Research on Magnetic Particle Imaging Zero Magnetic Field Based on Permanent Magnet and Coil Composite Structure
In the magnetic particle imaging(MPI)process,due to different topologies of the selection field and the driving field,significant differences exist in the zero magnetic field region,which has a key influence on the MPI imaging effect.This paper proposes an MPI topology based on the permanent magnet and coil composite structure for large aperture,low power consumption,and high resolution.The feasibility of its application in 3D imaging is verified.The selection magnetic field structure of the MPI device is designed based on the Maxwell configuration.When the excitation current is symmetric,the field-free line(FFL)is distributed along the z-axis;otherwise,the FFL is translated along the x-axis.The driving coil set is designed based on the Helmholtz structure.When the AC excitation signal is input,the driving magnetic field is superimposed with the selection magnetic field,and the FFL is driven to scan along a certain plane perpendicular to the x-axis.Secondly,COMSOL simulation software simulates the magnetic field generated by the composite device,and the effects of current excitation,magnet material,and spatial position on the magnetic field are calculated to determine the maximum scanning range and trajectory of the FFL.Finally,the effects of magnetic field gradient and particle size on the image reconstruction results are analyzed.In the layered scanning simulation,due to the small inter-layer distance,the coupling of signals between adjacent layers is considered in the image reconstruction.The simulation results show that the structure produces a linear magnetic field gradient of 3.075 T/m and 3.064 T/m in the x and y directions.The scanning area of the magnetic field reaches 30 mm×70 mm×70 mm,and the magnetic field uniformity in the scanning area reaches more than 98.7%.MPI simulation and reconstruction using 3 T/m,2 T/m,and 1 T/m magnetic field gradients show a positive correlation between the gradient and the imaging quality.Finally,the 3D vascular model is imaged by layers.The minimum structural similarity(SSIM)value of the vascular model data index of each layer is 0.939 5,and the maximum root mean square error(RMSE)value is 0.039 6.The following conclusions can be drawn.(1)The magnetic field generated by the proposed topology has good uniformity,and the scanning trajectory of FFL can be kept in the same plane.The proposed topology can realize hierarchical scanning compared to the traditional electromagnetic coil structure.(2)The image reconstruction results show that the designed magnetic field realizes multi-layer two-dimensional imaging in space,and the maximum resolution reaches 1 mm.A higher magnetic field gradient is more conducive to improving MPI imaging quality.(3)A 3D model is used for 2D layered scanning,and the influence of signal between adjacent layers is considered,which is helpful for MPI 3D imaging study.
Magnetic particle imagingpermanent magnet and coil compositefield-free linemagnetic field gradientmagnetic field uniformity
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