Television-computer method for in vivo measurement of capillary diameter, based on the passage of red cells

Capillary diameter is generally measured by methods requiring a trained observer to determine the location of the vessel walls. We have developed a video computer method for measurement of capillary diameter, based on temporal fluctuations of light intensity within the lumen due to the passage of red cells. These fluctuations are quantified, at successive points along a line perpendicular to the capillary by sampling, with a microcomputer, the output of a video analyzer and calculating the variance of light intensity with time at each point. A profile of this variance, across the vessel, shows an abrupt change at each edge of the flowing red cell column. The edge of the lumen is "defined," for the computer program, as the last point before the variance of light intensity becomes significantly greater than in the surrounding tissue. The method thus assumes there is no detectable cell-free plasma layer adjacent to the vessel wall. Both in vitro and in vivo tests showed good agreement between luminal diameters measured by a trained observer and by the video computer method. Each diameter measurement requires 2.5 sec for data acquisition and 0.3 sec for computation. This automated technique applies a standard criterion to the measurement of capillary diameter and thus avoids subjective error. It may be used for sequential measurements, e.g., where the microvessel diameter is changing with time.