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Ongoing study of ultrasound stone treatment shows promise


Ultrasonic propulsion stone treatment is a long-studied form of kidney stone treatment that uses transcutaneous ultrasound pulses to help clear residual urinary stone fragments.

Mathew D. Sorensen, MD, MS, FACS

Mathew D. Sorensen, MD

In a recent study presented at the 2021 American Urological Association Annual Meeting,¹ Mathew D. Sorensen, MD, MS, FACS, and a team of investigators assessed the efficacy and safety of ultrasonic propulsion treatment on patients with kidney stones. Sorensen is an associate professor and residency program director of the department of urology at the University of Washington, Seattle, Washington.

Please discuss the background for this study.

This is a project that's been ongoing for over 10 years now. Back around 2008, we started working with using focused ultrasound to try to push and move stones around. It's changed a lot over that time period. We moved from the benchtop, to animals, to now humans. This particular study comes from our human randomized trial that we hope will be the final step before FDA approval for the technology.

Please describe how this technique works. What is the learning curve like?

Essentially, we use ultrasound at near diagnostic levels, and we have a special probe that we've made that focuses ultrasound at depth to push a stone or fragments. We image through the center with an off-the-shelf transducer to give us real time monitoring and feedback. With an ultrasound probe that's completely outside the body, we image the kidney, find any small fragments, especially in the lower pole of the kidney because those really struggled to clear. If we can see the stones, and there's a line of sight—we have to be able to see the direction that we're trying to push the fragments—then we try to get the fragments anywhere but the lower pole since that's the worst place they can be.

We've also used this technology in a couple other ways, too. We had some other abstracts at AUA where we were helping to push stones that were in the ureter, either pushing them back into the kidney to try and relieve a blockage or, if the stones were down low by the bladder, we tried to get them to shift around so that we could help facilitate passage.

I think the learning curve for practitioners is going to depend on the familiarity with ultrasound. I use ultrasound a lot in my practice. I use it in clinic and in many of my stone surgeries, but at least in the United States, many aren't quite as familiar with ultrasound of the kidneys. We anticipate that taking some training. Clinicians in Europe may be a step ahead of us because they use ultrasound all the time. So, that will be one of the barriers. We've been using this technology and are definitely the experts, yet we are still learning all the time—different things we can do with positioning or things that patients can do to help make the space a little bit bigger so there's more room for the stones to move around. We have learned to have the patients drink fluid before the session, and it's pretty amazing. If someone drinks a small water bottle, within 20 minutes you can see that the kidney gets a little bit fuller and a little bit plumper. That little bit of extra space probably helps us. We've also learned to position patients with their head just a little bit down. We're learning all the time, so we expect there will be some learning curve so we have developed some training.

What were some of the notable findings? Were any of them surprising to you and your co-authors?

For the overall study, I know my colleagues are anxious to hear about the full RCT results. We have been presenting this technology for several years now and there's always excitement about it. That study is almost done. But we learned so much from this case that we thought there was some real value in presenting it because it surprised us. In the recruitment for the RCT, we had been avoiding patients that had stones that had been in place for a long time. If they had many months to clear and the fragments remained there might be trapped or perhaps an anatomic barrier to clearing, or maybe they were bigger with a chance to grow for a while. This subject really demonstrated that if somebody looks like they are a good candidate, then they should not be excluded. This subject was 17 months out from his stone procedure, but he still looked really favorable. So, it changed our approach and expanded recruitment criteria for the study and the way that we will recommend it after the studies done.

Is there further research on this topic planned, and if so, what will its focus be?

There is always more to do. We are constantly trying to improve the whole platform, and this study demonstrates one piece of the ultimate vision, and that is a single platform that can image stones better than your off-the-shelf ultrasound machine to increase confidence in stone detection. We are working on our platform to break stones with the same probe. That technology started about 5 years behind the pushing technology, but we are already in human studies and are having great success thus far breaking symptomatic stones in the ureter and in the kidney. We are focused on trying to get these technologies in the hands of providers.

Ultimately, we see lots of places where this could be helpful and they really go hand in hand. We need to be able to see the stones. Then, in the clinic, without anesthesia have a non-invasive technology to break the stones and make them smaller, and then use the pushing pulses to encourage the stone fragments to clear. In the end game, they'll all go together. We're working on trying to optimize the pushing, trying to optimize the breaking. We're always trying to optimize things.

If approved by the FDA, how will this technology change stone treatment?

The main places where we see that this could be useful currently is with either trying to help with an acute episode to resolve more quickly or to push a stone back into the kidney. As you can imagine, NASA is really interested in this technology, and they have been very supportive of our research. Because of some of our results, NASA has changed the risk category and now views our technology to help mitigate some of the risk of stones in space, for example, for a manned mission to Mars. In clinical practice, residual fragments after surgery are common and we know that if they stay in place, even in practices where we're really aggressive about trying to put prevention efforts in place and to try to keep them from getting bigger, there is still a real significant portion of them that lead to trouble down the line. So we hope this will be another tool in our toolbox to help to clear fragments and stones out of kidneys while they are small.

What is the take-home message for the practicing urologist?

Hang with us. We're almost there. Start practicing with ultrasound in your practice.Ultrasound is the future. Our group is definitely making progress. We are getting close, and we hope to be able to offer some of these technologies on a wider scale soon. We believe they will change the way we take care of patients with kidney stones.

Is there anything else you feel our audience should know?

There's a lot of excitement about the potential for these technologies. Meetings like the AUA and the World Congress of Endourology help to provide us with motivation, excitement, and suggestions how to improve the technology. We really appreciate the support that we have received from urologists, the AUA, NIH, and NASA. We are working hard and doing our best.


1. Sorensen MD, Dai JC, Chen TT, et al. Randomized control trial of ultrasonic propulsion to facilitate clearance of chronic residual fragments. Paper presented at: 2021 American Urological Association Annual Meeting; September 10-13, 2021; virtual. Abstract LBA01-06

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