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Reverberation occurs when sound encounters two highly reflective layers. The sound is bounced back and forth between the two layers before traveling back to the receiver. The probe will detect a prolonged traveling time and assume a longer traveling distance and display additional reverberated images in a deeper tissue layer. The reverberations can get caught in an endless loop and extend all the way to the bottom of the screen as parallel equidistant lines, referred to as A‐lines (also see Chapters 22 and 23). This artifact most commonly extends beyond air‐filled structures within the thorax, (e.g., lung) and within the abdomen (e.g., gastrointestinal tract), with varying width (Penninck 2002) (Figure 3.5; see also Figures 3.1 and 3.2).

Figure 3.5. Reverberation artifact. (A) Reverberation artifact, also known as A‐lines (think of it as "A" for air). A‐lines are seen as regularly spaced parallel lines illustrated by the arrows (←) outlined in (C). The most proximal arrow in the near‐field denotes the lung's surface, evident within the intercostal space between two ribs on either side (ribs [bone] creating the "clean shadowing" through the far‐field). (B) The very tight and distinct band of reverberation artifact, referred to as a comet‐tail or ring‐down artifact, caused by sound waves reflecting off a metal needle used during abdominocentesis in the near‐field. (D) The same image but with an arrow overlay to show the tight band of A‐lines as the comet‐tail or ring‐down artifact. Any strong reflector of ultrasound waves produces this artifact that typically involves bone, stone, or metal, such as implants, needles, and foreign bodies.

Source: Courtesy of Robert M. Fulton, DVM, Richmond, VA.