Acoustic and Optical Characteristics of Cavitation Bubble Clouds Produced by Shock Wave Pulse

Abstract

A shock pressure pulse used in an extracorporeal shock wave treatment has a large negative pressure (<-5 MPa) which can produce cavitation. Cavitation is known to have therapeutic effects but its measurement is not easy. This thesis is to measure cavitation induced by shock wave through an optical hydrophone and optical visualization. The signals of shock pressure were recorded by an optical hydrophone (FOPH2000, RP Acoustics, Germany) submerged in water for several hundred microseconds at the focus for analysis. The signals are characterized by shock pulse followed by a long tail after several microseconds; these signals are regarded as a Cavitation-Related Signal (CRS). The CRS was found to contain characteristic information about the shock pulse-induced cavitation. The first and second collapse times (t1 and t2) were identified in the CRS. The collapse time delay (tc = t2 - t1) increased with the driving shock pressures. The signal amplitude integrated for the time delay was highly correlated with tc (adjusted R2 = 0.990). This finding suggests that a single optical hydrophone can be used to measure shock pulse and to characterize shock pulseinduced cavitation. Cavitation bubbles produced by a clinical shock wave system were optically visualized and their geometric features were investigated in relation to the driving shock wave field. Cavitation bubbles induced by the shock wave were captured by an ordinary industrial CCD camera under illumination of a micro-pulse LED light. The light exposure was set to last for the whole life time of bubbles from formation to subsequent collapses. It was shown that the cavitation bubbles appeared mostly in the vicinity of the focus. The bubbles became more and larger as approaching to the focus. Jet streams formed when collapsing of the cavitation bubbles became enlarged as the output setting of the shock wave device increased. The bubble cloud boundary was reasonably fitted to an elongated ellipsoid similar to the acoustic focal area of negative pressure. Grayscale intensity of visualized cavitation bubble was highly correlated with the amplitude of negative pressure (adjusted R2 =0.87). When the light exposure time was varied, we could visualize the focal point where the bubbles were concentrated and confirm the collapse time of the bubbles. The geometric features of the cavitation bubbles were characteristically similar to those of the focusing acoustic field, which has potential to provide the therapeutic focal information. The similarity of these characteristics enabled to visualization of the cavitation cloud image, which may provide the intensity field and location of shock wave irradiation. This result would be useful for the clinical quality assurance of therapeutic devices without time-consuming and causing damages of the expensive hydrophone sensor. The present study is limited to the static afterimages of the cavitation bubbles, and further investigation including the bubble dynamics is suggested to deliver the more realistic therapeutic area of the shock wave therapy.