|ECR 2013 / C-2288|
|Ring-Down versus Comet Tail: Two artifacts uncovered|
Imaging findings OR Procedure details
COMET TAIL ARTIFACT
We can consider this phenomenon as a form of reverberation:
We assume that sounds travels in a straight line, at constant speed. Therefore, we can determinate the origin of an echo that goes back if we quantify the time it takes to return. In the presence of two reflective interfaces, the echoes generated from the main beam may be repeatedly reflected, in repeated trips before go back to the transducer, where they may be detected.
Each echo is received erroneously transcribed as a band located at a greater depth, so we may see multiple parallel lines of decreasing intensity and equidistant from each other (Fig. 2).
In this artifact, the two reflective interfaces, and therefore the generated echoes are closely spaced. In the image, sequential echoes can be so close together that the individual signals cannot be seen.
In addition, delayed echoes have lower amplitude, secondary to attenuation process; this decrease in amplitude is shown as a width of echoes increasingly diminished. The result is an artifact caused by the principle of reverberation, but with a conical or triangular shape.
This phenomenon occurs when the beam comes into contact with metal objects (Fig. 3 ) such as metal clips of suture, needles, foreign bodies… or calcification (granulomas, cholecystolithiasis (Fig. 4), calcified myomas, nephrolithiasis…). A classic example is cholesterolosis of the gallbladder wall (Fig. 5 ).
In the past, it was thought this artifact was a variant of the previous. This statement was based in the similar appearance both artifact exhibit.
However, it is known that Ring-Down appears mostly due to gas (rather than metal, as in the case of comet tail). We need multiple gas bubbles to produce it. When the ultrasound beam reaches the gas bubbles, it is capable of exciting the liquid trapped between the bubbles, which causes the liquid resonates. These vibrations create a continuous sound wave which is transmitted back to the receiver. This phenomenon is shown as a line or series of bands extending parallel after the image corresponding to gas (Fig. 6 ).
Some examples we can found in our daily practice are abundant gas in the colon structure ( Fig. 7 ) or those produced after surgery, such as aerobilia after cholecystectomy or hepaticojejunostomy (Fig. 8), air bubbles after injections (Fig. 9) or in pathological processes for example in Fournier gangrene, where the display of gas adjacent to the scrotum is charasteristic of the disease (Fig. 10).
Considering air is the cause of this artifact, seen as “dirty shadows” in the image, it is possible to think this phenomenon is not useful, because you can hide deeper structures. Nothing is further from the truth.
If you find air in the thickness of the gallbladder wall, or in the context of pyelonephritis, this artifact can help us to diagnose emphysematous cholecystitis (Fig. 12) or pyelonephritis. Likewise, if we visualize the Ring-Down in the thickness of a fluid collection we probably are in the face of an abscess (Fig. 11) , or if we see that image in peritoneal cavity is probably we are in front of a perforated hollow viscera.
Thematically related posters
ECR 2013 / C-0849
LPAC syndrome (low phospholipid-associated cholelithiasis) : Spectrum of Imaging features