 Ringdown artifact is the R in the stream and ringdown artifact is a type of reverberation artifact. This occurs due to a ring of bubbles with fluid trapped centrally. Because of sound that is coming from this pocket of fluid there's a continuous source of sound energy that comes and goes back to the transducer for detection. This is seen as a very bright line and a dirty shadow almost that we call representing air from the bowel but it is because of that fluid that is trapped in between the air bubbles.