Focused ultrasound to expel calculi from the kidney

Abstract

Purpose: A persistent stone burden after renal stone treatment may result in future patient morbidity and potentially lead to additional surgery. This problem is particularly common after treatment of lower pole stones. We describe a potential noninvasive therapeutic option using ultrasound waves to create a force sufficient to aid in stone fragment expulsion.

Materials and methods: Human stones were implanted by retrograde ureteroscopy or antegrade percutaneous access in a live porcine model. The calibrated probe of a system containing ultrasound imaging and focused ultrasound was used to target stones and attempt displacement. To assess for injury an additional 6 kidneys were exposed for 2 minutes each directly to the output used for stone movement. Another 6 kidneys were exposed to more than twice the maximum output used to move stones. Renal tissue was analyzed histologically with hematoxylin and eosin, and nicotinamide adenine dinucleotide staining.

Results: Stones were moved to the renal pelvis or ureteropelvic junction by less than 2 minutes of exposure. Stone velocity was approximately 1 cm per second. There was no tissue injury when tissue was exposed to the power level used to move stones. Localized thermal coagulation less than 1 cm long was observed in 6 of 7 renal units exposed to the level above that used for ultrasonic propulsion.

Conclusions: Transcutaneous ultrasonic propulsion was used to expel calculi effectively and safely from the kidney using a live animal model. This study is the first step toward an office based system to clear residual fragments and toward use as a primary treatment modality in conjunction with medical expulsive therapy for small renal stones.

Figure 1

New experimental ultrasound system. Imaging probe is placed in therapy probe central aperture (A). Probes are held in water filled coupling cone with acoustically transparent window, which is placed in contact with skin. Electronics and probe with longer coupling cone (B). Ultrasound imager is not shown.

Figure 2

Superimposed frames of fluoroscopic movie tracking ultrasonic expulsion of bead (A and B). Images show 5 mm bead moved about 3 cm in 1.3 seconds (s), traveling from lower pole through UPJ into ureter.

Figure 3

Sections of porcine kidney not exposed to ultrasound as control (A), and exposed to 325 W/cm² for ultrasonic stone propulsion (B) and to 1,900 W/cm², well above that for stone propulsion (C). Note thermal injury (C). H & E and nicotinamide adenine dinucleotide stain, scale bar represents 100 μm.