Characterization of the HIFU-induced cloud cavitation for the optimization of high pressure concentration for lithotripsy

The bubble cloud is a highly scattering object; to the contrary, it is also a strong pressure wave enhancer, if the wavelength and amplitude of the wave is appropriate ones. We've been investigated the stone erosion enhancement in the existence of bubble cloud on the stone surface by using high frequency waveform that immediately followed by low frequency trailing pulse (C-C waveform; Cavitation Control waveform). For the optimization of the high pressure concentration it is needed to know, "how the enhancement of the pressure wave through the bubble cloud" and "how we can estimate the efficiency thorough the passive detection of the reflected signal from the bubble cloud". We measured the "Transmittance" and " Reflection" by two types of acoustic emission sensors, PCD (Passive Cavitation Detector) and DCD (Direct Collapse Detector). The results well depict the characteristics of the HIFU-induced bubble cloud responses. The response curves reveal that the local maxima of the "Transmittance" and "Reflection" occur at the different bubble cloud volume. The PCD signal is higher in the larger bubble cloud band. The DCD signal is larger in the smaller bubble cloud band. These tendencies imply the therapeutic effect through the bubble cloud is optimized in the small bubble cloud region and too much bubbles scatter the incoming ultrasound wave and the ultrasound wave does not efficiently propagates inward the bubble cloud. Thus, for the optimization of the bubble cloud collapse, the cavitation threshold can be the lower limit and the large scattering amplitude from bubble cloud can be the upper limit of the ultrasound conditions.