Magneto Acoustic Tomography Method Using Low Frequency Magnetic Excitation And Active Acoustic Detection

Malignant tumors are the major diseases that seriously threaten human health and social development.Recently,malignant tumors have shown an ascending incidence globally,and become the leading cause of death in China.Studies have shown that in the early stage of tumor development,the electrical conductivity of tumor tissue is increased due to vascular proliferation and cellular structural changes.Therefore,early diagnosis of malignant tumor by measuring its conductivity may be possible.Magneto-acoustic tomography with magnetic induction(MAT-MI)has been proposed as a novel non-invasive biological tissue conductivity imaging method.This method conducts a pulse to excite tissue to vibrate in a static magnetic field to generate ultrasound waves,and then acquire the acoustic pressure signal to reconstruct the tissue conductivity distribution.To obtain spatial resolutions in the millimeter range,the excitation pulse should be at a high voltage(in range of k V)and a short time(in range of ?s).MAT-MI suffers from several limitations: energy loss during short pulse excitation is very high,leading to the equipment being very large,heavy and costly,intense interference of the ultrasound receiver and risk of patient exposure to high voltage.Further,each excitation impulse produced a small part of data in all direction,leading to extremely long acquisition time.Due to the passive reception,the received signal is also easily interfered by the high-frequency electromagnetic fields.To address these limitations,we propose an active detection method in magnetic acoustic tomography,based on ultrafast color Doppler detection of the Lorentz force induced motion.Active detection allows the substitution of the high voltage impulse excitation with a low frequency waveform,greatly increasing the matching between the energy expended and energy converted by Lorentz force induction.Active detection also provides spatial resolution proportional to transmitted ultrasound frequency and independent of the magnetic induction waveform.Ultrafast imaging capability available from companies such as Verasonics,the raw data for a full resolution 2D image frame can be acquired from a single magnetic excitation pulse,giving a potential to greatly increased imaging speed and high spatial resolution simultaneously.In this thesis,a numerical electromagnetic-solid mechanics model was created based on Maxwell's equations and Navier-Stokes equations.The numerical simulation study was carried out using COMSOL Multiphysics finite element simulation software.A two-dimensional simulation model in time domain was built.A number of various size and shape conductive regions were designed into various models.The relation between magneto-acoustic vibration and factors such as the shape,volume and strength of the high conductivity region were investigated.A magnetic excitation subsystem was designed and optimized.Several coil designs and parameters were investigated using numerical analysis.The driving electronics were also tested and improved.Active detection was implemented on the Verasonics system,and many parameters were tested and optimized.Imaging phantoms with various embedded conductive regions were fabricated and tested by an impedance analyzer;Different types of phantom experiments were then carried out to verify the feasibility of our imaging method,also,to prove the correctness of the simulation results.Results:(1)the amplitude and shape of the tissue vibration caused by the magnetoacoustic effect are related to the volume size,conductivity value and conductivity boundary shape of the anomalous conductivity region in the tissue;(2)The propagation of shear wave can be obtained by the color Doppler detection technique,however,this phenomenon can only be seen clearly while the Lorentz force is strong enough.Conclusion: low frequency,low voltage excitation combined with active acoustic detection has been successfully demonstrated to image weakly conducting soft-tissue phantom with a spatial resolution in the range of mm and a time scale of ms.The feasibility of reconstructing the internal conductivity distribution of tissue with Low Frequency Magnetic Excitation and Active Acoustic Detection Magneto Acoustic Tomography Method was proved.