Doppler & Color Artifacts

Doppler Artifacts: Same Assumptions, New Display

Doppler and color-flow imaging inherit every propagation and beam-geometry artifact already covered for gray-scale imaging — reverberation, mirror image, refraction, side and grating lobes, and beam-width or slice-thickness effects can all distort a Doppler or color display exactly as they distort a B-mode image. On top of that, Doppler adds artifacts unique to sampling and displaying frequency shift.

Aliasing

Aliasing is the signature Doppler artifact (its mechanism and fixes are detailed in the pulsed-wave Doppler chapter): a pulsed system can unambiguously measure a Doppler shift only up to the Nyquist limit, PRF/2. When the true shift exceeds that limit, the pulsed system under-samples the true waveform and displays it as an impossibly high shift in the opposite direction — the spectral tracing wraps from the top of the display to below the baseline, or the color display wraps through white into an aliased color, from red-orange to blue-cyan or vice versa. It violates the sampling assumption that the Doppler shift is sampled at least twice per cycle. Continuous-wave Doppler cannot alias, because it transmits and receives continuously rather than at discrete pulse intervals — the trade-off is that CW has no range resolution.

Mirror-Image (Doppler) Artifact

The same strong-reflector mechanism that duplicates a gray-scale structure can duplicate a spectral or color Doppler signal across the zero-flow baseline, or produce a mirrored vessel signal beyond a strong reflector such as the diaphragm. It is often worsened by excessive Doppler gain or an under-aggressive wall filter, which lets a duplicate, weaker signal cross the baseline and display as apparent flow in the opposite direction.

Twinkling Artifact

Twinkling is a rapidly alternating, mosaic pattern of red and blue color signal appearing posterior to a stationary, rough, strongly reflective surface — most classically a renal calculus — even though there is no true blood flow present. The exact mechanism is still debated; leading explanations include intrinsic machine phase jitter amplified by the rough reflecting surface, and micro-reflectors such as trapped surface bubbles on the stone. The practical value is high: twinkling can flag a stone that produces little or no clean acoustic shadowing, aiding detection.

Blooming Artifact

Blooming occurs when color or power Doppler signal overflows beyond the true anatomic boundary of a vessel into the surrounding tissue. It results from excessive color gain or excessive transmit power, and it causes the vessel to appear wider than it truly is, which can overestimate the degree of stenosis or mimic turbulent flow where none exists. Reducing color gain to just below the point where random background noise appears corrects it.

Flash Artifact

Flash artifact is a sudden, transient burst of color filling large areas of the field of view, unrelated to true blood flow. It results from bulk tissue motion — transducer pressure, patient movement, respiration, or cardiac pulsation — being misread by the system as a Doppler shift. The wall (high-pass) filter, designed to reject slow-moving tissue signals, can be overwhelmed by large or rapid motion, letting the flash through. Flash is common in abdominal and cardiac scanning and is usually recognized because it fills tissue as well as vessels simultaneously.

Partial-Volume (Color) Artifact

Just as slice thickness produces spurious gray-scale echoes, the finite elevational width of the color box can pick up flow from a vessel that lies adjacent to, but not truly within, the structure being interrogated — displaying apparent flow signal inside a structure, such as a cyst wall, that contains no vessel of its own. The fix mirrors the gray-scale fix: narrow the region of interest and use the elevational focus where available.

Spectral Broadening Artifact

True spectral broadening — filling-in of the normally clear window beneath a spectral waveform — is a genuine finding of turbulent, disturbed flow, as at a stenosis. But broadening can also appear as a technical artifact with entirely laminar flow, most often because the sample-volume gate is set too large, spanning too much of the vessel's diameter and averaging the fast center-stream velocities with the slower near-wall velocities into one trace. Excessive Doppler gain and normal intrinsic spectral broadening — from the finite transit time particles spend within the beam and the inherent range of angles across a focused beam — also widen the trace without true turbulence. Reducing gate size, keeping the gate centered in the vessel, and using appropriate gain settings distinguish artifactual from true spectral broadening.

Exam tip: true spectral broadening signals pathologic turbulence; artifactual broadening signals a technique problem — gate size or gain. Twinkling and clean acoustic shadowing are complementary clues to the same finding, a calculus: twinkling can be positive even when clean shadowing is absent.