Measurement of erythrocyte velocity by use of a periodic differential detector.

An optical velocimeter employing a linear array of photodiodes has been developed and utilized for measuring erythrocyte velocities in the microcirculation. A magnified image of a microvessel is projected and aligned on a one-dimensional array of photodiodes. Photocurrent from odd-ordered diodes is summed, photocurrent from even-ordered diodes is summed, and a signal proportional to the difference between these two currents is produced by a differential amplifier. The center frequency of the output signal of the differential amplifier is proportional to the erythrocyte velocity. After lowpass filtering the output of the differential amplifier, a signal proportional to its frequency and therefore velocity is produced by a frequency-voltage converter. In vitro calibration with a moving dried smear of erythrocytes illustrated a linear relation between the output of the frequency-voltage converter and erythrocyte velocity for a wide range of velocities and magnifications. The system produces a stable zero output at zero velocity and had an estimated frequency response of greater than 40 Hz in vivo. Volumetric flow rates computed from velocity and diameter measurements at arteriolar bifurcations in the rat cremaster muscle were consistent with mass conservation.