Optical coupling control: an important step toward better shockwave lithotripsy.

Background: In modern "dry" lithotripters, shockwaves are generated in a membrane-covered water cushion that is then coupled to the patient.To limit energy loss, a coupling agent, usually ultrasound gel, is used in this acoustic interface. During the coupling process, air pockets are inevitably trapped in the coupling area, which subsequently remains invisible to the operator.