Image correlation method for measuring blood flow velocity in microcirculation: Correlation 'window' simulation and in vivo image analysis

To elucidate the function of the microcirculation system, it is very important to know the blood flow velocity and its distribution in the microvessels. We have developed an automated system for measuring blood flow velocity in microcirculation by image correlation. The 'window' in the image correlation method is equivalent to the sensors in various other measurement methods. We performed simulations with virtual blood flow images consisting of random dots before measuring actual ones, and examined the optimum window shape and size. We found that by reducing the size of a circular window to the size of erythrocytes we could measure in vivo blood flow images with high accuracy. We recorded them with a high-speed video camera system at high temporal resolution, and measured the velocity in microvessels of normal Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). SHR had higher blood velocity than WKY even though the vessel diameters were the same. Using this method to measure the blood flow velocity profile at the bent corner of SHR's arteriole at the heart systole, we found that erythrocytes flow faster at the inner side of the bend, so the vessel wall was exposed locally to higher shear stress in the hypertensive condition.