Doppler Shift: High vs. Low

The Doppler shift is a change in frequency and wavelength caused by the motion of objects. This Doppler effect is provided by the Doppler equation that deals with three situations: sound source, moving reciever, or moving reflector.

Doppler shift is equal to the recieved frequency (i.e. reflector) minus the source frequency (i.e. transducer/sound source). A difference is required between these two frequencies in order for doppler shift to occur. If the recieved frequency is greater than the transmitted, then a positive or high doppler shift occurs. In a high doppler shift, the reflected echo is going towards the sound source (sound and blood moving in opposite direction)thus an "upstream" condition is seen. If, however, the transmitted frequency is greater than the recieved, then a negative or low doppler shift occurs. In a low doppler shift, the reflected echo is seen going away from the transducer (sound and blood moving in same direction) thus a "downstream" condition is seen. Therefore, the recieved and transmitted frequencies must not be equal in order for positive or negative doppler shift to occur. An example of no doppler shift can be seen in a vessel that is completely occluded, in which the recieved and transmitted frequencies are equal thus no flow occurs.

Doppler shift is also equal to the velocity, cosine angle, and operator frequency. If the angle indicator is increased, then cosine angle decreases and depending upon the angle, doppler shift is decreased by half or one. If the angle is at 0, then cosine angle increases by 1, thus doppler shift is increased. The angle to avoid is 90 degree or right angle, which sets the cosine angle at 0 thereby causing no doppler shift. That is why we steer the box to avoid the 90 degree angle especially when taking waveforms during vascular studies. During the vascular studies, a 45-60 degree angle is required for doppler shift to occur. An angle of 60 will yield a cosine angle of 0.5 thereby causing the doppler shift to increase by half.

In relation to the operator frequency, if for example, a 2.5Mhz transducer is switched to 5Mhz, then a high doppler shift occurs (and vice-versa). If the velocity of blood is increased (as seen in stenosis), then a high doppler shift occurs as well (and vice-versa). Therefore, in high doppler shift, the velocity of blood is usually fast and will exhibit a bright color, whereas in low doppler shift the velocity of blood is slow and will exhibit a dark color.