Multi-Parametric Photoacoustic Microscopy of Cerebral Hemodynamics and Metabolism

There is no organ in the body as dependent as the brain on a continuous blood supply. Unsurprisingly, disruptions in cerebral hemodynamics and oxygen metabolism underlie a wide spectrum of brain disorders, including ischemic stroke, traumatic brain injury, epilepsy and Alzheimer’s disease. Capitalizing on the optical absorption of hemoglobin, the primary oxygen carrier in the circulation, photoacoustic microscopy (PAM) is uniquely capable of imaging all hemodynamic and oxygen-metabolic parameters with high spatiotemporal resolution in vivo in a label-free manner.

The first part of my dissertation, Chapter II, Chapter III, and Chapter IV, focuses on the design, instrumentation, and validation of the multi-parametric PAM, which are capable of simultaneously in vivo imaging the total concentration of hemoglobin, oxygen saturation, blood flow speed in the mouse brain. Combining these parameters, two key oxygen metabolism parameters, oxygen extraction fraction and metabolic rate of oxygen, can be quantified. After the validation of the multi-parametric PAM, the head-restraint apparatus has been designed to extend the capability of our system to image the awake mouse brain, avoiding the bias introduced by the anesthesia. Furthermore, complicated algorithms have been designed for data analysis, image processing, and vessel segmentation, allowing the comprehensive characterization of the cerebral vasculature in the awake mouse brain. Beyond the parameters mentioned above, comprehensive cerebrovascular characterization also includes vessel density, tortuosity, shear stress, resistance, blood-brain barrier permeability, and cerebrovascular reactivity. This PAM-based imaging and analysis platform enables comprehensive and quantitative characterization of obesity-induced structural, functional and oxygen-metabolic changes in the cerebral microvasculature, in particular, the increased response to vasodilatory stimulation.

The second part of my dissertation, Chapter V and Chapter VI, demonstrated the feasibility of using our multi-parametric PAM for the studies of brain diseases in both mice and rats. The influence of blast traumatic brain injury (TBI) on cerebrovascular reactivity was investigated using PAM. The results showed the impaired cerebrovascular reactivity due to the blast TBI, although there was no observable vasculature damage. This study is the first to comprehensively characterize the cerebrovascular responses to bTBI. The striking impairment of the cerebrovascular reactivity by moderate bTBI, as revealed by our study, may lead to increased vulnerability of the brain to metabolic insults such as hypoxia/ischemia and secondary injuries. Furthermore, the feasibility of using PAM to image the cerebral hemodynamics in acute ischemic stroke has also been demonstrated in awake mouse brain, which showed clear changes in response to the ischemia. This provides the opportunity for further ischemic stroke studies without the interruption of the neuroprotective anesthetic. In the end, the neuroprotective effect of Sphingosine-1-phosphate was investigated, which showed altered cerebral oxygenation under hypoxia. The decreased cerebral metabolic rate of oxygen under hypoxia was found in the animal with elevated Sphingosine-1-phosphate, which may explain the reduced infarct volume in the pre-treatment animals.