Effects of Sector Shock Wave Beaming and Focused Shock Waves on Brittle Targets in Water

Abstract

All shock wave lithotripsy systems currently in use employ focusing methods that concentrate shock wave energy at a point coincident with the stone. This report looks at the erosion effects of a Dornier elliptical focusing system on brittle targets and an alternate non-focusing method called sector shock wave beaming (SSB).¹

The targets for the experiments were plaster of Paris cylinders, 2.5 cm in diameter, with a controlled compressive strength of approximately 40 bar (comparable to a typical calcium oxalate stone). These targets had controlled characteristics with respect to size, shape, and strength. Results of the two methods were compared in regard to the mode of destruction and the degree of plaster target erosion. After the administration of shock waves, residual masses were tested for strength y depth using standard penetrometer methods.

The following conclusions can be drawn from these experiments: First, with elliptical focusing, the high pressure focal point located at the flat end of the target cylinder eroded the target surface progressively over hundreds of shock waves. With the target held rigidly, a concave, dish-shaped erosion pattern developed. With the target partially restrained in a water-filled plastic bag, a convex surface developed. Second, with SSB, unlike the focused shock wave method, near total destruction of the target to fine particles occurred with a single shock wave at approximately 0.85 Kbar peak shock wave pressure, and the onset of visible erosion occurred at approximately 0.5 Kbar. With three repeated shock waves of 0.34 Kbar, significant erosion occurred. These shock wave pressures are entirely controllable and are at an order of magnitude below potential kidney injury. Each shock wave erodes target strength over the full target diameter to a substantial depth, depending on the shock wave strength. Third, with SSB, it may be possible to pulverize a human kidney stone of a large size with a few shock waves while minimizing the risk of injury to the kidney and surrounding viscera and without the generation of large fragments that may obstruct the ureter.

formerly with Naval Surface Weapons/Warfare Center White Oak Laboratory Silver Spring, Maryland