Développement d'un système de tomographie par cohérence optique pour la mesure de la compliance vasculaire cérébrale in-vivo
Abstract (summary)
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 flow and neurovascular function. The current aim of research in the field is to develop imaging protocols and methods for studying specific 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 flow 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 flow 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 offer 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 flow images of the cerebral microvasculature in mice and rats. It has a maximum axial resolution of 9 μm, a penetration depth of 600 μm and a sensibility of 105 dB. The system was also able to produce accurate flow 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 μm 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 differences in arteriolar compliance between a group of normal mice and another one which spontaneously develops atherosclerotic lesions. Recent ex-vivo measurements have revealed that compliance of larger arteries in the brain is higher in the atherosclerotic group. The OCT study was able to corroborate this result in a non-invasive manner. Furthermore, OCT was able to estimate compliance in vessels that have thus far been too small for ex-vivo preparations. The same trend was observed for these smaller arterioles as that of the larger arteries where vessels of atherosclerotic mice would have a higher compliance then wild type vessels.
Following the group study, success in reaching the objectives of this work is evaluated. The system was found to offer satisfactory performance in terms of resolution, sensitivity and depth of field. Hardware and software improvements are suggested in order to achieve higher imaging quality. However, volumetric and cardiac reconstructions provide satisfactory results for compliance evaluation. Overall, every objective of this work is reached and OCT was successfully demonstrated to be a viable tool for evaluating cerebral arteriolar compliance in-vivo.
Indexing (details)
Optics
0752: Optics