Aliasing occurs with pulsed ultrasound and is most commonly encountered with color Doppler or pulse wave spectral Doppler. With pulsed ultrasound, there is an upper limit of the Doppler shift which can be displayed. This is known as the Nyquist limit and is defined as the pulse repitition frequency/2. With high velocity blood flow generating Doppler shifts above the Nyquist limit, aliasing occurs and is displayed as bright, turbulent appearing flow in color Doppler and in blood flow profiles which "wrap around" the displayed scale in pulse wave spectral Doppler (see panel A and B below).
Means of Eliminating Aliasing
1. Image At a Shallower Depth: Notice that the Nyquist limit is dependent on the PRF which in turn is the reciprocal of the pulse repitition period (PRP). If one images at a shallower depth, the listening time decreases and therefore the PRP decreases. A decrease in the PRP therefore leads to an increase in the PRF and therefore the Nyquist limit.
2. Increase Angle of Insonification: The Doppler shift is dependent on the cosine of the angle between the ultrasound beam and the direction of motion. When the beam and the object are parallel, the cosine of 0 degree is 1. However, if this shift results in aliasing, increasing the angle between the beam and the direction of motion of blood flow will decrease the Doppler shift (for example cosine of 30 degree is 0.8). This may eliminate aliasing and allow display of the flow, at the expense of underestimating the true velocity.
3. Use Low Frequency Transducer: Recall that the Doppler shift frequency is directly related to the fundamental frequency emitted by the transducer. Therefore, using a lower frequency transducer will generate a smaller Doppler shift for any given blood flow velocity and may eliminate aliasing.
4. Use Continuous Wave Imaging: With CW, there is one element always emitting ultrasound and another element always listening. Therefore, there is no "listening time" and no limitation on the Doppler shift which can be measured and displayed. This is why CW is used to measure all high velocity jets in echocardiography. This advantage of CW imaging unfortunately comes at the expense of range resolution.
5. Change the Scale: On rare occasion, the scale for color or spectral Doppler is set inappapropriately low such that the color flow or spectral Doppler aliases even at a velocity which one would not expect it to. This is usually remedied by increasing the color scale. However, one can only increase the Doppler scale a relatvely remall amount.
6. Change the Display Baseline: This is not a true means of reducing aliasing because in both color and spectral Doppler the baseline can only be shifted so much and you are really only changing the how Doppler shift detected is displayed rather than changing the properties of the Doppler shift itself.