| Magnetic particle imaging(MPI)is a novel dynamic targeted imaging method for the human body.Compared to existing traditional imaging methods such as CT,MRI,PET,and SPECT,it has advantages such as high sensitivity,high temporal resolution,high spatial resolution,and safety.However,the main MPI research teams currently use mostly enclosed magnetic field scanning structures,which seriously limits the clinical application of MPI.In response to this issue,this article designs an open electrical scanning narrowband magnetic particle imaging system,which has the advantages of open imaging space and high-speed electrical scanning imaging.Through image reconstruction in the later stage of MPI,high spatial resolution can be achieved,providing new ideas for the research of open MPI technology.The thesis first discusses the basic principle of MPI,deduces the magnetization curve of Superparamagnetism magnetic nanoparticles based on Langevin function from the magnetization theory of paramagnetism materials,then explains the principle of third harmonic signal generation in MPI,and introduces the basic concepts of various magnetic fields and image reconstruction used in MPI system.Furthermore,the characteristics of the third harmonic of the nonlinear response of magnetic nanoparticles were studied by means of experimental measurement and analysis,and the dependence of the third harmonic on the amplitude and frequency of the AC magnetic field and the size and direction of the DC magnetic field was obtained.The magnetic field structure mode adopted by the open MPI system was determined,and the mathematical model of the third harmonic signal was established by numerical fitting,The theoretical calculation formula for the spatial resolution of the MPI system has been obtained.Secondly,based on the determined magnetic field structure,the entire MPI system is designed.An open spatial positioning magnetic field based on fieldless lines was formed using 8 DC coils,and a fieldless electric scanning method was designed.The gradient of the magnetic field and the fieldless electric scanning method were verified through simulation calculation and actual magnetic field measurement,achieving a fieldless electric scanning of a 0.276 T/m gradient magnetic field.The AC excitation coil and detection coil of the MPI system were also designed and validated through simulation calculations and actual magnetic field measurements.The AC magnetic field frequency used in the system was determined to be 20.7 k Hz,and a differential structure detection coil was used to reduce system noise.Further noise analysis was conducted on the system and some methods for reducing noise were proposed.Finally,three samples were made for the MPI system experiment and imaging experiments were conducted.The spatial resolution of the original image was obtained by measuring the standard sample points of sample one and compared with the theoretical values.The system function matrix generated from the imaging data of sample 1(i.e.point spread function)and the original images of sample 2 and sample 3 are used for MPI image reconstruction and the results are analyzed.The results indicate that the current system can achieve 30 mm × 30 mm range MPI imaging,the imaging speed is about 7 second,the original image spatial resolution of the system is about 15 mm.After image reconstruction,the spatial resolution can reach about 2 mm,the similarity can reach above 0.8,and the system can detect 20 μg magnetic nanoparticles of iron at a detection depth of 15 mm.In summary,the open magnetic particle imaging system designed in this project is successful,and the design scheme proposed in this article is feasible and worthy of further system optimization and research. |
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