HIFU Non-invasive Temperature Monitoring And Efficacy Evaluation Based On Magneto-acoustic-electric Detection Technology

High intensity focused ultrasound(HIFU)thermal therapy is a new non-invasive tumor treatment approach,which has been developed rapidly in recent years.Temperature monitoring in the target area is the key issue.However,the resolution and contrast of temperature measurement used clinically are still limited in currently.It was found that the temperature and electrical conductivity in biological tissue have a certain relationship.With increasing temperature,the electrical conductivity of biological tissue increases under HIFU.Especially,when the temperature reaches the threshold of thermocoagulation,irreversible coagulative necrosis occurs to achieve the purpose of tumor treatment while the conductivity of tissue mutates at the same time.Therefore,the temperature can be monitored indirectly using the detection of the conductivity of biological tissues.Among a number of conductivity detection methods,magneto-acousto-electrical(MAE)measurement is a new approach based on the conductivity variation.Based on the theory of acoustic propagation and electromagnetic induction,it combines acoustic,magnetic and electric field that not only has the advantage of high resolution of ultrasonic imaging but also has high contrast characteristic of electrical impedance tomography(EIT).In this study,MAE nleasurerment was applied to HIFU therapy to monitor the temperature variation and evaluate the curative effect.First,based on the ultrasonic vibration and propagation theory,Hall effect and magneto-electrical induction,the MAE voltage explicit formula was conducted by the excitation of the plane piston transducer and the plate receiving electrodes.A 1D conductivity mutation model was established to simulate the layered biological tissue with different conductivity and the MAE voltage signal was estimated.It turns out that the amplitude of signal is determined by the conductivity gradient along the direction of acoustic propagation,the sound pressure and the static magnetic.The polarity of the signal can reflect the direction of conductivity gradient.To illustrate the relevance between the signal and conductivity distribution,single layer gel whose conductivity is similar to biological tissue was tested.Then,the conductivity reconstruction algorithm based on Wiener inverse filter and Hilbert transform was proposed.A 3-layer gel models with conductivity variation were tested and the conductivity distributions had been reconstructed by the new algorithm.Afterwards,the MAE measurement was applied into HIFU thermal therapy.Based on the KZK equation and Pennes equation,a two-dimensional axisymmetric model was established to simulate the nonlinear acoustie and temperature distribution by finite difference method in frequency domain and finite element methods(FEM).The real-time distribution of electrical conductivity in biological tissues during HIFU therapy was obtained based on the temperature-conductivity variation factor of biological tissues.On this basis,magneto-acousto-electrical measurement three-dimensional model was established to simulate the MAE voltage signal during the treatment.The favorable results demonstrate that the MAE signal contains two opposite-polarity wave clusters.The amplitude indicates the temperature changes in target area and time interval of the wave packets indicates the efrective treatment size in tissue.This study proves the feasibility of real-time noninvasive temperature monitoring and therapeutic effect evaluation for HIFU therapy using MAE measurement based on the relationship among the physiological status,temperature and electrical conductivity of biological tissue.