1. Division of Gastroenterology, Department of Medicine, University of Washington, Seattle, Washington 98125, USA
2. Department of Acoustics, Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russia
3. Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, Washington 98105, USA
4. Department of Urology, School of Medicine, University of Washington, Seattle, Washington 98195, USA