Neurovascular coupling and cerebrovascular reactivity analysis through near-infrared spectroscopy (PhD)

Abstract

Near-infrared spectroscopy (NIRS) is a portable non-invasive tool to analyze the organic composition of a sample by the application of near-infrared light. For instance, in a routine medical check-up, a small clip: pulse oximeter is placed on the index finger which employ NIRS technique to determine the oxygen in the blood. In brain imaging, NIRS detect the local hemodynamic response with reasonable spatial resolution, better temporal resolution and comfort compared to other brain imaging modalities like magnetic resonance imaging (MRI) and electroencephalogram (EEG). It is an optical imaging technique that is sensitive to light and skin optics parameters. The usage of optode (light emitter-detector pair) for detecting the NIRS signals to estimate the cerebral oxygenation levels is hindered considerably by surface roughness profile of skin, which necessitates the optodes to be positioned at certain angle with respect to the skin surface and not perpendicularly on the skin surface. In the present study, first an attempt has been made to optimize the placement of NIRS optodes in reflection photoplethysmographic sensor system by analyzing the roughness of surfaces of skin layers. Its implications on cerebral cortex imaging down the line have been presented to obtain accuracy in cerebral hemodynamic response. The second part of the work focuses on cortical excitability through various electrode configurations of anodal transcranial direct current stimulation (tDCS). tDCS method involves application of weak direct current using non-invasive electrodes placed on the scalp. tDCS have shown to modulate both cortical neural activity and hemodynamics and is used in various neuro-rehabilitation strategies. However, the underlying mechanism of tDCS is not completely known and there is a huge inter-individual variedness in the responses to tDCS. This study evaluates the effect of various anodal tDCS designs with regard to electric field and voltage distribution using subject’s specific anatomy in a computational framework. The study evaluates the effect of conventional and high definition transcranial direct current stimulation (HD-tDCS) by utilizing synthetic magnetic resonance image volumes for normal and lesioned brain. The study next focuses on the application of NIRS to analyze the correlation between cerebrovascular reactivity (CVR) and neurovascular coupling (NVC). Cerebrovascular reactivity represents the responsiveness of cerebral blood vessels to vasoactive stimuli And, neurovascular coupling is a phenomenon through which neuronal activity elicits a local change in cerebral blood flow (CBF). This leads to the notion of neurovascular unit (NVU) consisting of the vascular smooth muscle, perivascular space, synaptic space and, astrocyte glial cell. Several neural disorders are associated with impaired NVC. The study postulates a correlation between neurovascular coupling and cerebrovascular reactivity under the effect of tDCS based on experimental NIRS data. The study presents a computational model to evaluate vessel volume response under anodal transcranial direct current stimulation considering a basic system of neurovascular unit. The proposed model based on neurovascular dynamics tracked vascular response elicited through neuronal activity via various signaling pathways during anodal tDCS. It was found that the stimulation can perturb the vessel response via both neuronal and non-neuronal pathways. The proposed model specifies the signaling pathways in a compartment model of neurovascular unit with respect to evoked hemodynamic responses in a computational framework which is otherwise extremely difficult to examine in wet lab environment. The computational modeling of neurovascular coupling and cerebrovascular reactivity measured through NIRS will possibly enable the understanding of underlying dynamics of non-invasive current stimulation.