Fluid-filled structures may also have artifactual echoes along the dependent wall that originate from off-axis portions of the ultrasound beam. Comet tail artifact is a feature of adenomyomatosis of the gallbladder and emanates from cholesterol crystals trapped in dilated Rokitansky-Aschoff sinuses in the gallbladder wall ( Fig.
Reverberation also accounts for comet tail artifact, a streak of tiny, bright, parallel bands tapering behind a punctate crystalline reflector, generated by sound echoing within the crystal itself. These echoes represent reverberation artifact, which occurs as the sound is repeatedly reflected between the highly echogenic anterior gallbladder wall and the transducer. Within fluid-filled structures such as the gallbladder, faint echoes are often seen paralleling the anterior surface. Although technically an artifact, acoustic enhancement is useful to characterize structures as cystic if they also lack internal flow on color Doppler.įIGURE 13.3 Gallstones ( curved arrow) layering in the gallbladder produce an acoustic shadow ( straight arrows). Such artificially bright areas beyond the fluid represent increased through transmission, or acoustic enhancement ( Fig. TGC results in needless amplification of echoes from areas deep to pure fluid structures, because the acoustic pulse is attenuated to a lesser extent by fluid than by tissue. This is called time-gain compensation (TGC) or depth-gain compensation. The ultrasound scanner normally compensates for the attenuation of sound that occurs with increasing depth within the tissue by amplifying echoes that return later from the far field. Spaces that contain fluid do not contain internal interfaces, so no sound is reflected from within them they appear uniformly black or anechoic.
Lesions that appear dark relative to the background tissue are termed hypoechoic those that are brighter than background are hyperechoic or echogenic ( Fig. Gray-Scale Ultrasound Terminology and Artifacts