Doppler shift and angle of incidence

Physics, Monitoring, & Devices

The Doppler shift occurs when a source of wave is moving toward or away from a receiver (or when the receiver is moving toward or away from a wave source). When the distance between a wave source and the receiver is getting smaller, the observed frequency of the wave is greater than the source frequency. When the distance between a wave source and the receiver is getting larger, the observed frequency of the wave is less than the source frequency. How much of a change in observed frequency is related to the velocity of the receiver/source and the angle of incidence. For application in the anesthesiology world, the ultrasound that we are using is usually a TEE probe. The ultrasound beam hits moving red blood cells and reflects the beam back at a different frequencies and different angles, based on how the beam is positioned. This is called the Doppler shift frequency. This allows for us to measure the velocity of blood through a vessel, for which the equation is:

(see image)

Where:

  • v=velocity of red blood cell targets

  • f=Doppler shift frequency

  • f0=transmitted ultrasound beam frequency

  • θ=angle between the ultrasound beam and the vector of red blood cell flow

  • c=velocity of ultrasound in blood (approximately 1570 m/sec)

As implied by the above equation, the impact of the angle on the Doppler shift will be greatest when the angle of incidence is 0 degrees (cos 0 degrees is 1) and least when the angle of incidence is 90 degrees (when the probe is exactly perpendicular to the source, the cos 90 degrees is 0).

null at https://www.openanesthesia.org/wp-content/uploads/2013/05/Shift.png

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2012

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