Study On Compressing Axial Length Of Acoustic Focal Region Using Variable Thickness Focused Ultrasound Transducer

Background:Thermal effect of ultrasound is mainly useed in HIFU to cause irreversible coagulative necrosis of the diseased tissue without damaging the surrounding normal tissue,thus achieving the goal of treating diseases.Single-frequency concave spherical shell focused ultrasound transducers,whose acoustic focal region(AFR),is ellipsoidal are mostly used in clinical treatment.The ratio of the axial length to the lateral length of the transducers is usually above 6.In clinical treatment,the safety can not be guaranteed because of some problems such as damage to the normal tissue around the lesion,skin burned,etc.Therefore,the rational design of the axial length of the acoustic focal domain of the focused ultrasound transducer is a prerequisite for safe and effective treatment in HIFU.Objective:When Conventional single-frequency concave spherical shell focused ultrasound transducers are used to treat the lateral thin layer of the lesion,the normal tissues outside the lesion will be damaged easily,which brings a major safety risk.Therefore,in order to improve the safety and efficacy of HIFU in the clinical treatment process,compressing the axial length of the acoustic focal area is of great importance.Methods:1.Based on the principle of multi-frequency ultrasonic superposition and the principle of axial length change in the acoustic focal field,a variable thickness(multi-frequency)focused ultrasound transducer was proposed,concave spherical shell and lens variable thickness focused ultrasound transducers were designed2.According to the Rayleigh integration principle,the calculation method of the radiated sound field of the variable-thickness focused ultrasound transducer was derived.Under different frequency differences,the axial length of the acoustic focal region of the two types of variablethickness focused ultrasound transducers was theoretically calculated and analyzed,and compared with the equal thickness transducer.In addition,the change of sound pressure with time at a certain point in space was analyzed.3.Variable thickness and equal thickness transducers of the lens were processed,and the sound fields of the two transducers were measured with a hydrophone.In addition,it was compared with the previous isolated bovine liver experiment of the research group.Results:1.Two types of variable thickness focused ultrasound transducers,concave spherical shell and lens,were designed,and the calculation equation of the sound field of variable thickness focused ultrasound transducer was derived.2.Numerical simulation results of concave spherical shell variable thickness focusing transducer: When the frequency difference was 0(equal thickness),50,100,150,and 200 kHz,the axial length of the acoustic focal area at-6dB was 9.5,5.6,3.9,3.2,and 2.7mm.Under the frequency difference,the axial length of the acoustic focal region of the concave spherical shell variable thickness transducer was compressed,and decreased as the frequency difference increased.The waveform whose sound pressure changes with time at a certain point in space was no longer a sine(cosine)waveform.But it was regular,and not only had a period,but also had maximum,and minimum.3.Numerical Simulation Results of Lens Variable Thickness Focused Ultrasonic Transducer: when the frequency difference was 0(equal thickness),50,100,150,and 200 kHz,the axial length of the acoustic focal area at-6dB was 15.7,7.1,4.4,3.5,and 2.9 mm.Under the frequency difference,the axial length of the acoustic focal region of the variablethickness lens transducer was compressed and decreased as the frequency difference increased.The change of sound pressure with time at a certain point in space was the same as the variable thickness of concave spherical shell.4.The sound field measurement results of the manufactured transducer and the experimental results of the isolated bovine liver injury in the early stage of the research group both showed that the theoretical calculation is consistent with the experimental results.Conclusions:1.Two types of variable-thickness focused ultrasound transducers are designed,and the calculation equation of the radiated sound field of variable-thickness focused ultrasound transducers is derived.2.The trend of the numerical simulation results of the two types of variable thickness focusing transducers is the same.Compared with the constant-thickness focusing transducer,the variable-thickness focusing transducer can effectively compress the axial length of the acoustic focal region,which decreases with the increase of the frequency difference;there is a regularity of the waveform of the sound pressure with time at a certain point in space.3.The sound field measurement results of the manufactured transducer and the experimental results of the isolated bovine liver injury in the early stage both show that the theoretical calculation is consistent with the experimental results,which proves the correctness of the sound field calculation equation of the variable-thickness focusing transducer.