In this article we will describe the following Doppler artifacts and how to avoid them: aliasing, blooming artifact, directional ambiguity, partial volume artifact, pseudoflow, flash artifact, mirror-image artifact, edge artifact, range ambiguity and twinkling artifact.

Aliasing

Aliasing is an incorrect display of velocities occurring when the velocity range exceeds the scale available to display it. It appears as reversed flow in high velocity areas, with a color change that wraps around from one end of the color scale to the other, without black/no flow area in between. In the Doppler spectrum, high frequencies (“peaks”) are cut off from the top and displayed below the baseline, at the bottom aimed upwards. The velocity scale is limited by the pulse repetition frequency (PRF) - number of pulses per second that can be transmitted and received by the transducer. Aliasing arises when the Doppler shift of the moving blood is greater than the Nyquist frequency (Nyquist frequency = pulse repetition frequency (PRF)/2). Aliasing is useful because it enables quick identification of high velocity flow in stenosed vessels or arteriovenous fistulas.

How to avoid:

1. Increase the PRF (careful – this reduces scan depth)
2. Shift the baseline lower – Doppler spectrum only
3. Choose a shallower sample volume
4. Increase the doppler angle (careful – this decreases the accuracy of velocity measurement)
5. Lower the transducer frequency (careful – this reduces spatial resolution)

Blooming artifact

Blooming artifact is a color Doppler phenomena appearing as color „overflowing“ beyond the vessel margins, making the vessels appear larger and obstructing the view of the vessel wall. It is a consequence of gain set too high.

How to avoid:

1. Lower the gain

Directional ambiguity

Directional ambiguity is a Doppler spectrum artifact presenting as symmetrical Doppler spectrum waveform above and below the baseline, as a mirror image. It is caused by insonating the vessel at an angle of ninety degrees and most commonly occurs in small vessels.

How to avoid:

1. Decrease the insonation angle

Partial volume artifact

Partial volume artifact presents as visualization of a vessel and blood flow within a structure that does not contain that vessel. It is the result of a slice thickness that is not infinitely thin. As a result, a vessel adjacent to the visualized structure appears as a part of it. For example, part of an iliac artery adjacent to the ovary mimicking flow within an ovarian cyst.

How to avoid:

1. Visualize the area in question in a plane 90 degrees perpendicular to the current plane

Pseudoflow

Pseudoflow is an artifact caused by movement of fluid that is not blood. It mimics blood flow on color and power Doppler. Pseudoflow artifact can present in ascites, amniotic fluid and urine. It is useful because it allows visualization of bladder jets.

How to distinguish from true flow:

1. Doppler spectrum displays no true flow

Flash artifact

Flash artifact occurs as a result of motion of the patient or the transducer and presents as a sudden flash of color filling the color box and obstructing the grayscale image. It can be useful as „color bruit“ that presents as a color mosaic in tissue adjacent to a vessel with turbulent flow. “Color bruit” is caused by vascular tissue vibration and can point to a severe stenosis, arteriovenous fistula or a shunt.  This artifact can also be seen in spectral Doppler as a small signal, symmetric above and below the baseline, most prominent during systole.

How to avoid:

1. Don't move.

Mirror-image artifact

Mirror-image artifact is a phantom image of the insonated vessel located deep to the actual vessel. This artifact occurs when a vessel is adjacent to a strong reflector such as the diaphragm, pleural surface, aortic wall, bone. The ultrasound pulse travels from the transducer to the reflector, deflects to the target vessel, then from the vessel back to the reflector and to the transducer. The longer time it took for the pulse to come back to the transducer is interpreted as a deeper location of the structure. In Doppler ultrasound, the most common location of mirror-image artifact is in the supraclavicular region when displaying the subclavian vessels adjacent to the highly reflective pleura. Doppler spectrum in both “vessels” is identical. It is important to emphasize that the phantom vessel is always displayed deeper to the actual vessel in the field of view.

How to avoid:

1. Try to visualize the vessel from a different angle
2. Decrease gain
3. Increase gain

Edge artifact

Edge artifact presents as color and power Doppler signal at the edge of specular reflectors such as calcified structures – gallstones or cortical bone; or manmade structures – Foley balloon. The Doppler spectrum is equal and symmetric above and below the baseline representing noise.

Range ambiguity

Range ambiguity is a consequence of high PRF (pulse repetition frequency). This artifact occurs when the second signal is sent before the echo from the first one returns to the probe. The ultrasound machine interprets the echo from the first signal as an early echo from the second signal and places it superficially. Thus, in color Doppler, a deep vessel can be displayed as shallower than it is.

How to avoid:

1. Lower the PRF

Twinkling artifact

Twinkling artifact is a very useful artifact presenting as a mosaic of rapidly changing red and blue color located deep to a strong reflector with color Doppler. With power Doppler, a uniform signal is displayed in the same location. The Doppler spectrum is equal and symmetric above and below the baseline representing noise, not flow. Common reflectors producing twinkling artifact are renal and bladder calculi, cholesterol crystals in the gallbladder wall and gas. Small calculi that don’t cast an acoustic shadow may produce a twinkling artifact, helping with their recognition. This occurs in 83% of calculi.