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In 1843, Christian Doppler, a physicist, described an effect which explained a change in the perceived frequency of sound emitted by a moving source. The same was later acknowledged as the ‘Doppler effect’. The principle is extended to the field of ultrasound imaging wherein the insonation frequency of the transducer undergoes a change when reflected by moving objects, which is perceived by the sonologist in an audio mode. The formula governing the ‘Doppler effect’ is as under:-
Current applications of Doppler ultrasound are in assessment of arterial and venous systems and cardiology. System wise, they can be summarised as follows:
There are five methods of imaging by employing Doppler principle :
Because PW ultrasound does not provide the information about the structure / site of origin of an echo, it is customarily combined with a B mode / 2-Dimensional mode imaging, which is then called duplex scanning.
There are disadvantages also, which are :
Thus, Doppler ultrasound applications form an important imaging tool today in a variety of clinical conditions. However, it does have limitations also. As for any ultrasound study, fat, flatus, faeces, motion (respiratory, peristaltic, cardiac or restless patient) cause hindrance to image quality. Secondly, changes in flow pattern occur proximal or distal to stenosis of a vessel only if the degree of stenosis is significant. Thirdly, multiple level stenoses / obstructions result in obscure patterns of flow. Fourthly, the flow patterns are subject to individual variations as also to other physical / physiological factors, thus making the diagnosis that much more difficult. In view of all these limitations, the learning curve is also flat and long before adequate degree of proficiency is achieved.
In conclusion, current Doppler ultrasound applications in clinical situations like Deep vein thombosis, Peripheral vascular disease (arterial), portal hypertension, Intra-uterine growth retardation, stroke etc need proper selection of cases on sound clinical judgement and the interpretation of results is guided by correlation of both.
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