| Kidney stone is a common disease whose incidence of occurrence increases rapaidlyin China. The current therapeutical methods such as surgery and extracorporeal shockwave lithotripsy (ESWL) have significant side effects, can cause severe damages tosurrounding tissues. In recent years, more and more researchers started to use ultrasound-induced cavitation for kidney stone fragmantations.HIFU (High Intensity Focused Ultrasound) can induce strong cavitation cloud inliquids. As a result, high temperature, high pressure and the micro jet induced bycollapsing bubbles will bring a rapid abrasion to stones when the liquid is acousticallyirradiated. The production powder can reach micron size so that it will be carried outeasily. Considering the irregular shape of real kidney stones, a parameter ξ is defined bystones’ surface cuvature (ξ<1means convex stone, and smaller ξ means more convexstone, and the stone surface is flat when ξ=1) for study. Series of experiments have beencarried out to study the relationship between the parameter ξ and the behavior ofcavitation cloud during the stones erosion.A complete set of cavitation test bench was established in this study. Series of curvedartificial stones were fabricated with ceram-Begostone material. HIFU transducer wasused to produce the focused ultrasound field and induce the cavitation cloud. The volumeloss curve was measured by3D scaning technology. High speed camera was used torecord the migration of cavitation cloud near a series of curved stainless steel samples inthe degassed NaCl solution (20g/L). The cavitation nuclei was induced by a focused highintensity pulsed laser beam. In another experiment, luminol solution (0.01mol/L luminoland0.1mol/L NaOH) was prepared for observing the cavitation sonoluminescence. ADSLR (Nikon D7000) digital camera and a PMT (photomultiplier tube) were used formeasurement of distribution and the strength of the sonoluminescence. The mainconclusions are as follows:(1) The cavitation erosion of the artificial stones had two distinct stages. In the firststage, the erosion occured at the edge of the curved stone at the beginning and expandedto center afterwards. As a result, the stone sectional area increased gradually until thatreached sectional area of cavitation cloud (sectional area of ultrasound field). In the second stage, the abrasion concentrated in the middle of stone and the edge of stoneremained unchanged. At the fisrt30s, smaller ξ led to more volume loss, however thedistinction of volume losses due to ξ was no longer apparent.(2) A standing wave appeared near the stainless steel samples because the ultrasoundrebounded on the obstacles. Pulsative phenomenon in the cavitation cloud rising processwas discovered. When the cloud contact the top of the stone sample, the cloud woulddistributed along a circle ring near the top due to the curved sample induces the shuntingeffect to acoustic streaming. The value of ξ had an influence on the velocity of cavitationcloud in the center. Bigger ξ led to smaller velocity.(3) Due to the existence of curved stainless steel samples, the cavitation strengthbecame stronger and a recirculation migration of cloud was discovered. As a result, ablock field for clouds was formed below the stone sample. It would be more obviouswhen the value of ξ became larger. The maximun strength of cavitation appeared at thepoint3-6mm further to the center of foucus region. |
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