The split image artifact in pelvic ultrasonography: the anatomy and physics.

Transverse ultrasound scans of the pelvis often demonstrate double images or an apparent widening of the structures deep to the bladder. The phenomenon occurs when the real-time transducer is placed on the midline of the abdominal wall. Most of the widening effect or split image is caused by refraction of sound at the interface between the abdominal wall muscle and the collection of fat deep to the muscle. Using Snell's Law, it is possible to calculate the amount of splitting for a given anatomic configuration. The phenomenon is most exaggerated when the object is furthest away from the transducer and when the curvature between muscle and fat is largest.