SAN DIEGO, CA USA (UroToday.com) - Therapeutic ultrasound has an increasing number of applications in urology, including shockwave lithotripsy, stone propulsion, tissue ablation, and hemostasis.
The group from the University of Washington developed a therapeutic focused ultrasound prototype that can transcutaneously propel kidney stones. However, considering that the threshold of tissue injury using focused ultrasound is unknown, the goal of their study was to determine the threshold for kidney injury for a range of intensities between diagnostic and ablative therapeutic ultrasound.
In this study, the kidneys of 13 domestic swine were targeted using a custom-built image-guided ultrasound system in an in vivo model. Ultrasound was coupled directly to the kidney through a water-filled cone, which removed attenuation and aberration from intervening tissue. Up to six regions in each kidney were treated with 10 different treatment intensities (0-30,000 W/cm2) with 100 µsec duration pulses triggered every 3 msec over 10 minutes. The kidneys were then perfusion-fixed, embedded, sectioned, and stained with HE and periodic acid Schiff (PAS) for histologic analysis by 3 blinded independent pathology experts.
The results of their study revealed that the threshold for significant tissue injury was greater than 20.000 W/cm2. Histopathologic evaluation revealed emulsification, necrosis, and hemorrhage at this level of intensity. Lesion size was on the order of millimeters. Below this threshold, the probability that injury was observed remained constant at less than 0.1. Almost all tissue injury that was observed below the threshold of 20.000 W/cm2, presented as focal cell and tubular swelling and/or degeneration. Most importantly, at 3.3% duty cycle, the intensity range required to propel kidney stones (<2.500 W/cm2) lies well below the 20.000 W/cm2 threshold.
In conclusion, focused ultrasound appears to have a low probability of causing kidney injury at a wide range of intensities. However, at intensities greater than 20.000 W/cm2, which is approximately the intensity of shockwave lithotripsy pulses, a higher probability of injury and more significant injury was observed. An IRB is currently submitted and under review in order to start with the initial clinical experience in 15 patients at the University of Washington.
Presented by Yak-Nam Wang, MD at the American Urological Association (AUA) Annual Meeting - May 4 - 8, 2013 - San Diego Convention Center - San Diego, California USA
Department of Urology, University of Washington School of Medicine, Seattle, USA
Reported for UroToday.com by Achim Lusch, MD; UC Irvine Medical Center, Orange, CA USA