Nakagami Parameter Imaging Of Ultrasonic Harmonic Envelopes For Microwave Ablation Monitoring

Microwave ablation is an important means for the therapy of hepatocellular carcinoma.The nonlinearity of biological tissue increased with the accumulation of heat in the ablation process.Based on this feature,the tissue ablated by microwave could be monitored using the quantitative imaging of parameters related to second-harmonic ultrasonic envelope signals to improve the validity of monitoringIn the present study,imaging based on Nakagami parameters of the second-harmonic ultrasonic envelope signals echoed from tissue is proposed to improve microwave ablation monitoring.First,the ultrasonic radiofrequency echo signals are filtered with a high pass filter to yield the second-harmonic radiofrequency components.The values of parameters of Nakagami distribution are estimated based on the windowed envelope signals.Finally,the Nakagami parametric images are synthesized using the window-modulated compounding imaging algorithms.In the experiments,the Nakagami parameters are estimated from simulated fundamental and harmonic envelope signals using the nonlinear ultrasound simulator CREANUIS to evaluate the behavior of different nonlinear coefficient versus Nakagami parameters of fundamental and second-harmonic components.Then,the spectra of second-harmonic radiofrequency signals and Nakagami parameters of second-harmonic ultrasonic envelope signals for the fresh porcine liver samples heated by a temperature-controlled water bath are calculated to evaluate the variations influenced by temperature in real tissues.Finally,second-harmonic ultrasonic envelope signals are obtained from ablating fresh porcine liver samples with a microwave antenna for different time durations,and window-modulated compounding Nakagami imaging is used to evaluate the microwave ablation monitoring performance.The results show that the Nakagami parameters of second-harmonic ultrasonic envelopes are separated and increased with increasing nonlinearity of the heated tissues.The ablated regions regarding the scaling parameter in harmonic B-mode image and shape parameter in harmonic color-coded images can be visualized accurately and effectively than those based on fundamental images.The normalized root mean square errors for long and short axes of oval thermal-lesion in the harmonic Nakagami images over the 5 tests are 0.073± 0.56 and 0.085± 0.74,whose mean errors are 0.052 and 0.090 less than those based on the fundamental images.,respectively.The mean and standard deviation of the measurement error with the harmonic-based parametric images are 6.1 ± 3.1%and 6.9± 3.7%,whose average error are 6.0%and 9.2%smaller than the measurement result in fundamental images.In conclusion,the second-harmonic ultrasonic envelopes are sensitive to the tissue nonlinearity that increases with an increasing temperature.Therefore,window-modulated compounding imaging based on the Nakagami shape parameter of second-harmonic envelope signals can improve the monitoring performance for the area and degree of microwave-induced thermal lesions.The technique can provide a better alternative to the existing methods regarding monitoring and evaluating microwave ablation processes in clinics.