DOPPLER BLOOD FLOW/VELOCITY METER

Typical Number in Hospital: 2

Cost Bands: 3,4

References: 1,3

Flow velocity in blood vessels can be estimated from outside the body using ultrasound. An ultrasonic transducer comprising two small piezoelectric elements, one for transmit and one for receive, is placed on the skin over the blood vessel in question so that a narrow beam of continuous wave ultrasound passes through it. Some of the energy is back-scattered to the receiving transducer from the blood and the frequency of the returning waves is shifted in frequency (by the doppler effect) in proportion to the velocity of the blood. Unfortunately blood flow is complicated by the fact that it is pulsatile, and because the velocity is not uniform across the vessel (velocity is higher at the centre). Thus there are many different frequencies in the returning signals and these are changing rapidly.

To calculate the blood flow velocity the mean frequency of the doppler signal can be calculated and this is related to mean blood velocity. The mean frequency can be obtained using analogue circuits or a spectrum analyser and computer. Absolute measurement of velocity is difficult because the angle between the blood flow and ultrasound beam is not known, and flow rate calculation requires a knowledge of the internal diameter of the vessel. The zero-crossing detector provides a simple and inexpensive method of obtaining a rough estimate of flow velocity.

The main use of such systems is in detecting flow abnormalities in peripheral arteries occurring in disease of the vascular system. Although the methods are imperfect, they are sometimes more desirable than using X-ray contrast techniques (which show the anatomy) because of the risks and pain associated with invasive procedures.

Doppler blood flow signals can now be linked to ultrasonic B-scanning so that the image formed is gated to show only those parts of the anatomy which are moving. The image formed has some of the advantages of X-ray imaging but is safer to obtain and can provide quantitative information about the flow patterns at particular points on the image. Arterial strictures can be identified thus reducing the need for exploratory surgery and also permitting the repeated assessment of the diseased artery after treatment.

A typical system might include the transducer, a high- frequency oscillator (typically between 5 and 15 MHz), a receiving amplifier, a detector (often bi-directional), a frequency spectrum analyser, and a meter, display or recording device. They would be found in the surgical ward or a special vascular diagnostic clinic.