DOI: 10.1148/rg.242035130
RadioGraphics 2004;24:497-506
© RSNA, 2004
Slow Flow or No Flow? Color and Power Doppler US Pitfalls in the Abdomen and Pelvis1
Shannon C. Campbell, MD,
Jeanne A. Cullinan, MD and
Deborah J. Rubens, MD
1 From the Department of Radiology, University of Rochester Medical Center, 601 Elmwood Ave, Box 648, Rochester, NY 14642. Presented as an education exhibit at the 2001 RSNA scientific assembly. Received May 12, 2003; revision requested June 24 and received August 8; accepted August 14. Address correspondence to S.C.C. (e-mail: gadogrrl@hotmail.com).
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Abstract
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Color and power Doppler ultrasonography allow rapid assessment of tissue vascularity. Motion sensitivity to phenomena such as flow along the edge of gallstones, urinary jets, and placentation abnormalities can result in artifactual flow patterns that require corroboration with spectral analysis. Similar artifacts have also been noted at the edge of bone and along the bowel wall secondary to bowel motion. These artifacts fall into four categories: (a) twinkle artifacts, which occur behind reflectors; (b) edge artifacts, which occur along strongly reflective interfaces; (c) flash artifacts, which occur due to tissue or fluid motion; and (d) pseudoflow, which occurs due to nonvascular flow of other fluids such as urine or ascites. Knowledge of the most common sites of color and power Doppler artifacts, familiarity with the typical appearances of these artifacts, and liberal use of spectral analysis will help prevent misinterpretation of these findings and may lead to improved diagnostic accuracy.
© RSNA, 2004
Index Terms: Abdomen, US, **.12983,2 **.12984 • Pelvic organs, US, **.12983, **.12984 • Ultrasound (US), artifact, **.12983, **.12984 • Ultrasound (US), Doppler studies, **.12983, **.12984 • Ultrasound (US), power Doppler studies, **.12983, **.12984
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Introduction
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Color and power Doppler ultrasonography (US) with or without spectral analysis are common and invaluable tools in the assessment of various organ systems in multiple disease processes. Because these modalities are used to assess vascularity, it is tempting, when one encounters a positive Doppler signal, to ascribe the color in the image to blood flow. Clearly, such an assumption could lead to misinterpretation of a real Doppler signal from a source other than blood flow or of a Doppler-related artifact. A variety of Doppler artifacts have been documented in the literature. Although many of these artifacts are related to instrumentation settings (eg, aliasing) (1) or anatomic factors (mirror image artifacts) (2), there are several types of artifacts that are unrelated to vascular structures and blood flow (3). Among these artifacts are twinkle artifact, edge artifact, flash artifact, and pseudoflow.
In this article, we discuss and illustrate examples of color and power Doppler US findings that mimic vascular structures in the abdomen and pelvis. In many cases, spectral analysis was used to confirm the absence of true vascular flow; consequently, we focus on the importance of such analysis in identifying these US findings as artifacts and not pathologic entities.
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Twinkle Artifact
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Twinkle artifact occurs behind a strongly reflecting granular interface such as urinary tract stones or parenchymal calcification and appears as a rapidly fluctuating mixture of Doppler signals (red and blue pixels) that imitate turbulent flow (Figs 1–5). However, the Doppler spectrum is absolutely flat, a finding that is characteristic of noise. In a study of 32 patients by Lee et al (4), 86% of urinary calculi demonstrated this artifact. The authors suggested that the presence of twinkle artifact may aid in stone detection, especially if the stone lacks a strong echo or discrete shadowing.
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Figure 1a. Twinkle artifact behind a bladder calculus. (a) Longitudinal post-voiding US image of the bladder shows a hyperechoic, shadowing bladder calculus (arrow). (b) Transverse color Doppler US image shows twinkle artifact posterior to the stone. (c) Transverse power Doppler US image again shows the twinkle artifact. (d) Doppler spectrum shows noise.
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Figure 1b. Twinkle artifact behind a bladder calculus. (a) Longitudinal post-voiding US image of the bladder shows a hyperechoic, shadowing bladder calculus (arrow). (b) Transverse color Doppler US image shows twinkle artifact posterior to the stone. (c) Transverse power Doppler US image again shows the twinkle artifact. (d) Doppler spectrum shows noise.
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Figure 1c. Twinkle artifact behind a bladder calculus. (a) Longitudinal post-voiding US image of the bladder shows a hyperechoic, shadowing bladder calculus (arrow). (b) Transverse color Doppler US image shows twinkle artifact posterior to the stone. (c) Transverse power Doppler US image again shows the twinkle artifact. (d) Doppler spectrum shows noise.
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Figure 1d. Twinkle artifact behind a bladder calculus. (a) Longitudinal post-voiding US image of the bladder shows a hyperechoic, shadowing bladder calculus (arrow). (b) Transverse color Doppler US image shows twinkle artifact posterior to the stone. (c) Transverse power Doppler US image again shows the twinkle artifact. (d) Doppler spectrum shows noise.
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Figure 2a. Twinkle artifact behind atherosclerotic plaque. (a) Longitudinal US image of the right internal carotid artery shows two areas of atherosclerotic plaque (arrows). (b) Color Doppler US image of the same region shows typical twinkle artifact behind the plaque. (c) Doppler spectrum reveals noise in the region.
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Figure 2b. Twinkle artifact behind atherosclerotic plaque. (a) Longitudinal US image of the right internal carotid artery shows two areas of atherosclerotic plaque (arrows). (b) Color Doppler US image of the same region shows typical twinkle artifact behind the plaque. (c) Doppler spectrum reveals noise in the region.
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Figure 2c. Twinkle artifact behind atherosclerotic plaque. (a) Longitudinal US image of the right internal carotid artery shows two areas of atherosclerotic plaque (arrows). (b) Color Doppler US image of the same region shows typical twinkle artifact behind the plaque. (c) Doppler spectrum reveals noise in the region.
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Figure 3a. Twinkle artifact associated with adherent gallstones. (a) Transverse color Doppler US image of the gallbladder shows multiple areas of twinkle artifact associated with gallstones that have adhered to the mucosa. (b) Doppler spectrum of the artifact demonstrates noise but no flow.
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Figure 3b. Twinkle artifact associated with adherent gallstones. (a) Transverse color Doppler US image of the gallbladder shows multiple areas of twinkle artifact associated with gallstones that have adhered to the mucosa. (b) Doppler spectrum of the artifact demonstrates noise but no flow.
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Figure 4a. Twinkle artifact due to gallstones. (a) Decubitus US image shows small calculi (arrow) floating in a sludge-filled gallbladder. (b) Applied power Doppler US image shows twinkle artifact behind the calculi. (c) Doppler spectrum demonstrates no flow in the region of the artifact.
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Figure 4b. Twinkle artifact due to gallstones. (a) Decubitus US image shows small calculi (arrow) floating in a sludge-filled gallbladder. (b) Applied power Doppler US image shows twinkle artifact behind the calculi. (c) Doppler spectrum demonstrates no flow in the region of the artifact.
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Figure 4c. Twinkle artifact due to gallstones. (a) Decubitus US image shows small calculi (arrow) floating in a sludge-filled gallbladder. (b) Applied power Doppler US image shows twinkle artifact behind the calculi. (c) Doppler spectrum demonstrates no flow in the region of the artifact.
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Figure 5a. Twinkle artifact owing to renal calculi. (a) Longitudinal gray-scale US image shows a faintly echogenic region in the lower pole sinus but no definite renal stones. (b) Color Doppler US image depicts three calculi with twinkle artifact (arrows).
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Figure 5b. Twinkle artifact owing to renal calculi. (a) Longitudinal gray-scale US image shows a faintly echogenic region in the lower pole sinus but no definite renal stones. (b) Color Doppler US image depicts three calculi with twinkle artifact (arrows).
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Edge Artifact
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Edge artifact is related to strong specular reflectors and appears as steady color along the rim of calcified structures such as gallstones or cortical bone (Figs 6, 7). Again, the Doppler spectrum demonstrates a straight-line noise pattern with no pulsatility. Edge artifact is more commonly seen with power Doppler US than with color Doppler US (5).
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Figure 6a. Edge artifact caused by bladder encrustation. (a) Transverse US image of the bladder (midline view) shows extensive bladder wall calcification (arrows). (b) Power Doppler US image demonstrates signal along the edges of the calcification. (c) Doppler spectrum fails to show any evidence of flow in the affected region.
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Figure 6b. Edge artifact caused by bladder encrustation. (a) Transverse US image of the bladder (midline view) shows extensive bladder wall calcification (arrows). (b) Power Doppler US image demonstrates signal along the edges of the calcification. (c) Doppler spectrum fails to show any evidence of flow in the affected region.
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Figure 6c. Edge artifact caused by bladder encrustation. (a) Transverse US image of the bladder (midline view) shows extensive bladder wall calcification (arrows). (b) Power Doppler US image demonstrates signal along the edges of the calcification. (c) Doppler spectrum fails to show any evidence of flow in the affected region.
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Figure 7a. Edge artifact in a large gallstone. (a) Longitudinal US image of the gallbladder shows a large, shadowing gallstone. (b) Applied power Doppler US image and Doppler spectrum show a rim of signal along the edge of the gallstone with no evidence of flow.
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Figure 7b. Edge artifact in a large gallstone. (a) Longitudinal US image of the gallbladder shows a large, shadowing gallstone. (b) Applied power Doppler US image and Doppler spectrum show a rim of signal along the edge of the gallstone with no evidence of flow.
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Although both twinkle and edge artifacts occur in association with echogenic interfaces such as bone and other calcium deposits, the physics of the cause of these artifacts has yet to be fully elucidated. With further study, these two artifacts may prove to be different manifestations of the same interaction between the Doppler signal and an echogenic material or interface.
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Flash Artifact
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Flash artifact manifests as a color signal caused by tissue motion. This motion may originate within any structure but is most commonly seen in hypoechoic areas such as cysts or fluid collections (6). Flash artifact occurs due to a machine setting that suppresses color pixels that would otherwise overwrite gray-scale echoes (the "color-write" priority). In the absence of gray-scale echoes, color pixels take priority, so that a "flow" is seen in nearly stationary fluid. The transitory appearance of flash artifact may also coincide with transducer or patient motion (most often respiration or cardiac pulsation) and therefore is usually easily interpreted (Figs 8–10).
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Figure 8a. Flash artifact due to motion of the fetal head. (a) US image of the cervix (C) and fetal head (H) obtained with fundal pressure being applied (left) shows a flash of color across the internal os simulating a vasa previa. On a US image obtained with no fundal pressure being applied (right), no color is demonstrated. (b) Doppler spectrum demonstrates the normal arterial flow pattern in a true vasa previa (compare with spectral noise in Figs 1d and 2c).
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Figure 8b. Flash artifact due to motion of the fetal head. (a) US image of the cervix (C) and fetal head (H) obtained with fundal pressure being applied (left) shows a flash of color across the internal os simulating a vasa previa. On a US image obtained with no fundal pressure being applied (right), no color is demonstrated. (b) Doppler spectrum demonstrates the normal arterial flow pattern in a true vasa previa (compare with spectral noise in Figs 1d and 2c).
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Figure 9a. Flash artifact due to fluid motion. (a) Longitudinal US image shows an enlarged, twisted right ovary. (b) Color Doppler US image shows flash artifact in moving fluid adjacent to the ovary, not in a peripheral vessel.
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Figure 9b. Flash artifact due to fluid motion. (a) Longitudinal US image shows an enlarged, twisted right ovary. (b) Color Doppler US image shows flash artifact in moving fluid adjacent to the ovary, not in a peripheral vessel.
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Figure 10a. Flash artifact in a patient with polycystic kidney disease. (a) Longitudinal US image of the superior pole of the right kidney shows a large, complex renal cyst (C). (b) Color Doppler US image shows multiple areas of color in the cyst that represent flash artifact. Note also the normal vessels in the adjacent liver. (c) Doppler spectrum of the vessel-like color pattern in the cyst fails to demonstrate a true vascular flow pattern.
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Figure 10b. Flash artifact in a patient with polycystic kidney disease. (a) Longitudinal US image of the superior pole of the right kidney shows a large, complex renal cyst (C). (b) Color Doppler US image shows multiple areas of color in the cyst that represent flash artifact. Note also the normal vessels in the adjacent liver. (c) Doppler spectrum of the vessel-like color pattern in the cyst fails to demonstrate a true vascular flow pattern.
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Figure 10c. Flash artifact in a patient with polycystic kidney disease. (a) Longitudinal US image of the superior pole of the right kidney shows a large, complex renal cyst (C). (b) Color Doppler US image shows multiple areas of color in the cyst that represent flash artifact. Note also the normal vessels in the adjacent liver. (c) Doppler spectrum of the vessel-like color pattern in the cyst fails to demonstrate a true vascular flow pattern.
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Pseudoflow
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Pseudoflow is also related to motion, but to that of fluid rather than of blood within a vessel. Pseudoflow is closely related to flash artifact and appears similar to real blood flow at color or power Doppler US, but in the absence of a vascular structure. The color or power Doppler signal will appear as long as the fluid motion continues. Pseudoflow may be caused by the motion of ascites or urine (Figs 11–14). Spectral analysis reveals flow that is atypical for a normal vessel.
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Figure 11a. Pseudoflow due to flowing urine within a bladder diverticulum. (a) Transverse US image demonstrates a bladder diverticulum (D). (b) Color Doppler US image shows pseudoflow secondary to flowing urine within the diverticulum. (c) Doppler spectrum shows a pattern that is inconsistent with vascular flow.
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Figure 11b. Pseudoflow due to flowing urine within a bladder diverticulum. (a) Transverse US image demonstrates a bladder diverticulum (D). (b) Color Doppler US image shows pseudoflow secondary to flowing urine within the diverticulum. (c) Doppler spectrum shows a pattern that is inconsistent with vascular flow.
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Figure 11c. Pseudoflow due to flowing urine within a bladder diverticulum. (a) Transverse US image demonstrates a bladder diverticulum (D). (b) Color Doppler US image shows pseudoflow secondary to flowing urine within the diverticulum. (c) Doppler spectrum shows a pattern that is inconsistent with vascular flow.
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Figure 12a. Pseudoflow due to ascitic motion adjacent to the ligamentum teres. (a) Color Doppler US image shows a simulated vessel coursing along the falciform ligament. (b) Doppler spectrum fails to document any flow.
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Figure 12b. Pseudoflow due to ascitic motion adjacent to the ligamentum teres. (a) Color Doppler US image shows a simulated vessel coursing along the falciform ligament. (b) Doppler spectrum fails to document any flow.
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Figure 13a. Pseudoflow due to ascitic motion in a patient with cirrhosis. (a) Color Doppler US image shows pseudoflow within ascites. (b) Doppler spectrum fails to show any evidence of normal blood flow. (c) Doppler spectrum demonstrates a unidirectional venous flow pattern (arrows) in the adjacent true collateral vessel.
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Figure 13b. Pseudoflow due to ascitic motion in a patient with cirrhosis. (a) Color Doppler US image shows pseudoflow within ascites. (b) Doppler spectrum fails to show any evidence of normal blood flow. (c) Doppler spectrum demonstrates a unidirectional venous flow pattern (arrows) in the adjacent true collateral vessel.
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Figure 13c. Pseudoflow due to ascitic motion in a patient with cirrhosis. (a) Color Doppler US image shows pseudoflow within ascites. (b) Doppler spectrum fails to show any evidence of normal blood flow. (c) Doppler spectrum demonstrates a unidirectional venous flow pattern (arrows) in the adjacent true collateral vessel.
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Figure 14. Pseudoflow caused by bladder jets. Transverse US image of the bladder (midline view) shows a jet of urine flowing from the left ureter, a finding that indicates that the ureter is not obstructed.
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Discussion
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There are many situations in which Doppler artifacts typically occur. Whether the question is medical or surgical or the problem is acute or chronic, the image patterns must be critically analyzed to determine the veracity of the Doppler signal and the possible contribution of artifact. To do otherwise is to risk introducing disease where none exists. Had the artifacts illustrated in this article not been identified, the findings could have been misinterpreted, leading to further (unnecessary) diagnostic tests or interventions.
In many cases, a typical location and appearance for an artifact is sufficient to allow differentiation from true disease. However, if any question persists or documentation is needed, evaluation of the spectral pattern becomes increasingly important. In none of our cases does spectral analysis show a pattern that could be interpreted as blood flow. Several of these artifacts produce a signal that clearly represents spectral noise, whereas in other artifacts, spectral analysis may demonstrate findings that appear as atypical flow. In both situations, failure to identify an expected vascular flow pattern is the key. In contrast, disease with true vascularity manifests with spectral findings that are easily recognizable as blood flow (Figs 8b, 13c).
Although the focus of this article has been identification of artifact, not all nonvascular Doppler signals are ineffectual in diagnosis. It has been hypothesized that, like shadowing or through transmission, the presence of twinkle artifact could be used to help identify disease entities such as kidney stones (5). Our experience has paralleled that of other authors, with improved detection and confidence in diagnosis of renal calculi, gallbladder calculi, and wall calcification (5). Although US artifact caused by bladder jets falls under the heading of pseudoflow, it also represents a type of nonvascular flow that is diagnostically helpful (Fig 14).
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Conclusions
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There are many sites in which color and power Doppler artifacts commonly occur. Knowledge of these sites, familiarity with the typical appearances of these artifacts, and liberal use of spectral analysis will help prevent misinterpretation of these artifactual pitfalls and may lead to improved diagnostic accuracy.
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Footnotes
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2**.multiple body systems 
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References
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- Taylor KJ, Holland S. Doppler US. I. Basic principles, instrumentation, and pitfalls. Radiology 1990; 174:297-307.
- Mitchell DG. Color Doppler imaging: principles, limitations, and artifacts. Radiology 1990; 177:1-10.[Free Full Text]
- Keogh CF, Cooperberg PL. Is it real or is it an artifact? Ultrasound Q 2001; 17:201-210.[Medline]
- Lee JY, Kim SH, Cho JY, Han D. Color and power Doppler twinkling artifacts from urinary stones: clinical observations and phantom studies. AJR Am J Roentgenol 2001; 176:1441-1445.[Abstract/Free Full Text]
- Rahmouni A, Bargoin R, Herment A, Bargoin N, Vasile N. Color Doppler twinkling artifact in hyperechoic regions. Radiology 1996; 199:269-271.[Abstract/Free Full Text]
- Pozniak MA, Zagzebski JA, Scanlan KA. Spectral and color Doppler artifacts. RadioGraphics 1992; 12:35-44.[Abstract]
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Abstract
Introduction
