In ultrasound, sonographers use Doppler Effect to measure a patient’s blood flow velocity. Red blood cells reflect sound waves produced by a Doppler. The waves & frequency varies with the flow velocity.
Continue reading about Doppler Effect and how it plays a vital role in measuring blood flow velocity.
What is Doppler Effect?
Have you ever been standing on a street corner when a motorcycle passes? The zooming noise from the engine seems to increase in frequency or pitch as the motorcycle approaches and decrease in frequency as the motorcycle departs. The change in frequency is due to the Doppler Effect. Doppler shift is also called Doppler frequency.
The frequency of sound changes when the sound source and the receiver move closer together or farther apart. The frequency does not change when the distance between the two remains constant. This physical principle is used to measure the velocity of blood in the circulation.
How Does Doppler Effect assist in Measurement of Blood Velocity?
As the blood flows towards a transducer, its echoes are reflected. The echoes have a higher frequency than those produced by the transducer. As blood flows away from the transducer, its echoes have a relatively low frequency compared to the emitted ones. A Doppler frequency shift is the frequency difference between the echoes received and transmitted. It is directly proportional to your blood flow velocity.
Doppler shift (Hz) = reflected frequency – transmitted frequency
Speed, which is purely a magnitude, indicates the distance that a red blood cell moves in 1 second.
Velocity is defined by a magnitude and a direction. Think of velocity as an arrow where the length indicates magnitude and the angle the direction.
The Doppler Equation
The “2” represents the fact that there are actually two Doppler shifts during a clinical ultrasound exam:
- When the sound wave from the transducer strikes the moving blood cells
- When the reflected sound wave returns to the transducer.