Cloud Cavitation Control for a Therapeutic Ultrasound Application

In the therapeutic ultrasound field, the cavitation frequently forms a bubble cloud that consists of many microbubbles. A method to control the violent collapse of cloud cavitation is being developed. It is comprised of two frequency ultrasound. The first is a higher frequency ultrasound waveform that makes cloud cavitation at the surface of the object. The second is a lower frequency waveform that produces a violent collapse of the cloud cavitation. If the phenomena is well controlled in time and space, it can be utilized for the therapeutic benefit such as sonoporation, HIFU, and lithotripsy. From a numerical simulation, the controllability of the cloud cavitation is predicted [1,2]. A stable bubble cloud at the solid surface and the shock wave from the violent cloud cavitation collapse are observed by using high-speed camera photography. The shock wave pressure is estimated to be in the order of a few GPa at the solid surface. And the occurrence time of the phenomena is controlled within 65 ns and the area is controlled within 1 mm. Using the method, stones are chipped away such that scoop-like indentations are achieved with the efficiency that is comparable to the conventional Shock Wave Lithotripsy. INTRODUCTION In a HIFU application, high intensity ultrasound causes acoustic cavitation near the focal area. The violent collapse of cavitation bubble has a potential of causing tissue traumas [3], especially in the case in which the bubbles form a cloud. The maximum pressure in the cloud that reaches order of GPa is reported both in numerical [4] and experimental [5] studies. On the other hand in the study of SWL (Shock Wave Lithotripsy), the complex effect of cavitation has been known in the early stage of its research history [6] and many researchers have investigated the role of the cavitation in SWL. The studies were conducted both as the factor of tissue damage [3], and of stone comminution accelerator [7]. In recently, cavitation control techniques by applying skillful shock wave combinations have been proposed and effective results have been achieved [8,9]. However, the main force that breaks the stone is still considered to be the incident plane shock wave that has a 10 ~ 40 mm focal region. Moreover cavitation collapse is utilized only to accelerate the stone comminution. By utilizing two frequency focused ultrasound, extracorporeal lithotripsy method, Cavitation Control Lithotripsy (CCL) is being developed [10], that can erode and chip away the renal stone solely by the violent collapse of the cavitation that is induced by HIFU. If the cavitation phenomena are well controlled in time and space only at the stone surface, the extremely high-energy and high-pressure concentration can be utilized as a main factor of renal stone disintegration. In this paper, the concept of the method and the phenomena in the CCL protocol are explained and the results of the stone crushing are also discussed. SCHEMATIC OF CLOUD CAVITATION CONTROL Fig. 1 shows the schematic of Cavitation Control Lithotripsy (CCL). CCL method is comprised of two different of ultrasound frequencies. First, higher frequency ultrasound is focused at the stone surface (Fig. 1-1). It has a range about 1 ~ 5 MHz in its frequency for a shorter wavelength than the characteristic length of the renal stone. It creates a hemispherical bubble cloud consisting of very tiny bubbles only at the stone surface (Fig. 12). Immediately after the higher frequency is stopped, a short pulse of lower frequency ultrasound that has 100 kHz ~ 1 MHz in its frequency is focused at the hemispherical bubble cloud (Fig.1-3). The lower one resonantly forces the cloud to oscillate (Fig. 1-4). Accompanied with the bubble cloud forced