| BackgroundHigh intensity focused ultrasound(HIFU)is a new non-invasive method for the treatment of benign and malignant tumors.The ultrasound energy in vitro can be gathered in vivo,and the tissue of the target absorbs the ultrasonic energy to produce an irreversible coagulation necrosis without damaging the normal tissue outside the target area.However,the focused methods of acoustic wave such as lens,phased array and spherical shell.They will affect the accuracy of focusing in the process of sound propagation directly.Moreover,the focal region formed in the propagation wave field is 10 wavelengths on the acoustic axis,and the vertical acoustic axis is 1~2 wavelength.However,the focal area is so large that damage normal tissue.In recent years,many researchers try to improve the focusing accuracy.However,the fine focusing can't be achieved due to the limitation of transducer materials and the acoustic pressure at the focus can't reach 108 Pa.The spherical transducer with open ends is composed by a certain number piezoelectric ceramic sheets,which is phase array transducer.Among them,the acoustic emission surface is also the reflection of acoustic waves.When the acoustic wave of the center of sphere and the acoustic wave reflected are the same phase,the standing wave in the center of sphere will become more intensity.The cavitation cloud is constrained in the center of the sphere.Therefore,the spherical transducer can achieve high pressure in the sub wavelength focal region.Compared with the same size spherical focus transducer,the positive pressure of the latter is obviously higher and the accuracy of focal area is more accurate,-6d B focal field only half a wavelength.Therefore,this technique has potential application value in improving HIFU conformal ablation.In addition,the cavitation threshold will increased and the activity of cavitation cloud will be more intense at the focus,when hydrostatic pressure increased.According to the article: when the hydrostatic pressure is 30 MPa,the pressure at the center of the bubble reaches a staggering 4400 Gpa,while maintaining the 35 ps.The density is about 8 times that of the water,and the temperature is 200000 K.When hydrostatic pressure is 20 MPa,the pressure at the center of the bubble reaches a staggering 250 GPa,while the temperature reaches 100000 K.Moreover,the cavitation bubble with the energy is greater,as hydrostatic pressure increased.When the hydrostatic pressure is 10 MPa,it takes only a few seconds to cut the steel wire with a diameter of 0.9mm.When the hydrostatic pressure is 20 MPa,it takes less than half a second to cut the steel wire with a diameter of 0.9mm.This gives us a new understanding of the phenomenon of acoustic cavitation.The energy,violent activity of the cavitation cloud generated,can be controlled in time and space,was used in medical field and industrial field.If this energy can be used in the field of medicine and industry in time and space,it will improve the scope of ultrasound in the clinical application and promote industrial development.In the first part of this paper,the activity of cavitation cloud under different hydrostatic pressures is captured by Nikon camera,which reflects the influence of hydrostatic pressure on the range of cavitation bubble.In the second part,the luminous intensity of cavitation cloud was studied by using em ICCD and spectrometer,and the influence of hydrostatic pressure on the activity of cavitation cloud was clarified.ObjectiveThis paper illustrates the different hydrostatic pressure and the different driving power can restrain the activity of the multi-bubble and the luminescence of the spherical focus,and deepen the understanding of the acoustic cavitation phenomenon.Methods1.In this part,the frequency 0.6MHz,diameter 500 mm,open 256 mm,high 474 mm of both ends of the spherical focused transducer.The activity of cavitation cloud at the focal point of spherical focused transducer is captured under different static pressure(the water solubility of medium water is 0.91mg/L)and different driving power 1000 W ?2000W and 3000 W.The photos of cavitation cloud processed by Matlab,the area of cavitation cloud can be obtained by comparing with the ruler.The bubble cloud luminescence condition under different static pressure(the water solubility of medium water is 0.91mg/L)and different driving power 1000W?2000W and 3000 W were taken by the Nikon camera in the darkroom.2.In this part,the parameters of the spherical transducer are same.The multi-bubble luminescence were captured at different static pressures(the water solubility of medium water is 0.91mg/L)and different driving power 1000 W ? 2000 W and 3000 W by em ICCD camera.The multi-bubble luminous intensity were measured by Lightfield software.The images of em ICCD were grayed,binary and noise was reduced by MATLAB,and the number of pixels at the focus reflected the luminescence zone with the static pressure changes.According to the formula of roundness,the roundness of multi bubble under different hydrostatic pressures is calculated,which reflects the influence of hydrostatic pressure on the bubble shape.By means of the software Origin,the data of the luminous region are displayed in plane and three-dimensional to reflect the energy distribution and change of the violent activity inside the cavitation cloud.3.In this part,the parameters of the spherical transducer are same.The spectrum of the multi-bubble luminescence at the focal point under different static pressure(the water solubility of medium water is 0.91mg/L)and different driving power is obtained by the spectrometer.In the same time,the luminous intensity of cavitation bubbles is reflected by the integral area of the spectral curve.Results1.The range of cavitation cloud decreases gradually with the hydrostatic pressure was increased from 0.1MPa to 10.0MPa.The area of the cavitation bubble cluster is the largest,when the static pressure is 0.1MPa,which is "fireworks".The area of cavitation cloud is the smallest,when the static pressure is 10 MPa,which is "punctiform".The area of cavitation cloud was not obvious,when static pressure increased from 2MPa to 10 MPa.The driving power is large,the area of cavitation cloud formed in the liquid is obviously larger.2.The average luminous intensity of cavitation cloud were gradually increased,luminous shape were gradually tend to a circle,the luminous region becomes larger and the energy distribution of cavitation cloud is more dense,when hydrostatic pressure was increased from 0.1MPa to 10 MPa.The center of cavitation cloud is not the most intensity activity.The luminous intensity of the cavitation group gradually increases and the luminous area becomes larger as the driving power increases when the hydrostatic pressure is constant.3.The greater the hydrostatic pressure,the larger the integral area of the spectral curve,which indicates that the static pressure enhances the intensity of the multi-bubble luminesence at the focal point of the spherical transducer,when the driving power is 2000 W.The larger the driving power,the larger the integral area of the spectral curve,and the stronger the intensity of the multi-bubble luminesence,when the hydrostatic pressure is constant.Conclusion1.As the hydrostatic pressure increased,range of activities of cavitation cloud are gradually decreased,the luminous range of multi-bubble become larger,luminous region tends to be round,the luminous intensity of multi-bubble are gradually increased,the energy cavitation cloud have higher energy and intensive energy distribution.In a certain wavelength range,the integrated area of the spectral curve is larger,the photon energy produced by multi-bubble luminescence is increased.This is related to the enhancement of cavitation threshold.2.As the driving power increased,the range of the violent activity and luminous range of cavitation cloud are becomes larger,the luminous intensity are gradually increased,the region tends to be round.The little effect the driving power on the range of cavitation bubbles,when the hydrostatic pressure is larger.In a certain wavelength range,the integrated area of the spectral curve is larger,the photon energy produced by multi-bubble photoluminescence is increased. |
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