Design And Application Of Magneto-acoustic-electrical Tomography Detection System

Early diagnosis plays an important role in the prevention and treatment of cancer.In the process of cancer growth,although the morphological structure of the tissue does not change significantly,the amount of the internal charge and the spatial distribution of charge vary obviously,resulting in the alterations of the conductivity characteristics of the tissue.Therefore,the detection of the electrical conductivity in tissues is expected to become a robust imaging method for the early diagnosis of biological cancerous tissues.Magneto-acousto-electrical tomography(MAET)is a novel imaging technique that can be used to detect the conductivity distribution in body tissues.The imaging principle is that: an ultrasound pulse excitation wave is injected into a target sample placed in a static magnetic field to vibrate the local particles of an imaging target vibrate.The local particles with positive and negative charges are separated under the Lorentz force generated by a permanent magnetic field and ultrasonic wave that generate a local source of electrical current in tissue.Thereby forms a corresponding electric field distribution under the action of Lorentz force.The magneto-acousto-electrical(MAE)signal can be obtained by the detected voltage signal,and the conductivity distribution of the target imaging body can be reconstructed using a reconstructed algorithm.Even though the MAET method has advantages such as real-time,in-vivo,non-invasive detection,the vibration amplitude of tissue generated by the ultrasonic excitation is very small,and only the microvolt-level MAE signal can be detected on the surface of the electrodes.The magnitude of the intensity of electromagnetic interference(EMI)signal caused by the instantaneous excitation of the probe can reach the millivolt level,and the EMI signals caused by the interface reflection are stronger than the MAE signals,and the MAE signal is susceptible to interference such as contact impedance and external noise.Therefore,the existing MAET detection systems often have drawbacks such as the low detection signal-to-noise ratio(SNR),poor conductivity detection resolution,and inability to quantitatively obtain the conductivity distribution in biological tissues.Based on the above reasons,this dissertation focuses on the construction and application of MAET detection system to solve problems such as shielding of external noise and the removal of the EMI signals.The effects of the frequency modulation period,multi-point focus method,excitation frequency and number of short pulses on the detection resolution of conductivity are discussed.The main research work carried out in this dissertation is as follows:1.Construction and analysis of MAET detection system against external noise and EMI signal.Many effective methods to remove EMI signals and external noise signals were applied in our proposed MAET detection system.Two MAET detection system were designed and constructed to realize the measurement of the discontinuous area of conductivity.The effectiveness of the improved methods,the accurate and repeatable measurement of the MAET detection system were verified by a large number of experiments.Moreover,several methods for improving the conductivity detection resolution are proposed.2.Study on the construction of MAET detection system based on the linear frequency modulation and multi-point focus theory.For the problem of the large power of the instantaneous excitation of the probe,the focus point of the probe has a great influence on the amplitude of the MAE signal,and the accuracy of the conductivity detection is thus low.The method of the linear frequency-modulated MAET imaging based on multi-point focusing is firstly proposed.To avoid the large instantaneous excitation power of the probe,the chirp signal is used as the excitation source.The correctness of the linear frequency modulation theory is verified in experiments.It is demonstrated that chirp-pulse duration is a key factor in the improvement of the imaging resolution of electrical conductivity.3.Researches on reconstruction method based on short pulse MAET detection system and its B-scan reconstruction method.In view of the difficulty of obtaining the conductivity distribution in the existing MAET methods,a MAET distribution reconstruction method using short pulse and its B-scan reconstruction method is proposed.Firstly,based on the COMSOL numerical simulation data and the MAE signals measured in experiment,the verification of the conductivity distribution reconstruction process is carried out.Reconstruction of the relative distribution of conductivity is achieved by de-convolving the received MAE signal and the sound pressure signal of the probe.The numerical simulation on the phantom embedded with high concentration material is carried out and the correctness of the theory and method based on short pulse MAET detection was verified.Secondly,the measurement of the conductivity accuracy and the resolution are conducted with the uniform phantom,layered uniform phantom and pork isolated tissue.Both the MAET detection method using short pulse and 1D reconstruction method of conductivity distribution are demonstrated correctly and feasibly.Finally,a B-scan imaging experiment was performed with the isolated tissue of pork and the contour of pork tissue was obtained.In summary,through the above research,the MAET detection system based on the multi-point focus method and linear frequency modulation theory,and the MAET detection system with chirp pulse stimulation has been successfully built.The problems that the SNR of the MAE signal is low,the resolution of single-point focus imaging is not high and the conductivity distribution and its B-scan reconstruction are difficult to achieve,are partly solved.In this dissertation,the magnetic acoustic imaging method is systematically studied,which has important meaning for the construction and application of MAET.