The development of a multi-model optical imaging system to investigate hemodynamic responses to tissue injury in vivo
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Ischemic injury is caused by blockage or rupture of a blood vessel supplying an organ with oxygen and nutrients. A reduction of blood flow in the brain below critical values leads to a series of metabolic, functional, and structural changes resulting in irreversible cell death (1). Ability to non-invasively monitor and quantify functional blood flow, tissue morphology, and blood oxygenation is important for improvement of diagnosis, treatment and management of ischemic injury. Decreased blood flow above the critical value leads to permanent cell death, but a portion of the hypoperfusion region has the potential for recovery. Over the past several decades, much effort has been put forth in investigating endogenous mechanisms involved in salvaging the hypoperfusion region but has focused on neuroprotective mechanisms with little attention given to the vasculature (2). This is most likely due to the lack of a technology capable of elucidating real time macro- and micro-vascular dynamics in vivo. Moreover, there are no techniques that can satisfactorily extract the aforementioned parameters from in vivo macro- and micro-circulatory tissue beds, with sufficient resolution at defined depth and without any harm to the tissue. The ability which could localize macrovasculature and subsequently visualize the detailed microvasculature networks with multiple parameters is crucial to determine the blood supply status under the pathophysiologic and metabolic conditions. In this thesis, we developed a multi-model imaging system to achieve a more comprehensive understanding of macro- and micro-vascular responses and hemodynamic parameters during and after ischemic injury, which would greatly improve our ability to develop therapeutic interventions to improve vascular function. There are three specific aims. (I). Development of the 1st generation multi-model imaging system capable of providing hemodynamic and morphological information of macro- and micro-circulatory tissue beds in vivo. We developed a multi-model imaging system that integrates together the functions of multi-wavelength laser speckle contrast imaging (LSCI) and optical microangiography (OMAG), so that it can rapidly image hemodynamic within living tissues in three dimensions (3D), providing important physiological parameters, e.g., functional blood flow, blood oxygenation, and tissue morphology. For multi-wavelength LSCI, we used two laser sources at different wavelengths, in which a mechanical chopper was used to modulate the illumination light to provide the measurement of blood flow and blood oxygenation. For OMAG, we used the ultrahigh sensitive OMAG (UHS-OMAG) system because of its sensitivity to image functional microcirculations with capillary details. We evaluated the performance of the system using an in vivo mouse pinna model through the imaging of blood vessel networks before and after a burn injury. (II). Improvement of the imaging speed of multi-model imaging system The imaging speed is a key to the investigation of fast hemodynamic responses (in milliseconds) in an injury. We envisioned that the first generation (1st G) system would have limited imaging speed due to the use of mechanical chopper to control the multi-wavelength LSCI. To mitigate this problem, we advanced the 1st G system, by the use of two synchronized cameras to concurrently capture the laser speckle images at 780-nm and 825-nm wavelengths; so that the rapid changes of blood flow rate and hemoglobin concentration can be provided. We also developed novel algorithms that will enable the direction of blood flow to be monitored. (III). Validation of the utility of multi-model imaging system for serial monitoring of macro- and micro- vasculature changes following ischemic injury Using the system we developed, we defined the characteristics of blood flow and oxygenation variations during baseline, occlusion, and reperfusion in an experimental ischemic stroke model. We also correlated measured macro- and micro-vascular responses with the results obtained by conventional histopathology. The immediate outcome of this research is that we can simultaneously monitor blood flow, blood oxygenation, and tissue morphology in ischemic injury in vivo. This technology would be valuable in future research studies that aim to improve our understanding of vascular involvement under pathologic and physiological conditions, and ultimately facilitate clinical diagnosis, monitoring, and therapeutic interventions of neurovascular diseases.
- Bioengineering