Accurate assessment of cerebral microvascular flow is crucial for understanding brain functioning and neurovascular diseases. Dynamic Light Scattering Optical Coherence Tomography (DLS-OCT) has been used to obtain blood velocity measurements in a large number of microvascular segments, including arterioles, capillaries, and venules in anesthetized mice. However, anesthesia induces large changes in the microvascular blood flow. Imaging awake animals by DLS-OCT is preferable, but very challenging due to motion artifacts. Here, we present the first DLS-OCT measurements of cortical microvascular blood flow in awake mice, made possible by an innovative algorithm based on Vertical Displacement at Inflection (IVD) in velocity distribution.
SignificanceThe accurate large-scale mapping of cerebral microvascular blood flow velocity is crucial for a better understanding of cerebral blood flow (CBF) regulation. Although optical imaging techniques enable both high-resolution microvascular angiography and fast absolute CBF velocity measurements in the mouse cortex, they usually require different imaging techniques with independent system configurations to maximize their performances. Consequently, it is still a challenge to accurately combine functional and morphological measurements to co-register CBF speed distribution from hundreds of microvessels with high-resolution microvascular angiograms.AimWe propose a data acquisition and processing framework to co-register a large set of microvascular blood flow velocity measurements from dynamic light scattering optical coherence tomography (DLS-OCT) with the corresponding microvascular angiogram obtained using two-photon microscopy (2PM).ApproachWe used DLS-OCT to first rapidly acquire a large set of microvascular velocities through a sealed cranial window in mice and then to acquire high-resolution microvascular angiograms using 2PM. The acquired data were processed in three steps: (i) 2PM angiogram coregistration with the DLS-OCT angiogram, (ii) 2PM angiogram segmentation and graphing, and (iii) mapping of the CBF velocities to the graph representation of the 2PM angiogram.ResultsWe implemented the developed framework on the three datasets acquired from the mice cortices to facilitate the coregistration of the large sets of DLS-OCT flow velocity measurements with 2PM angiograms. We retrieved the distributions of red blood cell velocities in arterioles, venules, and capillaries as a function of the branching order from precapillary arterioles and postcapillary venules from more than 1000 microvascular segments.ConclusionsThe proposed framework may serve as a useful tool for quantitative analysis of large microvascular datasets obtained by OCT and 2PM in studies involving normal brain functioning, progression of various diseases, and numerical modeling of the oxygen advection and diffusion in the realistic microvascular networks.
SignificanceIt has been hypothesized that abnormal microcirculation in the retina might predict the risk of ischemic damages in the brain. Direct comparison between the retinal and the cerebral microcirculation using similar animal preparation and under similar experimental conditions would help test this hypothesis.AimWe investigated capillary red-blood-cell (RBC) flux changes under controlled conditions and bilateral-carotid-artery-stenosis (BCAS)-induced hypoperfusion, and then compared them with our previous measurements performed in the brain.ApproachWe measured capillary RBC flux in mouse retina with two-photon microscopy using a fluorescence-labeled RBC-passage approach. Key physiological parameters were monitored during experiments to ensure stable physiology.ResultsWe found that under the controlled conditions, capillary RBC flux in the retina was much higher than in the brain (i.e., cerebral cortical gray matter and subcortical white matter), and that BCAS induced a much larger decrease in capillary RBC flux in the retina than in the brain.ConclusionsWe demonstrated a two-photon microscopy-based technique to efficiently measure capillary RBC flux in the retina. Since cerebral subcortical white matter often exhibits early pathological developments due to global hypoperfusion, our results suggest that retinal microcirculation may be utilized as an early marker of brain diseases involving global hypoperfusion.
Chronic inflammation is one of the most prominent features of Alzheimer's disease (AD). Little is known about how brain hemodynamics and oxygenation are affected by AD-related inflammation. Here, we use two-photon phosphorescence lifetime imaging with an oxygen-sensitive dye “Oxyphor 2P” to measure the partial pressure of oxygen (pO2) before and during endotoxin-induced neuroinflammation in cortical vessels of a mouse model of AD. Capillary red blood cell flux (RBC flux) was measured through two-photon phosphorescence intensity microscopy. To induce chronic inflammation, we injected lipopolysaccharide (LPS) intraperitoneally, daily for two weeks, in female APPswe: PS1dE9 mice and age-matched wild-type (WT) controls. Intravascular pO2 and RBC flux were measured in the somatosensory cortex before the LPS injection, on week 1 (day 7), and week 2 (day 15) during the LPS injection. Our results demonstrate that LPS-induced systemic inflammation leads to significant decreases in cortical intravascular pO2 while showing a negligible effect on capillary RBC flux. Moreover, AD mice are more susceptible to inflammation with more pronounced cortical pO2 reduction in comparison to WT mice. Our findings suggest that inflammation plays a key role in AD-related disruptions of cerebral tissue metabolism of oxygen.
Although the beneficial effects of regular physical exercise on brain aging and neurodegenerative diseases are well recognized, a clear understanding of how exercise leads to such benefits remains elusive. In this work, we investigated the effects of normal aging on cortical microvascular oxygenation, perfusion, and morphology and the impact of four months of voluntary wheel running on cortical microvascular oxygenation in 20 months old mice. We used two-photon microscopy to assess age-related and exercise-induced changes in the distributions of capillary oxygen partial pressure (PO2) and red-blood-cell flux across cortical depth in awake mice. Our finding suggests the mitigating effect of exercise on the progression of age-related changes in capillary oxygenation in deeper cortical layers which may be related to health-enhancing benefits of exercise in elderly individuals.
We present a framework to coregister a large set of blood flow measurements from the cortical microvascular segments obtained by DLS-OCT with the microvascular angiogram obtained using TPM. By using this framework, we estimated the distributions of the mean RBC velocities in arterioles, venules, and capillaries, and as a function of the branching order from over 1,000 microvascular segments. The developed tools will help with more quantitative analysis of the large data sets obtained by OCT and TPM in the studies involving normal brain functioning, progression of various diseases, and numerical modeling of the oxygen transport in the microvascular networks.
Quantifying intestinal microvascular oxygenation may shed light on origin and progression of intestinal diseases as well as the role of the gut microbiota. In this work, we developed a method for depth-resolved microscopic measurements of partial pressure of oxygen (pO2) and Red Blood Cell (RBC) flux in the capillaries of intestinal wall in mouse cecum. Capillary pO2 and RBC flux were measured in healthy mice as a function of depth during normoxia and hyperoxia. Our results show low mean capillary RBC flux and relatively uniform capillary pO2 across the cecum wall depth.
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