Magnetoacoustic tomography with magnetic induction (MAT-MI) is a multiphysics imaging technique that combines electrical impedance imaging with ultrasound imaging. In order to study the influence of parameters on the source of MAT-MI, such as radius and permeability of magnetic nanoparticle clusters, the paper is divided into the following stages. Firstly, this paper analyzes the electromagnetic and acoustic properties of MAT-MI after adding magnetic nanoparticles. Secondly, to determine suitable simulation conditions, a two-dimensional model is constructed. Thirdly, use the finite element method to solve physical processes of electromagnetic field and acoustic field under conditions of different magnetic nanoparticle clusters' radii and permeabilities, then obtain the magnetic flux density image. Consequently, make the qualitative and quantitative analysis according to the theory and simulation results. The results show that magnetic nanoparticle clusters interact with each other and distort the magnetic field to different degrees; its radius increases with the degree of flux density distortion around it, so does its permeability and magnetoacoustic signal intensity. The research results can play a guiding role in the parameter selection of magnetic nanoparticle clusters in practical applications to a certain extent.

Magnetoacoustic concentration tomography of magnetic nanoparticles (MNPs) with magnetic induction (MACT-MI) is a method that has been propsed recently for MNP concentration tomography for use in targeted therapy. This procedure has the advantages of being noninvasive, having high-resolution and providing quantitative estimation of MNPs. In order to further fundamentally increase the signal-to-noise ratio of the magnetoacoustic signal, the forward problem of MACT-MI in the saturation magnetization state was studied, especially the magnetization characteristics of MNPs, including the derivation of the force formula of MNPs in the saturation magnetization state and the relationships among the gradient of the external magnetic field, magnetic force, number concentration and sound pressure. Firstly, the magnetic force formula of MNPs in the saturation magnetization state was derived according to the classical Langevin theory of paramagnetism. Then, a 2D model was constructed to calculate the magnetic flux density, magnetic force and sound pressure. Finally, the 2D distribution diagram of magnetic force and sound pressure and its corresponding 1D curve were obtained. The simulation results showed that the saturation magnetic field produced by the Maxwell–Helmholtz coils made the MNPs magnetically saturated. Compared with the MNPs in an unsaturated magnetization state, the magnetic force of MNPs in the saturation magnetization state was stronger, and the stronger magnetic force could generate stronger magnetoacoustic signals, which made it possible to measure magnetoacoustic signals with higher signal-to-noise ratio. The research results can provide a research basis for the design of equipment for further MACT-MI experiment and clinical application.

No abstract

Magnetoacoustic concentration tomography with magnetic induction (MACT-MI) is a noninvasive imaging method that reconstructs the concentration image of magnetic nanoparticles (MNPs) based on the acoustic pressure signal generated by the magnetic properties of MNPs. The performance of MNPs is of great significance in MACT-MI. To study influences of the uniaxial anisotropy of MNPs on MACT-MI, firstly, based on the static magnetization curve, the force characteristic that the MNPs with uniaxial anisotropy experienced was analyzed. The magnetic force equation with the space component separated from the time term was deduced. The acoustic pressure equation containing the concentration of the MNPs with uniaxial anisotropy was derived. Then, a two-dimensional axisymmetric simulation model was constructed to compare magnetic force, acoustic source, and acoustic pressure before and after considering the uniaxial anisotropy of MNPs. The effect of scanning angle and detection radius of ultrasonic transducer on the acoustic pressure was studied. Finally, the concentration image of the MNPs with uniaxial anisotropy was reconstructed by the time reversal method and the method of moments (MoM). Theoretical considerations and simulation results have shown that the magnetic force has a triple increase after taking into account the uniaxial anisotropy of MNPs. The take-off time of acoustic pressure waves is only related to the position of the uniaxial anisotropy MNPs region. From the reconstructed image, concentration distribution and spatial location and size information of the uniaxial anisotropy MNPs region can be distinguished. The research results may lay the foundation for MACT-MI in subsequent experiments and even clinical applications.