Four window differential capillary velocimetry.

A video red blood cell velocimeter was implemented with four photometric windows in such a fashion that the upstream and downstream signals are the difference between spatially separated window pairs. The performance of this system was compared with that of a conventional dual window photometric video velocimeter. Tests were made with artificial patterns of red blood cells that simulated long trains of contiguous cells, or large plasma gaps. It was found that the four window system produces a correlogram that is better suited for delay to maximum cross-correlation detection. Similarly, when the responses of the two methods were compared in terms of ability to detect changes of velocity, the time constant for a test step velocity change was found to be 1.2 +/- 0.7 sec for the four window system vs 2.3 +/- 0.6 sec for the two window system. It is concluded that this modification of the capillary red blood cell velocimetry methodology is better suited for detecting the spontaneous flow variations due to vasomotion.