Small hyperechoic foci within the liver can be further characterized with the color comet-tail artifact. The artifact produced can be used to differentiate adenomyomatosis from isolated echogenic polyps on the nondependent wall of the gallbladder, especially when the typical gray-scale comet-tail artifact is absent or difficult to visualize (Figs. The color comet-tail artifact can be useful in the evaluation of adenomyomatosis. The artifact is also helpful in imaging of patients with cystic fibrosis in whom intrahepatic biliary stones are suspected clinically but there are few or no gray-scale findings (Figs. The presence of the color comet-tail artifact is sometimes critical to identifying such stones (Figs. The identification of stones is even more challenging in mildly dilated or normal-size common bile ducts. It is often difficult to identify stones within the common bile duct, even when the duct is dilated. The diagnosis of common bile duct stones is more difficult than that of gallstones. Although the color comet-tail artifact is not important diagnostically, it is important to recognize this sign to avoid confusion with flow in a gallbladder mass (Figs.
Biliary sludge also commonly engenders a prominent color comet-tail artifact. Gallbladder stones are readily diagnosed with gray-scale imaging, so the color comettail artifact plays little or no diagnostic role (Figs. Liver And Biliary Tree: Cholelithiasis And Choledocholithiasis We do not understand why the artifact is absent or poorly seen in some cases, even when the object (calcification, stone, or surgical clip) is clearly visualized with gray-scale imaging alone. This effect may be related to the roughness of the stone surface. For example, we found that the artifact is more reproducible in some patients with gallstones than in others, even when the color Doppler settings are the same. In our laboratory, we attempted to reproduce the color comet-tail artifact in different patients with similar imaging findings. This effect likely occurs because the artifactual frequency shift of the color comet-tail artifact is much higher than that of real blood flow (Figs. We are not sure whether use of the high setting actually enhances the artifact, but it certainly makes visualization easier because the color signal from flow in nearby vascular structures is decreased or eliminated, leaving little but the color comet-tail artifact in the field of view. Use of the highest levels of color scale available on the sonography machine (i.e., increased filter and pulse repetition frequency) frequently improves visualization of the color comet-tail artifact. When we used a lower-frequency curvilinear probe, a clear color comet-tail artifact did appear (Figs. For example, we scanned an obvious calcified carotid plaque with a 14-MHz probe, and despite attempts using different parameters, we could not produce the artifact. We do not understand why this effect occurs. Lower color Doppler frequency seems to produce a more prominent color comet-tail artifact than does a higher frequency. Sometimes the artifact is enhanced by an increase in color gain and moving the transmit zones deep in relation to the object causing the artifact. Certain scanning parameters appear to accentuate production of and enhance the visibility of the color comet-tail artifact in clinical imaging. In any case, the nature and origin of the artifact are poorly understood. They suggested that the artifact depends on machine settings, color-write priority, pulse repetition frequency, and gray-scale gain.ĭiscussion of the physics of the color comet-tail artifact is beyond the scope of this article. suggested that factors other than merely the roughness of the surface of the object imaged influence production of the artifact. described the artifact as the “twinkling sign,” which resulted from reflections from rough surfaces. The artifact appears on images as a linear aliased band of color. The color comet-tail artifact is a rapidly alternating color Doppler signal that occurs immediately deep in relation to the object causing it.