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Effect of firing rate on the performance of shock wave lithotriptors
Author(s) -
Pishchalnikov Yuri A.,
McAteer James A.,
Williams Jr James C.
Publication year - 2008
Publication title -
bju international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.773
H-Index - 148
eISSN - 1464-410X
pISSN - 1464-4096
DOI - 10.1111/j.1464-410x.2008.07896.x
Subject(s) - cavitation , hydrophone , shock wave , shock (circulatory) , materials science , optics , physics , mechanics , acoustics , medicine
OBJECTIVE To determine the mechanism that underlies the effect of shock wave (SW) rate on the performance of clinical lithotripters. MATERIALS AND METHODS The effect of firing rate on the pressure characteristics of SWs was assessed using a fibre‐optic probe hydrophone (FOPH 500, RP Acoustics, Leutenbach, Germany). Shock waves were fired at slow (5–27 SW/min) and fast (100–120 SW/min) rates using a conventional high‐pressure lithotriptor (DoLi‐50, Dornier MedTech America, Inc., Kennesaw, GA, USA), and a new low‐pressure lithotriptor (XX‐ES, Xi Xin Medical Instruments Co. Ltd, Suzhou, PRC). A digital camcorder (HDR‐HC3, Sony, Japan) was used to record cavitation fields, and an ultrafast multiframe high‐speed camera (Imacon 200, DRS Data & Imaging Systems, Inc., Oakland, NJ, USA) was used to follow the evolution of bubbles throughout the cavitation cycle. RESULTS Firing rate had little effect on the leading positive‐pressure phase of the SWs with the DoLi lithotriptor. A slight reduction (≈7%) of peak positive pressure (P+) was detected only in the very dense cavitation fields (≈1000 bubbles/cm 3 ) generated at the fastest firing rate (120 SW/min) in nondegassed water. The negative pressure of the SWs, on the other hand, was dramatically affected by firing rate. At 120 SW/min the peak negative pressure was reduced by ≈84%, the duration and area of the negative pressure component was reduced by ≈80% and ≈98%, respectively, and the energy density of negative pressure was reduced by >99%. Whereas cavitation bubbles proliferated at fast firing rates, HS‐camera images showed the bubbles that persisted between SWs were very small (<10 µm). Similar results were obtained with the XX‐ES lithotriptor but only after recognizing a rate‐dependent charging artefact with that machine. CONCLUSION Increasing the firing rate of a lithotriptor can dramatically reduce the negative pressure component of the SWs, while the positive pressure remains virtually unaffected. Cavitation increases as the firing rate is increased but as the bubbles collapse, they break into numerous microbubbles that, because of their very small size, do not pose a barrier to the leading positive pressure of the next SW. These findings begin to explain why stone breakage in SWL becomes less efficient as the firing rate is increased.