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When we image a structure that is close to a curved, strong reflector such as the diaphragm (remember this is actually the lung–air interface following the curve of the diaphragm), a sound beam can reflect off the curved surface, strike adjacent tissues, reflect back to the curved surface, and then reflect back to the transducer. Because the processor only uses the time it takes for the beam to return home and cannot “see” the ongoing reflections, it will be fooled into placing (mirroring) the image on the far side of the curved surface. The classic place for a mirror artifact is at the diaphragm, and the classic mistake is interpreting the artifact as a diaphragmatic hernia (Penninck 2002) (Figure 3.4).

Figure 3.4. Mirror artifact. The gallbladder appearing to be on both sides of the diaphragm is the classic example of mirror artifact, created by a strong soft tissue–air interface. The mirror image artifact also may be generated under similar circumstances when the fluid‐filled urinary bladder lies against the air‐filled colon. (A) The white arrows illustrate the actual path of the sound beam reflecting off the curved lung–air interface against the diaphragm, while the black arrows illustrate the path perceived by the ultrasound processor. Note that the gallbladder falsely appears as if it is within the thorax, and should not be mistaken for a diaphragmatic hernia. (B) Mirror image artifact in which it appears that the liver and gallbladder are on both sides of the diaphragm (*). (C) Mirror image artifact in which it appears that liver (gallbladder not present in this view) is on both sides of the diaphragm (*).

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