A diameter-independent blood flow measurement technique.

A phase difference technique for measuring true volume flow rate has been developed. The concept utilizes the phase difference from unequal transit times between bursts of ultrasonic energy transmitted simultaneously against and with the blood flow. A theoretical analysis reveals that the volume flow rate sensitivity (V/ml/sec) for a point crystal illumination pattern directed diagonally through the center of the vessel is a function of the inverse of the diameter. But an illumination pattern orientated perpendicularly to the vessel axis, which spans the entire diameter of the vessel, yields a volume flow rate sensitivity independent of the diameter. Line illumination, in contrast to point illumination, does not require the vessel diameter to be measured in order to establish the sensitivity factor, and a change in diameter does not alter the sensitivity factor. Line illumination has a considerable advantage when used on small vessels in which the ratio of wall thickness to the lumen diameter is large and is often variable.