Tracking kidney stones during shock wave lithotripsy

Abstract

Shock wave lithotripsy is a non-invasive procedure by which kidney stones are fragmented by shock waves. Real-time monitoring during shock wave lithotripsy remains a challenge more than three decades after it was introduced into clinical practice. Currently, many shock waves are delivered to the body that do not impact the stone, but do result in tissue trauma. This work presents a monitoring system to locate kidney stones, with the goal of gating shock waves not aligned with the stone, and hence, reduce renal trauma during lithotripsy. A circular array, housing twenty-two 0.5 MHz transducers that can be mounted on a clinical lithotripter, was deployed in a water tank. An algorithm, consisting of threshold detection, including automatic rejection of weak signals, and triangulation, was developed to determine the location of stones. The accuracy of the system was tested using: a spherical steel ball and two stone models made from gypsum cement. The results show that within ±15 mm of the focus of the lithotripter, the accuracy was better than 5 mm in the lateral directions and 2 mm in the axial direction. Using off-the-shelf hardware, the algorithm can calculate stone position every 1.1 s, allowing for real-time tracking during lithotripsy. [Work supported in part by NIH through P01-DK43881.]