Research On Multimodal Optical Imaging Of Brain Function And Its Application

Brain is the pivot of information receiving and processing. However, because of its complexity, little has been known about the mechanisms of brain regulation and neuronal disorders. Brain function imaging techniques provide a useful tool for the study of the secrets of brain. Whereas single-mode imaging technique could not clarify the exact mechanism of brain regulation, for better understanding the mechanisms of neurovascular coupling and neuronal disorders, simultaneous monitoring of multi-parametric changes in the brain is needed. Optic imaging as a non-invasive method, allows high-resolution imaging of brain cortex and has the potential of multimodal imaging. In this thesis, we presented a fluorescence-corrected multimodal optical imaging system which could provide information about changes in intracellular pH (pHi), hemoglobin concentrations and cerebral blood flow (CBF) simultaneously. And then we exploited this system to monitor the dynamic changes associated with cortical spreading depression (CSD) and partial ischemia induced by carotid artery occlusion. The main contents are listed in the following paragraphs.(1) Given the fact that the changes in absorption in biological tissue would induce the fluctuation of fluorescence signals, we presented a method of fluorescence correction to reduce this interference and built up a fluorescence-corrected multimodal imaging system, which combined intracellular pH-sensitive fluorescence imaging, dual wavelength optical intrinsic signal imaging (OISI) and laser speckle contrast imaging (LSCI). With this system, the changes in pHi, hemoglobin concentrations, cerebral blood flow (CBF) and cerebral metabolic ratio of oxygen (CMRO2) could be detected simultaneously.(2) The dynamic changes related to CSD were monitored with this multimodal imaging system. We found that a transient intracellular acidic shift followed by a small alkalization occurred during CSD, which was accompanied by pronounced hyperemia and vasodilation, the changes in CMRO2 and concentrations of hemoglobin. After CSD, NR fluorescence remained elevated, indicating a prolonged intracellular acidification. The recovery of pHi to baseline took much longer time than those of hemodynamic response, which might be caused by the increased content of lactate after CSD. And the time courses of the first two phases of pH; concurred with that of direct current (DC) potential, which suggested that pHi was an effective indicator of neuronal activity. Besides, we investigated the effects of dimethylsulfoxide (DMSO) on hemodynamic response during CSD. Topical application of DMSO increased arteriolar resting diameter and resting blood velocity at all vascular compartments. In addition, both vasodilation and hyperemic response to CSD were attenuated by DMSO in a dose-dependent manner at doses from 0.1% to 4%. In contrast, the maximum value of blood velocity during CSD was not significantly affected by DMSO. Our results suggested that the attenuation in hemodynamic response during CSD could possibly be caused by increased baseline value of vessel tone and blood velocity and when investigators use DMSO to dissolve water-insoluble, topically applied drugs in the hemodynamic study of CSD, doses of DMSO should be kept below 0.1% in order to avoid false results.(3) The spatiotemporal patterns related to partial ischemia induced by carotid artery occlusion (CAO) were investigated with our system. After CAO, partial ischemia was observed on the whole cortex accompanied by a decrease in CMRO2. And then the pial arteries dilated slightly because of the activation of auto-regulation mechanism, which caused an increase in cerebral blood volume, while deoxy-hemoglobin took the majority. During ischemia, the activation of glycolysis led to an increase in lactate concentration, and thus resulted in intracellular acidification. After releasing carotid artery, a hyperperfusion occurred on the cortex. Besides, the occlusion of contralateral carotid artery alone had no significant effect on cerebral blood flow. However, when the ipsilateral carotid artery was occluded, contralateral carotid artery had contribution to CBF as revealed by our observation that occlusion of contralateral carotid artery would lead to a severe ischemia while releasing contralateral carotid artery could alleviate the ischemia. This suggested that CBF was mainly provided by ipsilateral carotid artery. However, when this supply was insufficient and partial ischemia appeared on the whole cortex, blood flow of contralateral carotid artery would be redistributed through Willis circle in order to meet the metabolic demands.