Measurement Of Acoustic Pressure Of The Focused Ultrasound At Focus Based On Acousto-optic Deflection

High intensity focused ultrasound(HIFU) has become an importantfield in the medical and engineering community, and become an emergingnoninvasive surgery technology in the21st century. With the promotionand application of the technology, the focused ultrasound technology isdeveloping rapidly. The acoustic pressure of the focus could be an orderof107Pa by the focused ultrasound technology currently, and the size ofthe focus is mm-level, even sub-millimeter level, so it is a new challengeto measure the sound field with such a high acoustic pressure andintensity.According to the drawbacks of the existing measuring methods ofsound field for high pressure, for example, during the detection processthe sound field easily suffered from disturbance caused by transducer, andthe sensor for detecting easily damaged, as well as complex relationshipbetween the parameters of the theoretical model for detecting and so on,in this paper we propose a new noninvasive method based onacousto-optic deflection effect to detect the acoustic pressure at focus and nearby the focus of the focused ultrasound. The basic principle ofacousto-optic deflection effect for sound field detecting is that in theinteraction between a narrow and parallel beam and sound field, theexistence of the sound field is causing the change of medium refractiveindex, and making the beam result in refractive phenomenon. Based ondeflection path of the ray, its offset can be calculated, and its relationshipwith the acoustic pressure could be acquired. Based on the theory andexperiment this paper includes four aspects of the study:(1) From the characteristics of the focused sound field distributionand basic theory of ray propagation in uneven medium, acousto-opticdeflection effect is used to detect focused sound field innovatively. Inconditions of focused ultrasound, the medium refractive gradient causedby side lobe of focal region is exactly in opposite direction, so thedeflection is approximate offset when the ray cross through the side lobe,namely the deflection is only related to the sound field of main lobe offocal region. By studying the relationship between acoustic pressure ofthe focus and the ray deflection, a relational model between the maximumdeflection distance of the ray and the change of the focus acousticpressure is established, thus the peak acoustic pressure of the focus iscalculated.(2) The law of the ray projection position change caused by thebeam crossing the focus in a cycle is discussed. The results show that the maximum deflection distance ‘dmax’ of the beam is caused by the raycrossing the focus exactly. Due to high frequency of sound wave, thefacula image in the far field is a superposition image averaged by time,that is, an elliptical facula with broadening ‘2dmax’.(3) By studying the law of the facula shape alteration caused by thebeam moving along the major or minor axis of the focal region, arelational model between the deflection distance of the ray and thechange of the acoustic pressure of the major or minor axis of the focalregion is established. Research shows that in the major or minor axis, themore the beam is close to the focus, the more the deflection is caused, sothe acoustic pressure calculated is the one where the ray is nearest thefocus. However, the way of calculation with amount of ray deflection isdifferent. In the minor axis, the amount of deflection is half the maximumlength of the facula; and in the major axis, it is the amount of increasingdisplacement when nearest ray from the focus relative to no sound field.(4) In order to verify the theoretical model, the con-sphere typefocused transducers are used for experiment research. By comparing withthe result measured by optical fiber hydrophone, the feasibility of thetheoretical model is proved. The results show that the facula imagesacquired in experiment are consistent with the law of theoretical analysis,and the relative error between the acoustic pressure of the focus measuredby current method and by using optical fiber hydrophone is less than15%, which proves the feasibility of the method, and it can detect the peakacoustic pressure of the focus quantificationally.The method mentioned in this paper not only have no disturbancefor detecting the focused sound field, with high accuracy for measuringpeak value of acoustic pressure, but also it is not necessary to detect thedistribution of the light intensity and the relationship between the variableand the acoustic pressure is very simple. This study provides theexperimental and theoretical evidence for that the acousto-opticdeflection effect can be used to measure the whole focused ultrasoundfield quantificationally.