Développement d'un système de tomographie par cohérence optique pour la mesure de la compliance vasculaire cérébrale in-vivo

Resume L'imagerie de la microvasculature cerebrale chez les petits animaux connait depuis les dernieres annees une avancee rapide. La tomographie par coherence optique (OCT) est un outil qui a permis recemment d'obtenir des images de haute resolution de celle-ci chez des rats et des souris. Elle permet egalement des mesures de ux sanguin et des etudes fonctionnelles sur le cerveau. Il est maintenant rendu possible d'appliquer cette technique a l'etude de problematiques reelles en recherche neurovasculaire. Une telle problematique est l'etude de la compliance des arterioles. La compliance est un sujet d'inter^et depuis que de nouvelles hypotheses sur le developpement des maladies neurodegeneratives pointent vers un facteur commun vasculaire. Le r^ole du reseau neurovasculaire est de fournir les nutriments necessaires a l'activite des neurones. Une dysfonction de la regulation sanguine pourrait g^ener cette fonction. La compliance arterielle joue un r^ole cle dans la regulation sanguine, mais jusqu'a maintenant les outils disponibles pour en faire l'etude ne permettaient que des mesures ex-vivo. Cette contrainte elimine toute possibilite d'etude longitudinale sur un m^eme animal. L'OCT peut s'averer ^etre un outil interessant pour la mesure de la compliance in-vivo. L'objectif de ce travail de ma^trise a donc ete de developper et de valider une technique d'estimation de la compliance basee sur l'OCT. L'appareil OCT developpe utilise une source de lumiere superluminescente a 870 nm et un interferometre de Michelson an de produire des images structurelles et de ux de la microvasculature cerebrale chez la souris. La caracterisation des performances de ce systeme donne une resolution axiale de 9 m, une profondeur de penetration de 600 m et une sensibilit e de 105 dB. L'appareil permet des mesures de ux entre 10 et 100 nL=s et une taille minimale de vaisseaux detectes de 30 m. Une technique innovatrice de reconstruction des images OCT permet d'obtenir l'evolution du ux sanguin sur un cycle cardiaque. Cette nouvelle information de dynamique arterielle permet d'evaluer la pulsatilite du ux sanguin. Un evaluateur de la compliance qui se base sur cette mesure est derive. An de tester le modele d'evaluation de la compliance, une etude comparative entre un groupe de souris normales et un groupe de souris developpant des plaques d'atheroscleroses est proposee. Le resultat d'une etude ex-vivo montre que les arteres plus larges du cerveau ont une compliance superieure chez les animaux atteints d'atherosclerose. L'utilisation de l'OCT permet de retrouver ce resultat de maniere non-invasive. De plus l'OCT permet----------Abstract Optical imaging of the neurovascular network has been recently evolving at a rapid pace. Optical Coherence Tomography (OCT) has been used to produce high quality angiograms of cortical microvasculature in mice and rat. Through this technique, precise measurements of blood speed can be made without the use of invasive markers opening the door for studies of blood ow and neurovascular function. The current aim of research in the eld is to develop imaging protocols and methods for studying specic neurovascular function. One such vascular function is arteriolar compliance. New hypothesis in the development of neurodegenerative diseases have revealed a link with vascular degeneration. Neurovascular regulation involves the supply of blood ow towards regions of neural activity. Blood supplies oxygen and nutriments to active neurons and improper supply of it can lead to neuronal dysfunction. Compliance is a characteristic of arteries describing their reaction to changes in pressure. It plays a key role in blood ow regulation and as such has been the subject of recent interest. The tools used for compliance evaluation require an extraction of the target vessel for ex-vivo characterization which eliminates any possibility of longitudinal studies on the same animal. OCT through it's ability to image the neurovascular network could oer an alternative way of measuring arteriolar compliance in-vivo. The aim of this work was therefore to develop and validate a technique for measuring compliance based on OCT measurements. The OCT system developed in this work is based on a superluminescent diode emitting light in the near-infrared range at 870 nm. The system produces structural and ow images of the cerebral microvasculature in mice and rats. It has a maximum axial resolution of 9 m, a penetration depth of 600 m and a sensibility of 105 dB. The system was also able to produce accurate ow measurements between 10 and 100 nL=s when tested on a fantom and produced accurate volumetric maps of blood vessels with arterioles as small as 30 m being imaged. Along with the standard protocols for imaging volumes and slices through the vasculature, a novel reconstruction technique was developed. This technique uses electrocardiography information to produce sequences of OCT slices over one cardiac cycle. These sequences reveal the changes in blood speed and vessel area over that cycle. A simple arterial model then uses this novel information to produce an estimate of vessel compliance. In order to test this new compliance evaluation method, a group study is presented. This study aims to reveal dierences in arteriolar compliance between a group of norma

Contributions and complexities from the use of in-vivo animal models to improve understanding of human neuroimaging signals.

Many of the major advances in our understanding of how functional brain imaging signals relate to neuronal activity over the previous two decades have arisen from physiological research studies involving experimental animal models. This approach has been successful partly because it provides opportunities to measure both the hemodynamic changes that underpin many human functional brain imaging techniques and the neuronal activity about which we wish to make inferences. Although research into the coupling of neuronal and hemodynamic responses using animal models has provided a general validation of the correspondence of neuroimaging signals to specific types of neuronal activity, it is also highlighting the key complexities and uncertainties in estimating neural signals from hemodynamic markers. This review will detail how research in animal models is contributing to our rapidly evolving understanding of what human neuroimaging techniques tell us about neuronal activity. It will highlight emerging issues in the interpretation of neuroimaging data that arise from in-vivo research studies, for example spatial and temporal constraints to neuroimaging signal interpretation, or the effects of disease and modulatory neurotransmitters upon neurovascular coupling. We will also give critical consideration to the limitations and possible complexities of translating data acquired in the typical animals models used in this area to the arena of human fMRI. These include the commonplace use of anaesthesia in animal research studies and the fact that many neuropsychological questions that are being actively explored in humans have limited homologues within current animal models for neuroimaging research. Finally we will highlighting approaches, both in experimental animals models (e.g. imaging in conscious, behaving animals) and human studies (e.g. combined fMRI-EEG), that mitigate against these challenges

Measurement of cerebral microvascular compliance in a model of atherosclerosis with optical coherence tomography.

Optical coherence tomography (OCT) has recently been used to produce 3D angiography of microvasculature and blood flow maps of large vessels in the rodent brain in-vivo. However, use of this optical method for the study of cerebrovascular disease has not been fully explored. Recent developments in neurodegenerative diseases has linked common cardiovascular risk factors to neurodegenerative risk factors hinting at a vascular hypothesis for the development of the latter. Tools for studying cerebral blood flow and the myogenic tone of cerebral vasculature have thus far been either highly invasive or required ex-vivo preparations therefore not preserving the delicate in-vivo conditions. We propose a novel technique for reconstructing the flow profile over a single cardiac cycle in order to evaluate flow pulsatility and vessel compliance. A vascular model is used to simulate changes in vascular compliance and interpret OCT results. Comparison between atherosclerotic and wild type mice show a trend towards increased compliance in the smaller arterioles of the brain (diameter < 80μm) in the disease model. These results are consistent with previously published ex-vivo work confirming the ability of OCT to investigate vascular dysfunction

Catechin Prevents Severe Dyslipidemia-Associated Changes in Wall Biomechanics of Cerebral Arteries in Ldlr-/-:Hapob(+/+) Mice and Improves Cerebral Blood Flow

Endothelial dysfunction and oxidative stress contribute to the atherosclerotic process that includes stiffening of large peripheral arteries. In contrast, our laboratory previously reported a paradoxical increase in cerebro-vascular compliance in LDLr(−/−):hApoB(+/+) atherosclerotic (ATX) mice (7). We hypothesized that prevention of cerebral artery endothelial dysfunction with a chronic dietary antioxidant intake would normalize the changes in cerebral artery wall structure and biomechanics and prevent the decline in basal cerebral blood flow associated with atherosclerosis. Three-month-old ATX mice were treated, or not, for 3 mo with the polyphenol (+)-catechin (CAT; 30 mg·kg(−1)·day(−1)) and compared with wild-type controls. In isolated, pressurized cerebral arteries from ATX mice, CAT prevented endothelial dysfunction (deterioration of endothelium-dependent, flow-mediated dilations; P < 0.05), the inward hypertrophic structural remodeling (increase in the wall-to-lumen ratio; P < 0.05), and the rise in cerebrovascular compliance (rightward shift of the stress-strain curve measured in passive conditions, reflecting mechanical properties of the arterial wall; P < 0.05). Doppler optical coherence tomography imaging in vivo confirmed these findings, showing that cerebral compliance was higher in ATX mice and normalized by CAT (P < 0.05). CAT also prevented basal cerebral hypoperfusion in ATX mice (P < 0.05). Active remodeling of the cerebrovascular wall in ATX mice was further suggested by the increase (P < 0.05) in pro-metalloproteinase-9 activity, which was normalized by CAT. We conclude that, by preserving the endothelial function, a chronic treatment with CAT prevents the deleterious effect of severe dyslipidemia on cerebral artery wall structure and biomechanical properties, contributing to preserving resting cerebral blood flow

Two-photon microscopy of cortical NADH fluorescence intensity changes: Correcting contamination from the hemodynamic response

Quantification of nicotinamide adenine dinucleotide (NADH) changes during functional brain activation and pathological conditions provides critical insight into brain metabolism. Of the different imaging modalities, two-photon laser scanning microscopy (TPLSM) is becoming an important tool for cellular-resolution measurements of NADH changes associated with cellular metabolic changes. However, NADH fluorescence emission is strongly absorbed by hemoglobin. As a result, in vivo measurements are significantly affected by the hemodynamics associated with physiological and pathophysiological manipulations. We model NADH fluorescence excitation and emission in TPLSM imaging based on precise maps of cerebral microvasculature. The effects of hemoglobin optical absorption and optical scattering from red blood cells, changes in blood volume and hemoglobin oxygen saturation, vessel size, and location with respect to imaging location are explored. A simple technique for correcting the measured NADH fluorescence intensity changes is provided, with the utilization of a parallel measurement of a physiologically inert fluorophore. The model is applied to TPLSM measurements of NADH fluorescence intensity changes in rat somatosensory cortex during mild hypoxia and hyperoxia. The general approach of the correction algorithm can be extended to other TPLSM measurements, where changes in the optical properties of the tissue confound physiological measurements, such as the detection of calcium dynamics