Measurement Of The Critical Acoustic Pressure Of Intense Cavitation Induced By High Intensity Focused Ultrasound

High intensity focused ultrasound(HIFU) is a new noninvasivetherapeutic technique for cancer which emerged in recent years, has beenapplied to clinical treatment of many kind of tumors. The mechanism ofHIFU treatment includes cavitation effect, thermal effect and mechanicaleffect. Cavitation bubble cloud which produced by the cavitation effect atthe focal region plays a very important role in the HIFU therapeutic process.On the one hand, it can make the target temperature rised, and improve thetreatment efficiency. But on the other hand, cavitation bubble cloud canscatter the sound waves and hinder the propagation of sound waves, itmakes the damage widened and shifted toward the sound source at the sametime, the shape of damage from the uniform distribution "cigar shaped" to"tadpole shaped", and influences the therapeutic precision. Consequently,the study of HIFU cavitation field can not only reveal its mechanism, butalso control and use the cavitation effect.Cavitation effect can divide into transient cavitation and steadycavitation. Transient cavitation always used in the related practicalapplication, it needs a large number of cavitation bubbles collapse in the liquid to achieve the purpose of application. Because of a large number ofcavitation nucleus with different radius in the liquid,and they obey a certainstatistical distribution. Therefore, cavitation nucleus with different radiusrequire different acoustic pressure to collapse. Many cavitation bubblesbegin to collapse when acoustic pressure exceed a threshold,and the activityof cavitation bubbles are complex in the whole cavitation field. Accordingto the drawbacks of the theoretical calculation and the existing measuringmethod of the critical pressure when many cavitation bubbles collapse,theoretical calculation just can obtain the critical pressure of a bubbles witha certain radius, and the existing method of quantitative detection under thecondition of high pressure can damage the sensor, and produce theinterference in acoustic field. So we propose the concept of intensecavitation, and the intense cavitation is the phenomenon of a large numberof cavitation bubbles collapse under a higher ultrasonic energy in the liquid.We observed an apparent difference of the liquid transparency between theregion where the intense cavitation occurred and the other region. Therewas a big noise when the intense cavitation occurred,and the activity ofcavitation bubbles were complex in the whole cavitation field. Intensecavitation occurs in specific liquid under a certain conditions when theacoustic pressure exceeds a certain value, and we call this specific pressurevalue as the critical acoustic pressure of intense cavitation, namely thethreshold pressure when intense cavitation occurs. Based on these,we proposed a method which use the acousto-optic deflection effect to detectthe critical intense cavitation pressure, and observed the cavitation bubblecloud in the focal region of HIFU field by the high-speed photographydevice. This paper includes the following several aspects:(1) At first, we proposed the concept of intense cavitation.We studiedthe formation and the structure change of the cavitation bubble cloud andthe influence of the driving power to the whole structure in the focal regionof HIFU field by the high-speed photography device. The results show thatno intense cavitation occurs and no observable cavitation bubble cloud inthe focal region in a certain period of time when the power is less than acertain value. When power acheived to the specific value, the intensecavitation occurred in the focal region with noise. Cavitation bubble cloudfirst emerges near the focus,and the change rule in the beginning and end ofeach cavitation process was basically same, the change of the middleprocess changed with the different power on the tranducer.The specificgravity of the times of cavitation bubble cloud with different structureappears in the whole cavitation process was different. The times ofcontinuous cavitation was less and cavitation duration time was shorter inlow powe. It increased firstly and then decreased with the increase of power,and just one continuous cavitation in higher power,the mean of thecavitation duration time was gradually increasing, and the the mean intervaltime between adjacent cavitation process was gradually decreasing. （2）Then,we proposed the concept of critical intense cavitationpressure.In order to measure the critical pressure when occurred intensecavitation in liquid, a method to detect the critical acoustic pressure ofintense cavitation which based on the acousto-optic refraction effect wasproposed. According to the distribution of the far-field spot which changingwith time to determine the occurrence of intense cavitation, can detect thevalue of the critical acoustic pressure of intense cavitation non-invasivelyby the relation between the maximum deflection distance of the ray and thechange of the focus acoustic pressure with a high repeatability.（3）By analysing the distribution of the cavitation bubbles in liquid,and the cavitation threshold pressure changing with the externalenvironmental pressure to calibrate the results, and prove the feasibility ofthis way to detect the critical acoustic pressure of intense cavitation.This paper proposed a method to detect the critical acoustic pressure ofintense cavitation which based on the acousto-optic refraction effect Thismethod had a simple operation, high repeatability and a certainfeasibility.And we studied the cavitation bubble cloud in the focal region ofHIFU field by the high-speed photography device, analyzed the reason ofthe generation and the change of cavitation bubble cloud. This studyprovides the experimental and theortical evidence for the future research on cavitation mechanism, and how to control and use cavitation.