Reverberation artifact occurs when an ultrasound beam encounters two strong parallel reflectors.
When the ultrasound beam reflects back and forth between the reflectors ("reverberates"), the ultrasound transducer interprets the sound waves returning as deeper structures since it took longer for the wave to return to the transducer.
Reverberation artifacts can be improved by changing the angle of insonation so that reverberation between strong parallel reflectors cannot occur.
Comet-tail artifact is a specific type of reverberation artifact. This results a short train of reverberations from an echogenic focus which has strong parallel reflectors within it (e.g. cholesterol crystals in adenomyomatosis). With comet tail artifact, the space between the two strong parallel reflectors may be less than 1/2 the space pulse length, causing the echoes to be displayed as triangular lines (the later echoes get attenuated and have a decreased amplitude, manifesting on the display as decreased width).
Although sometimes grouped with reverberation due to similar appearances, ring down artifact does not arise from the same mechanism and should be considered separately .