Local in vivo optical measurements of neurometabolic and neurovascular coupling mechanisms in the rodent cortex

Elena Migacheva
EPFL, 2012
EPFL thesis

This thesis introduces a novel approach for in vivo separation and quantification of spatio-temporal dynamics of optical coefficients (absorption, scattering), oxygen saturation (SO2) and haemoglobin concentrations within rat brain somatosensory cortex under baseline and neuronal stimulation conditions. The quantification of mentioned parameters became possible due to design of a novel optical setup for localized spatio-temporally-resolved (within ∼ 1 mm3 at 10 Hz) reflectance spectroscopy (STRReS) measurements at small source-detector separations. The possibility to automatically take into account the light propagation path lengths in tissue using Monte Carlo modeling, defines the novelty of this approach and its perspectives for brain research. This kind of corrections is currently limited or inaccurate with existing techniques like optical intrinsic signal imaging. In particular, the main aim of this thesis is to explore and better understand the local spatio-temporal characteristics of neurovascular and neurometabolic coupling mechanisms within in vivo rat cerebral cortex at initial and subsequent stages of neuronal stimulation with using STRReS technique. It has been proven theoretically (with Monte Carlo simulations) and experimentally (from in vivo rat brain cortex), that STRReS measurements are suitable from in vivo somatosensory cortex under neuronal stimulation conditions. As a part of this research, we have validated theoretically and experimentally the biological importance of new optical parameter ”gamma” (parameter of the phase function), which is nessesary to take into account for accurate optical coefficients extraction. On the other side, the STRReS technique was not found to be helpful to determine differential factors corrections for improvement of optical intrinsic signal imaging analysis due to differences in experiment’s geometries. Finally, it has been shown that temporal dynamics of reflectance is strongly influenced by light scattering variations within 500-900 nm range. Specifically, the shape of reflectance dynamics at 600 and 610 nm differs significantly from deoxy-haemoglobin concentration dynamics at initial stage of neural activation. For this reason, it is important always to take into account the variations of both components of reflectance signal: absorption and scattering. Additionally, an initial delay of 1 s was found in SO2 and haemoglobin concentrations dynamics. The interpretation of this delay, however, is strongly dependent on the origin of scattering signal. Our results are well correlated with ”metabolic” hypothesis of underlying neurovascular and neurometabolic coupling mechanisms, assuming that the scattering signal has a non-vascular origin at 500-599 nm. Nevertheless, the ”neurogenic” hypothesis is not completely excluded as far as the scattering signal has a vascular origin at 500-599 nm.

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