1. Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, Washington 98105, USA
2. Division of Gastroenterology, Department of Medicine, University of Washington, 1959 NE Pacific Street, Seattle, Washington 98195, USA
3. Department of Acoustics, Physics Faculty, Moscow State University, Leninskie Gory, Moscow 119991, Russia