| As a new kind of material combining the nanotechnology with traditional magnetic materials,magnetic nanoparticles(MNPs)respond quickly to the external magnetic field.It has unique magnetic properties,such as good biocompatibility,lowremanence and coercivity,specific distribution and non-viral vectors.It has beenwidely used in the field of the biomedical engineering.MNPs accumulation and localization can be realized by a static magnetic field.While,with the excitation of an alternating magnetic field,vibration or non-linear magnetization response can be produced by MNPs based on Brownian relaxation and Neel relaxation effects.Thus,medical imaging using the vibration characteristics and the nonlinear magnetization response play an important role in early detection of tumors.Therefore,based on the mechanisms of Brownian relaxation and Neel relaxation,the nonlinear response of MNPs under the excitation of an external magnetic field and the correspondingapplications in modern medical imaging are studied in this thesis.By taking Brownian relaxation as the main mechanism,the vibrationcharacteristics of MNPs in magnetoacoustic(MA)tomography are studied.First,based on the theory of sound propagation,the physical mechanism of vibration and the MA response characteristics of MNPs are analyzed in an alternating magnetic field.With parameter analyses of the magnetic field(magnetic field intensity,magnetic field gradient)and the MA response of MNPs,a high-precision MA detection system is established using a laser vibrometer.It is proved that,under the excitation of an alternating magnetic field,MNPs magnetize rapidly and vibrate accordingly in a small amplitude at its equilibrium position,producing MA harmonic response.The second harmonic response is linearly related to the concentration of nanoparticles,and is proportional to the square of magnetic field intensity and the gradient of magnetic field.Then,based on the second harmonic MA response of MNPs,a model for MA tomography with magnetic induction(MAT-MI)is established.Finally,numerical simulations for the forward and inverse problems of MA pressure and image reconstruction prove the feasibility of MAT-MI based on the second harmonic MA response.By considering Neel relaxation as the main mechanism,magnetic particleimaging(MPI)is studied based on the nonlinear re-magnetization behavior of superparamagnetic nanoparticles using the controllable magnetic field.First,bycombining the nonlinear magnetization response of magnetic particles based on the law of electromagnetic induction,the voltage signal is obtained.Then,by considering the sensitivity of the receiving coil,the voltage signal shows a linear relationship with the concentration of magnetic particles.The fast Fourier transform is used to analyze the influence of the spectrum characteristics and the frequency resolution on the system matrix.Moreover,the influences of the coding number,frequency component selection of the system function and the receiving direction on image reconstruction are studied systematically.It is proved that,the spatial structure of the system function can be optimized by selecting the high-frequency signals.The imaging accuracy and quality can be improved by increasing the number of the frequency component and reconstructing the matrix using signals in different directions.More frequency components can be used to construct the linear independent equations to obtain a unique solution,and the system matrix reorganized by signals with different receiving directions has a more accurate spatial structure.In conclusion,the characteristics of MA harmonic response and nonlinear magnetization response of MNPs under the two relaxation mechanisms are studied.It can be used to reconstruct the distribution of MNPs in biological tissues,which provides a theoretical foundation for the practical applications of MNPs in early tumor diagnosis and biomedical imaging. |
PDF Full Text Request