Doppler effect

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• Doppler Effect

Doppler effect represents a change in the detected frequency of waves when the source and detector are in relative motion. The physical essence of the Doppler effect is the summation of the wave's speed with the frequency of the relative speed of the source and the detector. This effect applies to waves of any type, i.e. mainly acoustic and electromagnetic. It was first described by Christian Doppler as a shift of spectral lines in rotating binaries, where the spectrum of a star moving towards us shifted towards the blue end and the spectrum of a star moving away from us towards the red end of the spectrum. For medicine, the most important application of the Doppler effect is the reflection of ultrasound from moving particles, especially red blood cells.

A typical situation of the application in medicine is a standing observer, i.e. a fixed detector, and a moving source, i.e. tissue reflecting the waves incident on it. For wavelength of the detected waves we can apply:

$\lambda =\lambda _{0}\pm {\frac {v_{zdr}}{f_{0}}}$

The sign is determined by whether the sound source is moving away (+) or closer (-) to the detector, λ₀ is the wavelength of the wave leaving the source, f₀ is the original frequency of this wave and v_zdr is the speed of the source.

The basic application of the Doppler effect is the detection of the blood flow. By the combining color-coded information of the blood flow with the ultrasound image in B mode we get the duplex ultrasound. In angiology may be also used coriolis flow meter, that detects only movement and signals it with an audio output

The arrangement is more complicated in this case. The wave is sent by the probe and falls on the tissue, which acts as a "detector". In the tissue, it is already reflected with a shifted frequency and returns as a wave from the moving source back to the probe, which also functions as a detector. So there are two frequency shifts. For the difference between the detected and transmitted frequency, the equation applies:

$\Delta f={\frac {2vf_{0}}{c}}\cos \alpha$,

where v is tissue movement speed, c is the wave speed and α is the angle that the vector of tissue movement speed makes with the axis of the probe.

Related articles

• Doppler ultrasonography in medicine • Doppler ultrasonography
• Doppler representation
• Doppler echocardiography • Transcranial doppler ultrasonography • Fetal Dopplerometry • Doppler flow meter

References

BENEŠ, Jiří – JIRÁK, Daniel – VÍTEK, František, et al. Základy lékařské fyziky. 4. edition. 2015. 322 pp. ISBN 9788024626451.