INTERNATIONAL CONGRESS ON ACOUSTICS MADRID , 2-7 SEPTEMBER 2007 FREQUENCY ANALYSIS OF SHOCK WAVE SCATTERING TO IDENTIFY KIDNEY STONE FRAGMENTATION IN SHOCK WAVE LITHOTRIPSY

Currently there is little feedback available in shock wave lithotripsy (SWL) to determine kidney stone fragmentation. The identification of fragmentation would aid a urologist in deciding to continue or stop treatment, and it could potentially reduce SW dose. Lithotripsy SWs strike stones with a broadband mechanical load. Reverberations excited within the stone are transmitted to surrounding fluid; a process termed resonant acoustic scatter (RAS). The frequency of RAS is inversely proportional to stone size. In experiment, variable SW treatments were applied to two types of stone models in vitro to produce different levels of fragmentation, which were measured by sieving dehydrated fragments and normalizing their mass to intact stone mass. RAS from selected SWs was measured with a broadband receiver and a new frequency analysis method was applied to display the redistribution of spectral energy. Mean percent mass for fragments smaller than 2 mm increased proportionally to the number of SWs applied. Amplitude of the frequency analysis output was directly proportional to fragmentation, and peak frequencies were inversely proportional to stone size. Results show promise that frequency analysis of RAS might provide feedback on fragmentation in SWL. INTRODUCTION Shock wave lithotripsy (SWL) is an acoustic method to remove kidney stones, and it has been in clinical practice for more than two decades. [1] Today, SWL is the most frequently used method to treat kidney stones sized between 2 mm and 20 mm. [2,3] Tissue damage accompanies most if not all SWL treatment, which can lead to acute and chronic complications. [4,5,6] Further, damage to renal tissue in the form of hemorrhagic lesion is proportional to SW dose, which is measured in part by the number of SWs applied. [7,8] However, there is currently little feedback available to urologists during SWL to help reduce SW exposure. Fluoroscopy imaging is used intermittently during SWL to target the stone, but the duration is limited by the associated radiation dose and it can be difficult to distinguish between an intact stone and a collection of stone fragments. B-mode ultrasound imaging is also used, but not all stones are detectable with ultrasound imaging and small stones can be difficult to distinguish from speckle. [9] The identification of stone fragmentation could aid a urologist in screening stones that are resistant to SWs and in determining the endpoint of SWL treatment. However, there is currently no method available during SWL to quantify stone size. Some work has been completed to provide feedback during SWL. Many systems have been proposed to account for stone mobility, which can cause many SWs to miss the stone [10], by tracking the stone with a novel lithotripter design or stone detection system. [11,12,13] In those in vitro works, the number of SWs necessary to fragment a stone is reduced because more SWs hit the stone. Less work has been completed to identify stone fracture and fragmentation. Fedele et al. [14] describe a diagnostic sensor and signal processing technique to assess the degree of stone fragmentation and stone location. The technique uses cavitation signals, where the secondary acoustic emission from bubble collapse and rebound is dependent on stone size. They are pursuing their method in vivo but that data has yet to be published.