Quantification of chromophore concentrations in reflectance mode remains a major challenge for biomedical optics. Spectroscopic Optical Coherence Tomography (SOCT) provides depth-resolved spectroscopic information necessary for quantitative analysis of chromophores, like hemoglobin, but conventional SOCT analysis methods are applicable only to well-defined specular reflections, which may be absent in highly scattering biological tissue. Here, by fitting of the dynamic scattering signal spectrum in the OCT angiogram using a forward model of light propagation, we quantitatively determine hemoglobin concentrations directly. Importantly, this methodology enables mapping of both oxygen saturation and total hemoglobin concentration, or alternatively, oxyhemoglobin and deoxyhemoglobin concentration, simultaneously. Quantification was verified by ex vivo blood measurements at various pO 2 and hematocrit levels. Imaging results from the rodent brain and retina are presented. Confounds including noise and scattering, as well as potential clinical applications, are discussed. Kleinfeld, "Chronic optical access through a polished and reinforced thinned skull," Nat. Methods 7(12), 981-984 (2010).

Abstract:We present volumetric imaging and computational techniques to quantify neuronal and myelin architecture with intrinsic scattering contrast. Using spectral / Fourier domain Optical Coherence Microscopy (OCM) and software focus-tracking we validate imaging of neuronal cytoarchitecture and demonstrate quantification in the rodent cortex in vivo. Additionally, by ex vivo imaging in conjunction with optical clearing techniques, we demonstrate that intrinsic scattering contrast is preserved in the brain, even after sacrifice and fixation. We volumetrically image cytoarchitecture and myeloarchitecture ex vivo across the entire depth of the rodent cortex. Cellular-level imaging up to the working distance of our objective (~3 mm) is demonstrated ex vivo. Architectonic features show the expected laminar characteristics; moreover, changes in contrast after the application of acetic acid suggest that entire neuronal cell bodies are responsible for the "negative contrast" present in the images. Clearing and imaging techniques that preserve tissue architectural integrity have the potential to enable noninvasive studies of the brain during development, disease, and remodeling, even in samples where exogenous labeling is impractical.

A method of measuring cortical oxygen metabolism in the mouse brain that uses independent quantitative measurements of three key parameters: cerebral blood flow (CBF), arteriovenous oxygen extraction (OE), and hemoglobin concentration ([HbT]) is presented. Measurements were performed using a single visible light spectral/Fourier domain OCT microscope, with Doppler and spectroscopic capabilities, through a thinned-skull cranial window in the mouse brain. Baseline metabolic measurements in mice are shown to be consistent with literature values. Oxygen consumption, as measured by this method, did not change substantially during minor changes either in the fraction of inspired oxygen (FiO2) or in the fraction of inspired carbon dioxide (FiCO2), in spite of larger variations in oxygen saturations. This set of experiments supports, but does not prove, the validity of the proposed method of measuring brain oxygen metabolism.

Our graphing and modeling data suggest that vascular architecture enables both local and lamina-specific control of blood flow in the inner retina. The imaging, graph analysis, and modeling approach presented here will help provide a detailed characterization of vascular changes in a variety of retinal diseases, both in experimental preclinical models and human subjects.

Although microvascular dysfunction accompanies cognitive decline in aging, vascular dementia, and Alzheimer's disease, tools to study microvasculature longitudinally in vivo are lacking. Here, we use Doppler optical coherence tomography (OCT) and angiography for noninvasive, longitudinal imaging of mice with chronic cerebral hypoperfusion for up to 1 month. In particular, we optimized the OCT angiography method to selectively image red blood cell (RBC)-perfused capillaries, leading to a novel way of assessing capillary supply heterogeneity in vivo. After bilateral common carotid artery stenosis (BCAS), cortical blood flow measured by Doppler OCT dropped to half of baseline throughout the imaged tissue acutely. Microscopic imaging of the capillary bed with OCT angiography further revealed local heterogeneities in cortical flow supply during hypoperfusion. The number of RBC-perfused capillaries decreased, leading to increased oxygen diffusion distances in the days immediately after BCAS. Linear regression showed that RBC-perfused capillary density declined by 0.3% for a drop in flow of 1 mL/100 g per minute, and decreases in RBC-perfused capillary density as high as 25% were observed. Taken together, these results demonstrate the existence of local supply heterogeneity at the capillary level even at nonischemic global flow levels, and demonstrate a novel imaging method to assess this heterogeneity. Keywords: angiography; capillary recruitment; Doppler optical coherence tomography; hypoperfusion; vascular dementia INTRODUCTION Chronic cerebral hypoperfusion induced by small-artery occlusion or critical arterial stenosis is a pathophysiologic hallmark of vascular dementia. Histologically characterized by white matter changes and multifocal infarctions, chronic cerebral hypoperfusion is associated with cognitive decline. 1-3 Rodent models of chronic cerebral hypoperfusion that use common carotid artery stenosis or occlusion mimic many aspects of vascular cognitive impairment in patients, including decreased cerebral blood flow, inflammation and blood-brain barrier abnormalities, brain atrophy, white matter degeneration, decreased metabolism, as well as impaired working memory. 4 Bilateral common carotid artery occlusion has been widely used in the rat to model chronic cerebral hypoperfusion. 5 Recently, mouse bilateral common carotid artery stenosis (BCAS) models have emerged. 6 In these models, although the degree of hypoperfusion is correlated with the extent of ischemic lesions and neurologic impairment, 7 the magnitude of flow reductions after BCAS were moderate at best (~30 to 40%), 8 at least relative to classic ischemic thresholds of 80 to 85%, 9-11 and lasted only a few days before collateral development and recovery of flow. The lack of classic ischemia raises questions about the precise mechanism of tissue loss and cognitive impairment. Previous studies have used laser Doppler Journal of Cerebral Blood