigated. Line source measurements were acquired in multispectral mode using the Sherbrooke PET simulator. Scatter parameters were extracted from data summed in energy windows with a lower threshold varying from 129 keV to 516 keV in steps of 42 keV, and a fixed upper threshold of 644 keV. Decreasing the lower threshold from 344 keV to 129 keV increases the trues by only 25%, but increases object scatter by 136% and almost triples detector scatter. A gain in efficiency by a factor of 2 or more would result from recovering the latter by restoration in the broad window. The intensity and shape of the scatter functions for both object and detector are shown to have a significant dependence on energy and position. This dependence needs to be taken into account in the design of kernels for accurate scatter correction over a broad energy range. >