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When the ultrasound waves strike the edge of a fluid‐filled structure with a curved surface (its wall), such as the stomach wall, urinary bladder, gallbladder, eye, or cyst, ultrasound waves are reflected to a small degree and these reflected sound waves therefore do not return to the transducer. As a result, a thin hypoechoic (darker) to anechoic (black) area lateral and distal to the edge of the curved structure is formed. For example, the novice may mistake this artifact for a “rent” in the urinary bladder wall when in fact it is an artifact created by the ultrasound machine (Nyland et al. 2002) (see Figure 3.2).

Figure 3.2. Edge shadowing artifact. (A) An edge shadowing artifact is seen arising from the curved edge on the left side of the stomach wall in this image, making its wall appear to extend distally as an anechoic (dark or black) line. A dirty gas shadow is also produced from gas within the stomach lumen that appears as "pseudo B‐lines." (B) An edge shadowing artifact at the apex of the urinary bladder makes it falsely appear to have a rent, which can fool the hasty sonographer into thinking the free fluid is from a ruptured bladder; however, note the smooth expected normal contour of the urinary bladder. In (C) the edge shadowing artifact is outlined with arrows (←). In (D) the edge shadowing creates a false rent in the wall of the urinary bladder (circled) and the superimposed curved line points out the edge shadowing from the curved surface of the urinary bladder wall extending through the far‐field.

Source: Courtesy of Dr Sarah Young, Echo Service for Pets, Ojai, California.