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Physical principles

Created: 13/8/2005
The oesophageal doppler relies on the Doppler principle.

Doppler Principle

There is an increase in observed frequency of a signal when the signal source approaches the observer and decrease when the source moves away.

Click here for more details regarding the Doppler principle.

Doppler systems are totally dependent on the changes in the frequency of the transmitted ultrasound that result from the encounter of the wavefront with moving red blood cells. If the transmitted sound waves encounter a group of red cells moving towards the source, they are ref lected back at a frequency higher than that at which they were sent producing a positive doppler shift. The opposite effect occurs when a given frequency sent into tissues encounters red cells moving away. The result is the return of a frequency lower than that transmitted, and the doppler shift is negative. Analysis of the ref lected frequencies allows determination of velocity of flow.

Doppler systems may utilise a continuous or pulsed ultrasound technique;

The continuous system

Can measure high velocities but averages the frequency shifts along the whole length of the ascending aorta so that the exact point at which the velocity is measured is unknown. It is therefore difficult to know where to measure the aortic diameter.

The pulsed doppler system

Uses the same transducer to generate and receive the ultrasound but this produces short pulses instead of a continuous stream of ultrasound. The great advantage of this system is that the beam can be focussed so that the operator knows the precise depth at which the
measurement is being made. However there is a limit to the velocity of blood flow which can be measured since high velocities lead to a large doppler shift. Increasing the pulse repetition frequency extends the range of velocity measurement, but decreases the possible depth of measurement since the signals must have time to return to the transducer before the next pulse is emitted.

The Doppler equation

Doppler Equation Where Ft is the transmitted Doppler frequency, V is the speed of blood flow, CosØ is the Cosine of the bloodflow to beam angle and C is the speed of sound in tissue.

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