The Research Of Tissue’s Ultrasonic Harmonic Characteristic Using Nakagami Statistics

As the first choice for clinical diseases,ultrasonic imaging technique provides a way to characterize biological tissue.At present,B-mode ultrasonic imaging mainly uses the amplitude information of echo signals reflecting the characteristics of acoustic impedance in biological tissue.However,the harmonic signal of the ultrasonic echo signals can reflect the nonlinear characteristics of the tissue.Combined two aspects above,more accurate organization characteristics could be obtained through the quantitative analysis of the distribution characteristics about the harmonic echo envelope using parameterized probability model.In present study,the relationship between the nonlinear coefficient and parameter values of Nakagami distribution from the envelopes of fundamental and second harmonic signals are investigated to obtain more accurate nonlinear quantitative characteristics of tissue.Simulation experiments and vitro experiments were carried out respectively.The steps of analysis and computation about the experimental datas are following.Firstly,we get the envelopes of fundamental and second harmonic signal separated from the whole echoed RF signals using Butterworth high-pass filter,then the Nakagami distribution parameters of the envelope signals are calculated and compared.The simulation results show that for different values of the nonlinear coefficients,the values of distribution parameters of the fundamental envelope signals are aliasing.On the contrary,the parameter values of second harmonic are mutually separating,especially for the nonlinear coefficients ranged from 3 to 6.However,with the further increase of the nonlinear coefficient,the smaller the difference results in the aliasing of shape parameter.The vitro experiments of three kinds of tissues healthy swines with difference nonlinear coefficients were carried out.Compared with fat tissue,the relative differences of Nakagami parameter ω and μ of the fundamental envelope signals for liver and brain tissue is 7.3%,0.03%and 2.0%,4.3%respectively;however the relative differences of 2nd harmonic envelope signals is 8.3%,19.4%and 7.0%,34.0%,respectively.Compared with the fundamental wave,the features of harmonic distribution parameters separate more prominent in the vitro experiments.The results of vitro experiments present a good consistency with those of simulation versions,which verifies the effectiveness and correctness of the simulation results.In conclusion,the parameter values of Nakagami distribution for the second harmonic envelope signals from different degrees of nonlinearity in tissues are significant different.According to this,we can quantitatively analyze the nonlinear characteristics of biological tissue.This could promote the research of the quantitative diagnosis with ultrasound for the nonlinear properties of biological tissues in clinical applications.