Research On A Travelling-wave And Standing-wave Combined Focused Ultrasound Transducer

BackgroundHIFU transducer is the core component of HIFU treatment equipment,and the selection of material,focused mode,frequency,shape and structure,as well as other performance parameters of the component will directly affect the acoustics focal region shape of the transducer when it works,thus affecting the treatment precision.In previous studies,HIFU transducers were mainly based on spherical self-focused,acoustic lens focused,multiple self-focused and electronic phased array focused,which were the travelling-wave FUTs?FUTs?.The-6dB acoustic focal region formed by these types of FUTs has a large dimension,and the overall focal region is shaped like a cigar.Usually,the long axis of the focal region is greater than 6 wavelengths,and the ratio of the long axis to the short axis is greater than 3,and the ultrasound energy is relatively dispersed.which makes the focus precision lower,and it cannot effectively avoid normal tissue damage.However,the existence of standing-wave in the acoustic field can effectively reduce the ratio of the long axis to the short axis of the focal region of the FUT,obtain a smaller focal region volume,make the ultrasound energy more concentrated,and even make the-6dB focal region dimension reach sub-wavelength scale.Therefore,this subject designed a travelling-wave and standing-wave combined FUT that can change the shape of the focal region,especially the ratio of the long axis to the short axis,and investigated the change law of the sound field under different structural dimensions in order to find the best structural parameters of the transducer.PurposeIn order to accurately design and optimize the FUT which can improve the focal region morphology,based on the drum standing-wave transducer,the acoustic pressure distribution and the changes of focal region morphology of the transducer under different structural parameters were studied.The structural parameters of the transducer were determined to improve the shape of the focal region and shorten the ratio of the long axis to the short axis.At the same time,a FUT with a focused angle greater than 180° was designed and manufactured.Methods1.On the basis of the theory of travelling-wave focusing,the theoretical formula for calculating the sound field of a transducer containing travelling-wave and standing-wave in the radiation sound field is derived,and the distribution of the sound field and the dimension of the acoustic focal region under different structure parameters are calculated.2.The COMSOL software was used to establish the simulation model,and the transducer was designed as a drum standing-wave FUT?focused angle 360°?.The variation trend of the acoustic pressure distribution and the shape of the acoustic focal region at different table heights was studied in order to find the optimal table height parameters.On this basis,the focused angle of the transducer is changed,so that the focused angle of the transducer changes in the range of 90°²70°and the interval is 30°.The variation trend of sound pressure and morphology of acoustic focal region are studied when the focused angle changes.3.A drum FUT is designed and manufactured based on the simulation model.The electrical performance is tested with an impedance analyzer,and the standing-wave acoustic field,travelling-wave and standing-wave confocal acoustic field of the transducer were measured by the focused ultrasound acoustic field scanning system.Finally,the theoretical,simulation and experimental results are compared and analyzed.Results1.Under linear conditions,the opening part of the drum standing-wave transducer was regarded as a hole by using Rayleigh integral on the basis of theoretical calculation of acoustic field of the concave spherical FUT with a hole in the center,and the upper and lower limits of the integral of the transducer were changed.,the theoretical calculation formula of the transducer in the three-axis direction was deduced.Through the numerical calculation with MATLAB,the acoustic field distribution formed by the traditional travelling-wave focused and the standing-wave focused was compared,and it was found that the focused accuracy of the standing-wave transducer could be improved by more than 7 times compared with the travelling-wave transducer.2.The COMSOL Multiphysics simulation software was used to simulate the drum standing-wave transducer.It was found that the acoustic focal region formed by the transducer gradually tended to be round from the long ellipsoid.When the table height is?1.4F_geo,the acoustic focal region dimension could reach the sub-wavelength scale.Selecting the height of the ball table 1.6F_geo and change the focused angle of the transducer,when the focused angle is?210°,the acoustic focal region dimension formed by the transducer could reach the sub-wavelength scale,and the ratio of the long axis to the short axis of the focal region of the main acoustic radiation surface is less than 2.3.According to the above theoretical calculation and simulation results,a drum FUT that could change focused angle was designed and manufactured,with an inner diameter of 160mm and a table height of 128mm(1.6F_geo),and the frequency of the piezoelectric ceramic array elements of the transducer was measured between 986±8KHz.Under the same focused angle,the maximum difference of resonance frequency in different sound-producing areas of the transducer is no more than 5KHz,and the maximum difference of impedance is no more than 3?.The experimentally measured focal region dimension is.073?×.060?×.066?when the transducer focal angle is 360°,and the focal region dimension at focal angle of 210°,240°,and 270°are0.1?×.07?×.093?,.087?×.06?×.097?,.08?×.053?×.092?,respectively.The results show that when the focused angle is?210°,the ratio of the major axis to the minor axis of the main acoustic radiation surface is less than 2,and the focal region dimension reaches the sub-wavelength scale.The actual measured values are similar to theoretical calculations and simulation results.ConclusionsAlthough the theoretical calculation formula of sound field derived is carried out under linear conditions,and the sound waves reflected from the acoustic emission surface of the transducer are not included in the calculation,the average deviation of the dimension of acoustic focal field is less than 5%compared with the results of COMSOL finite element simulation.This shows that under the condition of linear sound field,using Rayleigh integral method,it is effective to calculate the transducer containing this type of standing-wave sound field by theoretical formula.Then,the COMSOL software was used to establish the finite element simulation model of the drum-shaped standing-wave transducer,and the change trend of the sound field distribution and the acoustic focal region dimension of the transducers with different table heights were obtained.When the table height is 1.6F_geo and change the focal angle of the transducer,the focal region dimension decreases from.092?×.087?×.321?to.082?×.047?×.072?,the ratio of the long axis to the short axis of the main acoustic radiation surface's focal region is reduced from 3.69 to 1.53,When the focused angle is?210°,the focused precision can be improved to the sub-wavelength scale.Finally,through theoretical calculation,simulation and experimental verification,it is shown that the optimal table height and the optimal focused angle of the travelling-wave and standing-wave combined transducer with the focused angle?180°is 1.6F_geoeo and the optimal focused angle should be?210°.Compared with the travelling-wave transducer,the focal region morphology of the transducer can be significantly improved,and the focused accuracy can be improved to the sub-wavelength scale.