Imagine a noninvasive treatment for renal calculi that does not shatter the stones but instead employs low-intensity, ultrasound-generated pulses to move them to and through the ureter.

Principal investigator Jonathan Harper, MD, assistant professor in the Department of Urology at the University of Washington (UW) School of Medicine in Seattle, explains to Renal & Urology News how he and his collaborators are turning their “Rolling Stones” concept into a feasible office procedure.

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What do you call the low-power ultrasound device you are using in your studies?

Dr. Harper: We call the procedure ultrasonic propulsion. In the FDA application we called the device Propulse 1. Casually around UW, we refer to our work and our group as Rolling Stones. If this becomes a startup company, we like the name Sonomotion.

How does the device/intervention work?

Dr. Harper: Generally, sound waves are focused on the stone and they transfer momentum to the stone, which makes it move. Specifically, it looks like a diagnostic ultrasound machine with an ultrasound image. The user puts the probe against the skin and visualizes the stone and kidney, touches the image of the stone on the screen, and watches the stone move. Touching the screen sends the focused wave to the stone without interrupting imaging.

What inspired you to develop this process and device?

Dr. Harper: [Senior principal engineer and adjunct assistant professor of urology] Michael Bailey and others in the UW Applied Physics Laboratory have worked on shock wave lithotripsy (SWL) for a long time. Stones are fragmented with SWL, but often these pieces remain in areas of the kidney and do not pass. Many have wanted to find a way to help those pieces pass.

We knew ultrasound could be used to create a pushing force, and so applied that to this problem. Ultrasound engineers use the force to calibrate instruments. In our case we tried to use focused ultrasound to break stones like lithotripsy, but ended up moving the stones and had to chase them around a water tank.

Why do you think ultrasonic propulsion could be successful?

Dr. Harper: It is a practical solution to a real problem. The design and operation are pretty simple and elegant. We know the forces we can generate and have a good feel for the forces that are required. The outputs to achieve these forces have been shown to be safe and not cause pain. We have had success in multiple preliminary studies and are now performing the first clinical trial.

There also has been enthusiasm from many expert endourologists throughout the country, which is encouraging. In addition, we have had many patients contact us with questions and [express] interest in volunteering as subjects.

Approximately 300 urologists visited the hands-on demonstration  [conducted at the 2013 annual meeting of the American Urological Association (AUA)], and all survey respondents marked [that they would be] “likely” or “very likely” to use the technology. They had the chance to use the system to reposition stones in a mannequin or to drive the stone through a maze. [The group will be demonstrating the procedure again at the May 2014 AUA annual meeting in Orlando, Florida.—Eds.]

What advantages does low-power ultrasound have over extracorporeal SWL and such other treatments for kidney stones as flexible ureteroscopy and percutaneous nephrolithotomy?

Dr. Harper: Ultrasonic propulsion serves a different purpose. Most stones are small enough to pass naturally; however, many of them ultimately require surgery. Any of the above-mentioned surgeries breaks the stone into fragments that either pass naturally or remain in the kidney. These fragments may slowly grow and ultimately require another surgery.

The goal of our technology is to help the small stones or residual fragments pass by moving them out of the calyx and closer to the UPJ [ureteropelvic junction] or ureter. In turn, we would expect to avoid some surgeries and improve the outcomes of others.

There are other possible scenarios of moving a stone that would be of benefit, such as dislodging a large stone obstructing the UPJ. This could not only relieve a patient’s pain and obstruction, but also could avoid an urgent procedure and allow for scheduling an elective surgery.

Moving a stone before or during surgery could facilitate access to a hard-to-reach stone. Other uses are [related to] diagnostic feedback—for example, by inducing movement, one might be able to tell that two stones next to each other are in fact two stones and not one.