Wei Lu,    Bioengineering Grad. Student      Bryan Cunitz, Engineer      Peter Kaczkowski, Engineer      Oleg Sapozhnikov, Engineer      Mike Bailey, Engineer      Center for Industrial &          Medical Ultrasound      Applied Physics Laboratory      University of Washington      Anup Shah      Resident, Dept. of Urology      UW School of Medicine      NIH      NSBRI	THE PROBLEM In medical treatment, kidney stones are broken and either removed surgically or allowed to pass out of the body with the urine. Often fragments remain, specifically in the lower pole of the kidney, where they must overcome gravity to exit the kidney. Residual fragments act as a site of re-growth for future stone formation and 50% of patients with residual stones return for treatment within 5 years. The goal of this project is to use ultrasound to push stones and stone fragments within the kidney to move them toward the exit of the kidney and facilitate their passage. THE SOLUTION Our solution relies on using a pushing force created by an ultrasound beam. Ultrasound waves can create a force that pushes on an object in the direction the waves propagate by transferring momentum carried by the waves to the object such as a stone. This force called radiation force, or when measured over an area, radiation pressure, is increasingly being used in many applications—levitating objects for non-contact fabrication, characterization of ultrasound sources by the output force, and new ultrasound imaging techniques to record the stiffness of tissue by pushing on it. In fact, the kidney stone can be seen with x-ray or ultrasound imaging to jump around during lithotripsy because of the radiation force from the shock waves. Although ultrasound can thermally and mechanically damage tissue, after exposure to the radiation force ultrasound, injury to the kidney was not observed on visual inspection or in samples viewed under a microscope. Thus, our technique to push stones appears effective and safe. This is not unexpected as a calculation of the ultrasound pulse intensity required to lift a submerged 1-cm stone against gravity yields a pulse intensity within the range used safely in diagnostic ultrasound. And pulses need last only a fraction of a second to produce a pushing force sufficient to move a stone or stone fragment.