ULTRASONIC BLOOD FLOW RATE METER

An ultrasonic flowmeter may be used in place of an electromagnetic flowmeter to measure the instantaneous flow rate of blood. Ultrasound can be beamed through the skin, and thus measurements can be made from outside the body. Advanced types of ultrasonic flowmeters can also measure the flow profiles within blood vessels providing a potentially powerful tool for the assessment of vascular disease.

There are various methods by which blood flow can be recorded using ultrasound including transit time methods in which the blood velocity is calculated from the time taken to cross the vessel oblique to the direction of flow. More commonly a continuous wave of ultrasound is transmitted through the vessel and received on a transducer adjacent to the first which picks up the back-scattered ultrasound from the blood in a zone where the transmitting and receiving beams intersect. The received ultrasound contains doppler-shifted components which represent the velocity components of the blood along the direction of the ultrasound beam.

Doppler-shifted components can also be received using a pulsed waveform provided there are sufficient waves at the ultrasonic frequency in each pulse. This allows the principle of range-gating in which the receiving echo amplifier is only turned on for a short time during the receiving phase. This allows reception from one particular depth in the tissue by stepping or sweeping the time of the receiving amplifier gate. Blood flow velocity at a series of points across a blood vessel may be obtained to produce a velocity profile. These pulsed doppler flowmeters may be combined with ultrasonic B-scanners to provide a two-dimensional image of a blood vessel showing only the moving parts of the image (the blood).

The most practical form of ultrasonic blood flowmeter is the continuous wave doppler system with the doppler-shifted components being fed to a zero-crossing detector. This can produce a signal which corresponds approximately to the average blood velocity in the vessel. A slightly more complex version of this allows the independent identification of forward and reverse flow. Forward and reverse flow is represented by the doppler- shifted components above and below the ultrasonic frequency and these can be separated by established methods. The two signals may be fed to separate pen traces or combined in a bi-directional display.

Such devices are mainly used by vascular surgeons. See also Zero-crossing detector, blood flow meter, Doppler blood flow meter.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details