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Ultrasonic flowmeters can be used to measure and record instantaneous blood flow from outside the body, employing an ultrasound beam transmitted through the skin. Most are continuous wave (CW) flowmeters which provide little information about the flow profile, and so pulsed instruments have been developed which can identify the flow velocity at any point beneath the probe. The size and depth of the measurement point can be varied by adjusting the pulse length and timing of the range-gate. By examining the doppler shift at various depths we may obtain a velocity profile across a blood vessel. Typical parameters for this device would be an ultrasonic frequency of 8 MHz, pulse duration of 1 [u]s (which produces a travelling packet of sound 1.5 mm long), and pulse repetition frequency of 10 000 c/s.
The blood velocities recorded from each point across the profile can be displayed separately on a chart recorder after analysis by zero-crossing detector, and these signals may be combined to produce a figure for blood flow rate through the vessels. However, there are many variations on the end processing of pulsed wave doppler signals and these are not covered here.
Since range information is available as well as information about blood flow it is possible to combine it with an ultrasonic B-scan display (two-dimensional section of tissue) so that the point at which the measurement is being made in the tissue can be identified and manipulated. In more complex instruments a 'doppler scan' can be produced which is a B-scan, but showing only the moving parts of the picture (blood flow in arteries).
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details