Prototype ultrasound device images, moves kidney stones

October 1, 2011

A team at the University of Washington School of Medicine and Applied Physics Lab, Seattle, is refining prototype technology that should allow extracorporeal ultrasound to image and, more importantly, move stones in a kidney as if they were being nudged gently by a pool cue.

Key Points

Washington-A team at the University of Washington School of Medicine and Applied Physics Lab, Seattle, is refining prototype technology that should allow extracorporeal ultrasound to image and, more importantly, move stones in a kidney as if they were being nudged gently by a pool cue. To put polish to an already novel technique, the team has developed a computer program that automatically re-targets the stones as they move.

"The take home is that we are developing a device that can noninvasively move stones and residual fragments out of the lower pole and into the collecting system," first author Anup Shah, MD, told Urology Times. "We can image the stone as a colored object on a screen and track it in real time. We have also shown that the levels of energy needed to move stones are well below those associated with tissue damage.

"This should allow for spontaneous clearance of stones and residual fragments or move stones to a position where they can more readily be accessed," explained Dr. Shah, a urology resident at the University of Washington, working with Jonathan Harper, MD, Michael R. Bailey, PhD, and colleagues.

The concept is novel, but the instrumentation at this point in its development is readily available. In the porcine model, the researchers used commercial diagnostic probes to image the stones and deliver focused acoustic pulses at a 1% duty cycle at powers up to 8W. Dr. Shah reports that this is far below the 80W, 10% duty cycle that preliminary results indicate is the threshold for mechanical injury, disrupted vessels, and extravasation, and the 10W, 100% duty cycle that is the threshold for thermal coagulation.

The in vitro study found that stones moved at a rate of 1 cm/sec and that all stones could be moved but that full power was needed to move the larger 8-mm stone into the collecting system. The researchers reported that the technology could generally control stone displacement direction from any angle but that the focused ultrasound angle generating the greatest displacement was in line with the stimulated infundibulum, an angle they admitted might be difficult to determine extracorporeally.

The researchers listed several potential applications should clinical trials show the technology and technique to be efficacious. The system could be used as adjunctive therapy with primary expulsive therapy, and to manage obstructive ureteropelvic stones by moving them to a nonobstructive location. It could also be used in patients with known infectious stones requiring removal to reduce the risk of recurrent infection and prevent further stone formation. The technique would also have a role in pediatric and pregnant populations, where there is concern about the ionizing radiation used in conjunction with ureteroscopic, shock wave lithotripsy, and percutaneous stone management.

The continuing study is financed by grants from the National Institutes of Health and the National Space Biomedical Research Institute.