Acoustic coupling can be problematic in shock wave lithotripsy

Abstract

Research to improve lithotripsy has yielded considerable progress in determining mechanisms of SW‐action, has led to development of innovative SW‐delivery systems, and has offered practical strategies to enhance stone breakage and minimize renal injury. Little attention has been paid to how SWs are coupled to the body. In vitro tests were performed to assess the quality of coupling with a clinical Dornier DoLi‐50 lithotripter. A test tank with a Mylar window was coupled to the treatment head using lithotripsy gel. Application of gel as performed in patient treatment always trapped air pockets at the interface, and it took considerable manipulation to eliminate them. Stone breakage was less efficient when air pockets were present, with a linear correlation between percent area of air‐pocket coverage and reduction in breakage (p<0.001). Breaking contact and recoupling—as can also occur in patient treatment—reduced stone breakage to ∼10% of air‐free coupling. Air pockets created when gel was applied to both surfaces, or with coupling and recoupling, reduced the energy density of the pulse to ∼30%–50% of air‐free. Thus, even under controlled conditions, acoustic coupling in SWL can be inefficient. Poor acoustic coupling could be a significant source of variability in clinical shock wave lithotripsy.