Multiscale modelling of haemorrhagic transformation after ischaemic stroke

The brain occupies around 2 % of human adult bodyweight but accounts for nearly 20% of metabolism in the body and 14% of blood flow. With an increasingly elderly population globally, the impacts of cerebrovascular diseases, such as stroke and dementia, are becoming increasingly significant. Notably, the clinical challenge of stroke is growing. For example, the data collected by the Stroke Association show that there are approximately 17 million new stroke patients across the world and more than 100,000 strokes each year in the UK. Haemorrhagic transformation (HT) is one of the most common complications after ischaemic stroke caused by damage to the blood–brain barrier (BBB) that could be the result of stroke progression or a complication of stroke treatment with reperfusion therapy, causing bleeding in the brain. This can lead to further damage to the brain tissue and can increase the risk of disability or death. To better assist in understanding this, in this thesis a new intracerebral haemorrhagic transformation model is presented. This model is divided logically into three steps, starting from simulating haemorrhage in a single vessel to HT in a 3-dimensional vasculature model, and eventually applying this model within a whole brain model. In the first model, a mathematical model of HT is developed to simulate the consequence of HT over a range of vasculature length scales. Then in the second study, this model is developed further into an enlarged multi-scale microvasculature model in order to investigate the effects of HT on the surrounding tissue and vasculature. Next, this HT is applied into a computational whole brain model. The effects of capillary compression and tissue displacement are also considered in these three models. Finally, the volume of the haematoma is investigated in 15 subjects and used for validation against clinical imaging data. In addition, perfusion is calculated in the region of HT and used to compare with experimental data. This model is the first such to be able to simulate the correlation between bleeding regions and haematoma, which may be of assistance in future to assess the HT for clinicians

In vivo ultrafast Doppler imaging combined with confocal microscopy and behavioral approaches to gain insight into the central expression of Peripheral Neuropathy in Trembler-J Mice

Funding: Agencia Nacional de Investigación e Innovación (ANII), grant FCE_1_2019_1_155539. PEDECIBA, CSIC-UdelaR and the Institut Franco—Uruguayen de Physique (IFUP), LIA-CNRS-UdelaR. SNI-ANII (J.P.D., C.N., N.R., A.K. and J.BR.). ANII- POS_NAC_M_2020_1_164127 (M.A.F.).ANII- POS_FCE_2020_1_1009181 (M.M.B). CSIC I+D group grant CSIC2018—FID 13—Grupo ID 722 (N.R.). PID2019-110401RB-100 from the Spanish, Ministry of Science and Innovation and the Spanish CIBERNED network (M.C.). Acknowledgments: We would especially like to thank Dr. Cecilia Scorza, head of the Department of Experimental Neuropharmacology - Instituto de Investigaciones Biológicas Clemente Estable,Peer reviewedPublisher PD

Deep optoacoustic localization microangiography of ischemic stroke in mice

Super-resolution optoacoustic imaging of microvascular structures deep in mammalian tissues has so far been impeded by strong absorption from densely-packed red blood cells. Here we devised 5 µm biocompatible dichloromethane-based microdroplets exhibiting several orders of magnitude higher optical absorption than red blood cells at near-infrared wavelengths, thus enabling single-particle detection in vivo. We demonstrate non-invasive three-dimensional microangiography of the mouse brain beyond the acoustic diffraction limit (<20 µm resolution). Blood flow velocity quantification in microvascular networks and light fluence mapping was also accomplished. In mice affected by acute ischemic stroke, the multi-parametric multi-scale observations enabled by super-resolution and spectroscopic optoacoustic imaging revealed significant differences in microvascular density, flow and oxygen saturation in ipsi- and contra-lateral brain hemispheres. Given the sensitivity of optoacoustics to functional, metabolic and molecular events in living tissues, the new approach paves the way for non-invasive microscopic observations with unrivaled resolution, contrast and speed

Spectral Domain-optical Coherence Tomography for the Assessment of Cerebrovascular Plasticity

Vascular pathologies represent the leading causes of mortality worldwide, accounting for 31% of all deaths in 2012. Cerebral hypoxia is a condition that often manifests as a result of these medical conditions. Remarkably, the nervous system has evolved mechanisms to compensate for oxygen deprivation. The dilation of existing vessels and the growth of new blood vessels are two prominent physiological responses to hypoxia, both of which play a critical role in maintaining cerebral homeostasis. More recently, exercise has been shown to induce a mild state of hypoxia in the brain, leading to several robust morphological changes within the cerebrovascular system (e.g., angiogenesis, vasodilation). Thus, exercise serves as a viable model for investigating hypoxia-induced adaptations. The present study introduces spectral domain optical coherence tomography (SD-OCT) as a novel technique for examining these micro-level changes in the rat motor cortex. SD-OCT produces high resolution, three-dimensional angiograms, and allows for moderately invasive imaging within the same animal at multiple time points. The independent effect of exercise training on cerebrovascular structure and function has never been explored using SD-OCT. Thus, the primary goal of this study was to determine the relative efficacy of SD-OCT utility. To validate this novel technology, we employed SD-OCT in the examination of exercise-dependent blood vessel growth, as well as real-time capillary dilation in response to a laboratory-induced condition of hypoxia (i.e., 10% oxygen). In addition, histology data was collected to provide comparative measures for statistical analyses. At the start of this investigation, animals were pseudo-randomly assigned to one of two groups: 26-week voluntary exercise (VX), or an inactive control (IC). Upon completing the exercise treatment, animals were anesthetized and prepared for imaging. Vascular anatomy and blood velocity data was captured during three experimental conditions: [1] normal oxygen baseline, [2] hypoxia – 10% oxygen, and [3] normoxia, return to baseline. A two-way analysis of variance revealed a significant difference in total blood vessel density between treatment groups, independent of condition. That is, VX animals had a greater density of blood vessels in the scanned region of interest when compared to IC. These findings were confirmed using unbiased stereology techniques to analyze tissue in the scanned region of interest. Furthermore, statistical analyses revealed a significant increase in small arteriole diameter in both VX and IC animals. However, the dilation captured by SD-OCT was significantly greater in VX animals when compared to IC. In sum, exercise induces potent adaptations that promote greater flexibility during hypoxia. Moreover, these micro-level changes can be effectively probed using SD-OCT

Leptomeningeal collaterals regulate reperfusion in ischemic stroke and rescue the brain from futile recanalization.

Recanalization is the mainstay of ischemic stroke treatment. However, even with timely clot removal, many stroke patients recover poorly. Leptomeningeal collaterals (LMCs) are pial anastomotic vessels with yet-unknown functions. We applied laser speckle imaging, ultrafast ultrasound, and two-photon microscopy in a thrombin-based mouse model of stroke and fibrinolytic treatment to show that LMCs maintain cerebral autoregulation and allow for gradual reperfusion, resulting in small infarcts. In mice with poor LMCs, distal arterial segments collapse, and deleterious hyperemia causes hemorrhage and mortality after recanalization. In silico analyses confirm the relevance of LMCs for preserving perfusion in the ischemic region. Accordingly, in stroke patients with poor collaterals undergoing thrombectomy, rapid reperfusion resulted in hemorrhagic transformation and unfavorable recovery. Thus, we identify LMCs as key components regulating reperfusion and preventing futile recanalization after stroke. Future therapeutic interventions should aim to enhance collateral function, allowing for beneficial reperfusion after stroke

Spectral and Temporal Interrogation of Cerebral Hemodynamics Via High Speed Laser Speckle Contrast Imaging

Laser Speckle Contrast Imaging (LSCI) is a non-scanning wide field-of-view optical imaging technique specifically developed for cerebral blood flow (CBF) monitoring. In this project, a versatile Laser speckle contrast imaging system has been designed and developed to monitor CBF changes and examine the physical properties of cerebral vasculature during functional brain activation experiments. The hardware of the system consists of a high speed CMOS camera, a coherent light source, a trinocular microscope, and a PC that does camera controlling and data storage. The simplicity of the system’s hardware makes it suitable for biological experiments. In controlled flow experiments using a custom made microfluidic channel, the linearity of the CBF estimates was evaluated under high speed imaging settings. Under the camera exposure time setting in the range of tens of micro-seconds, results show a linear relationship between the CBF estimates and the flow rates within the microchannel. This validation permitted LSCI to be used in high frame rate imaging and the method is only limited by the camera speed. In an in vivo experiment, the amount of oxygen intake via breathing by a rat was reduced to 12% to induce the dilation of the vessels. Results demonstrated a positive correlation between the system’s CBF estimates and the pulse wave velocity derived from aortic blood pressure. To exemplify the instantaneous pulsatility flow study acquired at high sampling rate, a pulsatile cerebral blood flow analysis was conducted on two vessels, an arteriole and a venule. The pulsatile waveform results, captured under sampling rate close to 2000 Hz. The pulse of the arteriole rises 13ms faster than the pulse of the venule, and it takes 6ms longer for the pulse of the arteriole to fall below the lower fall-time boundary. By using the second order derivative (accelerated) CBF estimates, the vascular stiffness was evaluated. Results show the arteriole and the venule have increased-vascular-stiffness indices of 0.95 and 0.74. On the other side, the arteriole and the venule have decreased-vascular-stiffness indices of 0.125 and 0.35. Both vascular stiffness indices suggested that the wall of arteriole is more rigid than the venule. The proposed LSCI system can monitor the mean flow over function activation experiment, and the interrogation of blood flow in terms of physiological oscillations. The proposed vascular stiffness metrics for estimating the stroke preliminary symptom, may eventually lead to insights of stroke and its causes