DOPPLER ULTRASONIC SCANNER

Information about the motion of blood in the larger vessels may be obtained from the doppler shifted reflections received by a transducer outside the body. This is most commonly used to assess the quality of flow from an audible signal produced from the doppler shifted components or by processing of these signals to indicate the flow pattern or flow rate. Images can also be produced by combining the information derived from these doppler signals with B-scan images. There are two main types.

A continuous wave doppler transducer may be scanned back and forth across the skin and an image produced corresponding to the scan plane, which writes on the screen only when doppler signals are being received. Thus a plan of blood vessels immediately beneath the transducer may be produced.

A more complex type exists in which the continuous wave is interrupted to form short bursts of ultrasound which interrogate small sections of the tissue beneath the transducer in sequence. By moving the transducer across the skin a B-scan is produced. However, the B-scan is gated to prevent the presentation of any echoes arising from tissue which is not moving. In this way a section scan is produced showing only those sites which exhibit motion. The doppler signals also contain information about the flow rate at each moving point on the image and this may be presented on a colour display which shows different flow rates as different colours. This is particularly useful since a diseased artery will have turbulent flow (or flow separation) in which flow may exist in both directions at once. The colour display can provide unequivocal evidence of arterial disease and identify the site of any constriction or dilatation.

Such techniques are used by vascular surgeons in special clinics and they offer the potential of avoiding dangerous and painful X-ray contrast procedures.

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