Dynamic Flow Velocity Mapping from Fluorescent Dye Transit Times in the Brain Surface Microcirculation of Anesthetized Rats and Mice.

OBJECTIVE: This study aimed to develop a new method for mapping blood flow velocity based on the spatial evolution of fluorescent dye transit times captured with CLSFM in the cerebral microcirculation of anesthetized rodents. METHODS: The animals were anesthetized with isoflurane, and a small amount of fluorescent dye was intravenously injected to label blood plasma. The CLSFM was conducted through a closed cranial window to capture propagation of the dye in the cortical vessels. The transit time of the dye over a certain distance in a single vessel was determined with automated image analyses, and average flow velocity was mapped in each vessel. RESULTS: The average flow velocity measured in the rat pial artery and vein was 4.4 ± 1.2 and 2.4 ± 0.5 mm/sec, respectively. A similar range of flow velocity to those of the rats was observed in the mice; 4.9 ± 1.4 and 2.0 ± 0.9 mm/sec, respectively, although the vessel diameter in the mice was about half of that in the rats. CONCLUSIONS: Flow velocity in the cerebral microcirculation can be mapped based on fluorescent dye transit time measurements with conventional CLSFM in experimental animals.

Fluorescence Imaging of Blood Flow Velocity in the Rodent Brain.

An adequate supply of blood flow to the brain is critically important to maintain long-term brain function. However, many issues surrounding the regulatory mechanism of the blood flow supply to the brain remain unclear, such as i) the appropriate range of capillary flow velocity to keep neurons healthy, ii) the size of the vascular module to support a functioning neural unit, iii) the sensing mechanism for capillary flow control, and iv) the role of flow regulation in promoting neural plasticity. A fluorescence technique allows for visualization of the dynamic changes between cerebral microcirculation and neural activity concurrently and thus is capable of addressing these questions. Here, we briefly review the methodological aspects of measuring blood flow using fluorescence imaging in rodent brains and introduce a novel approach for mapping the flow velocity in multiple vessels with laser scanning fluorescence microscopy. The flow velocity was imaged by calculating the traveling distance and time of the instantaneously injected fluorescent tags through the vascular networks. Using the present method, we observed that the average flow velocity in the pial artery and vein was 3.0 ± 1.4 mm/s and 1.6 ± 0.5 mm/s, respectively (N = 6 mice). A limitation of the method presented is that the quantification is only applicable to the vascular networks laid in two-dimensional planes, such as pial vascular networks. Further technical improvement is needed to quantify three-dimensional flow through parenchymal microcirculation. Furthermore, it is also needed to fill a gap between the microscopically measured flow parameters and the macroscopic feature of the brain blood flow for clinical interpretation.