Study On The High-frequency Ultrasonic Field Characterization Based On The Spatial Averaging Correction Of Hydrophones

Characterization of high-frequency ultrasound field is the need for the development of advanced medical equipments and other strategic emerging industries.Hydrophone is the device to detect the acoustic pressure which is widely used in the field measurement of medical ultrasound equipments.With the development of the resolution and safety index in the application of high-frequency medical ultrasound equipments,there are more and more demands on the measurements of precison and frequency bandwidth for ultrasonic field parameters.According to the development demands of the medical ultrasound applications,the frequency should be extended to at least tens of Meghertz.With the increasing frequency,the required hydrophone diameter should be smaller and reach to the micrometers dimensions,which inevitably reduces the sensitivity further and increases the difficulty in the production process.On the other hand,the ultrasonic power is more lower that reaches the limit of existing estimating methods,and the valid estimating methods for radiation force are absent at the same time.This is a challenge for high-frequency ultrasound field characterization based on hydrophones.In this research "Study on the high-frequency ultrasonic field characterization based on the spatial averaging correction of hydrophones",on the basis of investigating the spatial averaging correction and sensitivitty calibration techniques of the hydrophone,the quantitative estimating method of high-frequency ultrasonic power and radiation force is studied.The main works are as follows:In order to solve the problem that the spatial averaging effects of the hydrophone cause the underestimation of ultrasonic pressure,and the characterization model is insufficient,the Rayleigh integral-Fresnel approximation model is put forward to account for it.It uses the Rayleigh integral to express the ultrasonic pressure and integral over the effective diameter of the hydrophone,and then utilizes series expansion and the Fresnel approximation to derive the analytical expression of the averaged ultrasonic pressure.The spatial averaging effects on ultrasonic pressure and beamwidth on the acoustic axis and off-axis of planar and focused transducers are analyzed separatedly.After that,this research extends the results to attenuating fluids and discusses the validity zone of the Fresnel approximation and the Rayleigh integral for focused transducers.As a result,the spatial averaging correction models of hydrophones for characterizing the high-frequency ultrasonic field are complemented,which lays a solid foundation for the precise measurement of ultrasonic pressure and beamwidth in the dynamic positions feedback scanning system.In order to solve the problem that the low frequency vibration causes the working point of the homodyne interferometer drifts,which results in signal fading and can not achieve credible calibration results over 40 MHz,this research proposes the sensitivity calibration method based on heterodyne interferometer.On the basis of analyzing the uncertainty budgets,for example,the nonlinearity error in the interferometer,the frequency response of the photodiode and the spatial averaging effects of the hydrophone in nonlinear ultrasonic fields et al,it utilizes the acousto-optic modulator to shift the optical frequency and processes the carrier signal based on the digital quadrature demodulation technique after high-speed sampling.This heterodyne method can avoid the working point drifts and feedback system oscillates probem in the homodyne calibration system,and extends the sensitivity calibration capability up to 60 MHz.The calibration method firstly analyzes the theory of reconstructing ultrasonic pressure based on particle vibration,and the relationship between the pellcile vibration displacement/velocity and ultrasonic pressure is revealed.Then the following performance of the pellicle to ultrasound is analyzed,and the strict model characterizing the pellicle thickness and pressure reduction is derived to correct the influence of pellicle thickness at different frequencies.The pellicle is placed on the water surface,then the heterodyne interferometer is used to measure the pellicle vibration displacement to reconstruct ultrasonic pressure.This experimental arrangement can avoid the acousto-optical interaction in the common-path setup.In order to solve the problem that the weak power of high-frequency ultrasound reaches the limit of existing estimating methods,and there is an absence of valid techniques for estimating the ultrasonic power and radiation force,on the basis of the spatial averaging of the hydrophone being corrected and the sensitivity being calibrated,this research puts forward a planar scanning ultrasonic power measurement method based on the dynamic positions feedback of the hydrophone,and a combined method based on measurement and modelling for radiation force.The method firstly uses the heterodyne interferometer to calibrate the hydrophone sensitivity,then the spatial averaging effects are corrected.In the high-frequency focused ultrasound,the ray acoustic theory is used to model the three-dimensional radiation force on particles.The scanning system is established to measure the ultrasonic power and beamwidth,which are then used in the radiation force model to sovle the quantitative problem for high-frequency radiation force.Based on the above research,the planar scanning system for ultrasonic field characterization and the hydrophone sensitivity calibration system based on heterdoyne interferometer are established,including the acousto-optic frequency shifter,the carrier signal demodulation module and the amplitude frequency response calibration for photodiodes et al.On the basis of the above modules,experiments are made to validate the proposed methods for the spatial averaging correction,the hydrophone sensitivity calibration and the ultrasonic power measurements.Experiments show that the proposed hydrophone sensitivity calibration system based on the heterodyne interferometer can extend the calibration capability up to 60 MHz with an expanded uncertainty budget better than 9.8%;in the established planar scanning system for ultrasonic field characterization,the resolution for ultrasonic beamwidth can be 0.1 ?m;the ultrasonic power can be measured in the calibrated frequency range of the hydrophone,and the lower limit can be smaller than 1 m W;the radiation force of n N level can be quantitatively analyzed.