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Introduction: The hemodynamic response following neural activation is the basis for most functional neuroimaging methods, including functional magnetic resonance imaging (fMRI). Consequently, in order to correctly interpret the results of neuroimaging experiments both the functional and the structural neurovascular coupling must be well understood. The degree and density of cortical vascularization should be compared with high resolution maps of blood flow changes induced by neural activation. In
this presentation, methods will be described that try to elucidate this coupling using ex-vivo and in-vivo approaches. Methods: Ex-vivo experiments: Formalin-fixed frozen sections of brain tissue (macaque monkey) were
processed for anti-collagen type IV fluorescence immunohistochemistry to stain for vessels. Digital image processing was applied to compute the length density [m/m3], surface density [m2/m3], volume fraction [m3/m3], and mean diameter [m] of the vessels. Scanning electron microscopic analyses of intravascular polymer fillings and synchrotron-based micro-CT (using a voxel size of 1.4
m) have been performed using monkey and rat tissue and the results were compared with the data obtained from the histological methods. In-vivo experiments: Laser speckle contrast imaging was applied in the rat barrel cortex and blood flow changes in response to single vibrissa deflection were recorded with sub-second and sub-millimeter resolution.
Results: Ex-vivo experiments: The immunohistochemical staining procedure yielded vessel-specific images of high quality and reproducibility within and between animals. The vascular density is in close relationship with the steady-state metabolic demand of the particular region, as can be seen in the high vascular density of layer IV (the layer with the highest cell density) in all visual cortices of the macaque monkey. The quantification of the synchrotron-based micro-CTs confirms the histologically obtained results and renders a true 3D-analysis possible. In-vivo experiments: Laser speckle contrast imaging
provides blood flow maps with a high signal-to-noise ratio, making single trial analysis feasible. The spatial resolution of the method is excellent as neighboring cortical columns could be differentiated.
Conclusion: Several high resolution approaches to study structural and functional aspects of the cortical
vasculature have been successfully implemented. The influence of differences in vascularization on the
neuroimaging signals is of utmost importance but remains largely unknown. Future work will involve modeling approaches on the basis of the three dimensional vascular architecture (obtained from synchrotron-based micro-CT) to understand the basic mechanisms that are used by the brain to locally regulate the blood flow. Precise in-vivo measurements of the flow changes (obtained from laser speckle contrast imaging) will in turn be necessary to validate these modeling results.
