Research On Forward Problem Of Imaging Of Magnetic Nanoparticles Based On Passive And Active Magnetoacoustic Effect

Magnetic nanoparticles concentration imaging based on magnetoacoustic effect is a highresolution,non-invasive magnetic nanoparticles number concentration imaging method,which uses the coupling effect of electromagnetic and acoustic multiphysical fields and has the characteristics of both electromagnetic technology and ultrasonic technology.In order to make the magnetoacoustic tomography method get in-depth research and application in the field of magnetic nanoparticles in vivo imaging,in this thesis,in view of the shortcomings of magnetoacoustic concentration tomography of magnetic nanoparticles with magnetic induction(MACT-MI,passive)in the forward problem theory,the forward problem of the MACT-MI under saturation magnetization state and ultrasound induced magnetic nanoparticles concentration imaging(UICI,active)were studied.Based on the existing MACT-MI theory,the forward problem theory of MACT-MI when magnetic nanoparticles were in saturation magnetization state was studied for the first time.Firstly,according to the classical Langevin theory of paramagnetism,the saturation magnetization of magnetic nanoparticles was studied,and the force formula of magnetic nanoparticles under saturation magnetization state was derived.The Maxwell-Helmholtz coils with double coil structure was designed and analyzed.Then,in the theory of sound field forward problem,the active linear sound pressure wave equation of magnetic nanoparticles in saturated magnetization state was established,and the integral analytical solution of Green's function was given.Based on this,the sound pressure amplitude of spherical model was estimated to be within the measurable range,which verified the feasibility of the proposed method in theory.Finally,the numerical simulation model was constructed,and the two-dimensional distribution of magnetic force and sound pressure and the corresponding one-dimensional curve were obtained;compared with the unsaturated magnetization state,the magnetic saturated magnetic nanoparticles were forced more under the external magnetic field,which could excite stronger magnetoacoustic signal,which has guiding significance for solving the problem of weak magnetoacoustic signal in the experimental process.Based on the inverse mode of MACT-MI,the forward problem of UICI was proposed.Firstly,the mathematical model of the motion of magnetic nanoparticles was established to solve the sound field forward problem;secondly,the assumption conditions of quasi-static approximation and time independent current distribution were analyzed for the forward problem of electromagnetic field.Based on the basic theory of electromagnetic field,the calculation formula of induced voltage of receiving coil based on reciprocity theorem was derived;thirdly,the acoustic-electromagnetic coupling field analysis was carried out,starting from the small imaging target,and then extended to the coupling field analysis in general volume,and the induced voltage was connected with the number concentration of magnetic nanoparticles;finally,the UICI forward problem numerical simulation model was constructed,and the sound field generated by ultrasonic transducer under the specific voltage excitation was analyzed.The simulation results showed that the magnetized magnetic nanoparticles could cause the change of magnetic flux through the receiving coil when the sound field moved,thus sensing voltage,and the amplitude of the induction voltage was positively correlated with the number concentration of magnetic nanoparticles,the validity of the formula of induction voltage calculation based on reciprocity theorem and the correctness of imaging theory were verified;the imaging target was placed on the axis of the far sound field of ultrasonic transducer,and better induction voltage signal could be obtained by changing the main frequency of ultrasonic transducer,modulating the voltage excitation waveform or changing the radius and position of the receiving coil,so as to improve the imaging effect.The research content made up for the deficiency of the theory of magnetic nanoparticle concentration imaging based on magnetoacoustic effect,and provided the theoretical basis for further imaging research.This thesis has 42 figures,2 tables and 73 references.