Ultrasonic refraction by the rectus abdominis muscles: the double image artifact.

Using a real-time mechanical sector scanner in a transverse plane of section, the authors noticed that a double image of an intrauterine gestational sac or an intrauterine contraceptive device can be obtained when the transducer is passed over the midline of the abdomen. It was suggested that this is caused by refraction of the sound passing through the medial edges of both rectus abdominis muscles. The authors were able to reproduce this artifact in vitro using isolated human cadaver rectus muscles and imaging a single object in a water bath.