Evaluation of Renal Stone Comminution and Injury by Burst Wave Lithotripsy in a Pig Model

Abstract

Introduction: Burst wave lithotripsy is an experimental technology to noninvasively fragment kidney stones with focused bursts of ultrasound (US). This study evaluated the safety and effectiveness of specific lithotripsy parameters in a porcine model of nephrolithiasis. Methods: A 6- to 7-mm human kidney stone was surgically implanted in each kidney of three pigs. A burst wave lithotripsy US transducer with an inline US imager was coupled to the flank and the lithotripter focus was aligned with the stone. Each stone was exposed to burst wave lithotripsy at 6.5 to 7 MPa focal pressure for 30 minutes under real-time image guidance. After treatment, the kidneys were removed for gross, histologic, and MRI assessment. Stone fragments were retrieved from the kidney to determine the mass comminuted to pieces <2 mm. Results: On average, 87% of the stone mass was reduced to fragments <2 mm. In three of five treatments, stones were completely comminuted to <2-mm fragments. In two of five treatments, stones were partially disintegrated, but larger fragments remained. One stone was not treated because no suitable acoustic window was identified. No injury was detected through gross, histologic, or MRI examination in the parenchymal tissue, although petechial damage and surface erosion were identified on the urothelium of the collecting system limited to the area around the stone. Conclusion: Burst wave lithotripsy can consistently produce stone fragments small enough to spontaneously pass by transcutaneous administration of US pulses. The data suggest that such exposures produce minimal injury to the kidney and urinary tract.

Keywords: burst wave lithotripsy; nephrolithiasis; renal injury; shock wave lithotripsy.

FIG. 1.

Photographs of all stones before implantation (top) and fragments extracted after 30-minute BWL exposures (bottom). BWL = burst wave lithotripsy.

FIG. 2.

Distribution of sizes for 211 fragments recovered from the five treatments, as calculated by image analysis. Nearly all fragments have <2 mm equivalent diameter.

FIG. 3.

Typical gross appearance (left, anterior, and center, posterior) of a freshly excised kidney exposed to 30-minute BWL treatment, showing no apparent injury to the capsular surface, but petechial injury to the urothelium (circular outline in right image), in the vicinity of stone fragments. The scale bar in the right image is 1 cm.

FIG. 4.

Hematoxylin and eosin histology slide showing a section distal to an exposed stone in the kidney. The bottom slide is a magnified view of the area outlined in the top slide. Focal mucosal injury is observed in the location of treatment (arrowheads), extending to the lamina propria of the wall, but not into the renal parenchyma.

FIG. 5.

MRI appearance of renal tissue in freshly excised kidneys. (a–c) Sample image slices show locations of stone fragments (arrowheads), but no apparent hemorrhagic injury using the T1-weighted TSE image (a), T2-weighted TSE image (b), and susceptibility-weighted image (c). Scale bar = 1 cm. TSE = turbo spin-echo.