High Resolution Optical Imaging Methods Of Hemodynamics And Vasculature In The Acute Phase Of Focal Cerebral Ischemia

Stroke is a leading cause of death and disability worldwide.Ischemic stroke accounts for over 80%of all strokes with limited treatment options.Both experimental and clinical studies have indicated that the early hours after stroke onset are the most critical window for therapeutic interventions.The early neurovascular changes after stroke have demonstrated great prognostic value in neuroimaging and electro-neurophysiologic studies.Animal stroke models of focal cerebral ischemia mimic common characteristics of human stroke,playing an important role in understanding the pathological mechanism of ischemic stroke,especially in the acute phase.During ischemic stroke,the perturbed vascular morphology causes abnormal cerebral blood flow?CBF?.Reduced cerebral blood flow results in a shortage of tissue oxygenation,causing irreversible brain injury.Neurovascular imaging provides a useful tool to evaluate the ischemic injury so as to direct effective therapy.Optical imaging techniques provide an alternative to current clinical imaging modalities such as computed tomography?CT?,positron emission tomography?PET?,single-photon emission computed tomography?SPECT?and magnetic resonance imaging?MRI?in the study of ischemic stroke.Several optical imaging techniques,e.g.laser Doppler imaging?LDI?,near-infrared spectroscopy?NIRS?,diffuse correlation spectroscopy?DCS?,laser speckle imaging?LSI?,optical intrinsic signal imaging?OISI?,photo-acoustic tomography?PAT?,two-photon microscopy?2PM?and optical coherence tomography?OCT?,have shown their great capabilities to resolve hemodynamics and vascular changes of functional brain response thanks to the relative high imaging resolution and distinct optical absorption properties of oxygenated and deoxygenated blood.Though many optical imaging techniques have been used in the study of ischemic stroke,there are still challenges remained in high resolution optical imaging in the acute phase of focal cerebral ischemia.1)Real-time high resolution imaging of hemodynamics and morphological changes in the acute phase of focal cerebral ischemia;2)high resolution depth-resolved vasculature and oxygenation measurement in the acute phase of focal cerebral ischemia;3)simultaneous and dynamic monitoring of hemodynamic and vascular changes with high resolution during the acute phase of focal cerebral ischemia.In this thesis,we proposed several optical imaging methods to tackle the above-mentioned challenges in the study in the acute phase of focal cerebral ischemia.A microscopic laser speckle imaging system was implemented to monitor the cerebral blood flow and blood vessel profile?BVP?during the acute phase of focal cerebral ischemia with high spatial and temporal resolution.A frequency-domain laser speckle imaging?FDLSI?method was developed to directly measure the absolute blood flow speed in laser speckle imaging.Visible-light optical coherence tomography?Vis-OCT?was applied to monitor the changes of vasculature and oxygen saturation?sO₂?in the acute phase of focal cerebral ischemia.A multi-modal optical imaging platform?laser speckle imaging/visible-light optical coherence tomography?was integrated to measure changes in cerebral blood flow,vascular morphology and oxygen saturation during the acute phase of focal cerebral ischemia.The major results and conclusions of the dissertation are summarized as follows.?1?We proposed laser speckle imaging related techniques to address the applications of laser speckle imaging in monitoring cerebral blood flow changes in the acute phase of focal cerebral ischemia.We presented a microscopic laser speckle imaging system to monitor the cerebral blood flow and blood vessel profile features of cerebral blood vessels during thrombosis with high spatial and temporal resolution.Using a high resolution and high speed CCD camera,online image registration technique,and automatic parabolic curve fitting,we obtained real time cerebral blood flow and blood vessel profile changes in cortical vessels.The analysis of our photothrombotic stroke model shows a multi-stage process in vascular occlusion,with different hemodynamic characteristics in the main and branch vessels.The results reveal details of vascular disturbance and different stages of blood coagulation in photothrombotic stroke,including changes of vessel morphological size,blood flow centerline velocity and cerebral blood flow spatiotemporal fluctuation.We developed a frequency-domain laser speckle imaging method that could directly measure the absolute flow speed in laser speckle imaging.We analyzed the speckle variation in the frequency domain to obtain the autocovariance function curve without the influence of static scattering or illumination intensity.We also built an analytical flow model,upon which absolute flow speed can be fitted from the autocovariance function.The phantom experiment showed that the flow speed obtained by frequency-domain laser speckle imaging was reliable within a 10%deviation from the preset actual values.Furthermore,in vivo blood vessel imaging showed that frequency-domain laser speckle imaging was robust to illumination changes.Frequency-domain laser speckle imaging enables quantitative comparison of blood flow imaging from different subjects at multiple time points,which could further extend the limits of traditional laser speckle imaging for chronic and longitudinal applications.?2?We implemented visible-light optical coherence tomography to monitor the changes of vasculature and oxygen saturation in the acute phase of focal cerebral ischemia.We utilized the capability of visible-light optical coherence tomography to image hemodynamic response in the microvasculature before and after focal cerebral ischemia.Using optical coherence tomography angiography rather than a structure optical coherence tomography signal,we simplified the required image segmentation algorithm,and,more importantly,enhanced the visibility of smaller vessel branches that are otherwise invisible.To improve the accuracy of visible-light optical coherence tomography oximetry on vessels embedded in highly scattering medium,i.e.,brain cortex,we developed and formulated a novel dual-depth sampling and normalization strategy that allowed us to minimize the detrimental effect of ubiquitous tissue scattering.We applied our newly developed approach to monitor the hemodynamic response in the mouse cortex after focal photothrombosis.We observed vessel dilatation,which was negatively correlated with the original vessel diameter,in the penumbra region.The oxygen saturation of vessels in the penumbra region also dropped below normal range after focal cerebral ischemia.?3?We proposed a multi-modal imaging system?laser speckle imaging/visible-light optical coherence tomography?to measure changes in cerebral blood flow,vascular morphology and oxygen saturation in the acute phase of focal cerebral ischemia.We integrated laser speckle imaging and visible-light optical coherence tomography to characterize the dynamic changes in the acute phase of the distal middle cerebral artery occlusion?dMCAO?model in mouse.Laser speckle imaging provides full-field,real-time imaging to guide visible-light optical coherence tomography imaging and monitor the dynamic cerebral blood flow changes.Visible-light optical coherence tomography offers depth-resolved angiography and oxygen saturation measurements at the capillary level.The results showed detailed cerebral blood flow and vasculature changes before,during,and after distal middle cerebral artery occlusion.After distal middle cerebral artery occlusion,we observed little oxygen saturation variation in arteries and arterioles and location–dependent oxygen saturation drops in veins and venules.We observed that higher branch-order veins had larger drops in oxygen saturation at the reperfusion stage after distal middle cerebral artery occlusion.In summary,we presented high resolution optical imaging methods based on laser speckle imaging and visible-light optical coherence tomography to investigate hemodynamics and vasculature changes in the acute phase of focal cerebral ischemia.The new imaging methods and systems potentially have a wide range of applications in the investigation of vascular-related diseases.