INFLUENCE OF TISSUE STIFFNESS ON CAVITATION CLOUD FORMATION BY HIGH INTENSITY FOCUSED ULTRASOUND IN AGAROSE GELS

The effects of tissue stiffness on the growth and collapse of a laser-induced bubble and the cavitation cloud formation by high intensity focused ultrasound (HIFU) in tissue-mimicking phantoms were investigated. First, the growth and collapse of a laser-induced bubble were observed with a high-speed video camera, which showed that the maximum bubble radius decreases with increase in the stiffness of agarose gels. The period of bubble collapse becomes shorter as the stiffness increases in agarose gels. However, the period in 0.3% agarose is longer than that in 7% gelatin, although both Young's moduli are similar to each other. In 0.3% and 0.5% agarose gels, the laser-induced bubble enlarged with the generation of wrinkles on its interface. Experiments were also conducted for the cavitation cloud formation due to the backscattering of HIFU from the residual microbubbles which remain after the laser-induced bubble collapses. In 0.3%, 0.7%, and 1.5% agarose phantoms, the cavitation clouds developed along with the propagation axis of HIFU, while a dense cavitation bubble cloud was not observed in 3.0% agarose. As the gel stiffness increases, the size of the cavitation cloud decreases due to the high stiffness of the surrounding medium. Also, the collapse time of a cavitation cloud becomes shorter with increase in the stiffness of agarose gels. Although the Young's modulus of 6% gelatin is smaller than 0.3% agarose, the collapse time of the bubble cloud is shorter than that in 0.3% agarose.