HIFU Non-destructive Temperature Monitoring And Efficacy Evaluation Based On Magnetic Induction And Magnetoacoustic Detection

High intensity focused ultrasound_HIFUis a new technique for noninvasive tumor therapy in vitro,of which the real-time temperature monitoring of the target and the treatment efficacy evaluation is the key.Magnetoacoustic tomography with magnetic induction_MAT-MIis a bio-tissue electrical impedance tomography modality,which can detect the conductivity difference between liver tumor and the surround healthy tissue.In the actual HIFU treatment,resistance changes arise in the occurrence of thermocoagulation.The great difference of electrical impedance before and after the treatment provides a good physical basis for the application of MAT-MI into HIFU therapy to realize precise noninvasive temperature monitoring and treatment efficacy evaluation.Due to the focused ultrasound and the heat conduction effect,the conductivity of the HIFU focal region varies gradually in space because of increased temperature,which should be considered as a conductivity gradual-varying model.In this paper,through combining the electrical impedance distribution of the focal region with MAT-MI,a new method of noninvasive temperature monitoring and treatment efficacy evaluation for HIFU therapy using MAT-MI is innovatively proposed.Firstly,based on the electromagnetic,acoustic and acoustic dipole radiation theory,the relationships between MAT-MI source,pressure and waveform of the conductivity gradual-varying model and the size of conductivity transition zone_SCTZare studied through theoretical analysis and numerical simulation in this paper.It shows that tissue can be regarded as the conductivity abrupt-varying model only when SCTZ is small and the contribution of inner source to MAT-MI signal can be ignored.Otherwise,it is necessary to consider both the contribution of the inner source and boundary source to MAT-MI signal and meanwhile,the wave clusters generated by the variation of the conductivity gradient will also be mixed with each other.Then,based on homogeneous Helmholtz equation,Pennes equation and the temperature-impedance variation factor_TIVF,the distributions of sound,temperature field and the electrical impedance of focal region exerted by the HIFU transducer with the same shape and size under four different acoustic powers are obtained and the according MAT-MI signals can be achieved.Due to the gradual-varying conductivity of the focal region,the received pressures are dominated by the inner pressure and the derivatives of the wave cluster peaks at the corresponding distance from the focal point to the magnetoacoustic transducer with regard to the treatment time receive an extremum.At the same time,the temperature of the focal region in the range of about ± 0.46 mm in radial direction and ± 2.2 mm in axial direction all reach 69 ? with the highest temperature about 80.2 ? under four different acoustic powers and the thermocoagulation occurs,which can be seen that the therapeutic efficacy of HIFU is fully achieved.As a result,the derivative variation of the wave cluster peak caused by the electrical impedance variation of focal region can be used as a real-time parameter for noninvasive temperature monitoring and treatment efficacy evaluation for HIFU therapy using MAT-MI to actualize precise dose control by observing the derivative extremum.The favor results provide a new idea and method for the noninvasive temperature monitoring and treatment efficacy evaluation for HIFU therapy.