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When the ultrasound wave strikes bone (and stone), most of the waves are reflected back so there will be an area of intense hyperechogenicity (whiteness) at the soft tissue–bone (stone) interface. Because the surface of bone is often smooth, there is little scattering or reverberation of the ultrasound wave and a nice, clear‐cut, anechoic (blackness) “clean shadow” is produced beyond the reflector (bone or stone) (see Figure 3.1).

Figure 3.1. Dirty and clean shadowing. (A) "Dirty shadowing" created by air, a gas bubble within a fluid‐filled distended loop of small bowel. "Dirty shadowing" is generated because some ultrasound waves pass through the structure. Contrast the "dirty shadowing" with the "clean shadowing" of the cystourolith (urinary bladder stone) in (B) in which all ultrasound waves are reflected back to the transducer. (B) "Clean shadowing." The smooth surface of the cystourolith (urinary bladder stone) generates the "clean shadowing" typical of bone or stone with a hyperechoic (bright white) reflective surface in the near field, completely blocking all echoes and thus resulting in an anechoic (dark or black) shadow extending from it through the far‐field. In (C) and (D), the images have arrows (←) pointing out the artifact.

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