Study On Signal Detection Method Of Magnetic Nanoparticles Based On Free Magnetic Field

Magnetic particle imaging（MPI）is a new medical imaging method.It has high sensitivity to the distribution of magnetic nanoparticles in space,overcomes the influence of ionizing radiation,and can obtain high-quality images of magnetic particles with high temporal and spatial resolution.The detection accuracy of magnetic nanoparticle signal in MPI directly affects the image resolution.Aiming at the key problem of magnetic nanoparticle signal detection in MPI,a nonlinear magnetization signal detection method of magnetic nanoparticle based on second harmonic is proposed.Firstly,according to the magnetization theory of magnetic nanoparticles,the static selection magnetic field,excitation magnetic field and receiving device required by MPI scanning system are constructed.A selective magnetic field structure that can produce large magnetic field gradient is constructed,which is combined with the excitation magnetic field to form a linear zero magnetic field system.The coupling field of AC magnetic field and DC bias field is designed as the excitation magnetic field to enhance the detection signal energy of magnetic nanoparticles and effectively avoid the thermal damage of a single high vibration AC magnetic field to tissue and nerve.The optimal relationship between AC excitation field strength and DC bias field strength is studied.Secondly,based on the signal analysis in the detection coil,the particle signal processing circuit is designed to separate the harmonic signal in the signal,filter the fundamental frequency information in the detection coil and amplify the particle signal.Several methods of magnetic nanoparticle signal compensation in MPI are discussed.A biplane gradient detection coil is designed to detect the second harmonic signal of particle signal,remove the interference of background signal,and obtain a particle signal with signal intensity amplified by 4 times.The DC bias coil is used to realize the translation and rotation scanning function,optimize the system structure and improve the particle signal strength.Finally,the experimental results show that when the ratio of AC excitation field strength to DC bias field strength reaches 2:1,the second harmonic signal strength of magnetic nanoparticles is the best.The signal-to-noise ratio of particle signal obtained by this method is1.52 times higher than that of the original third harmonic detection method,which effectively enhances the signal-to-noise ratio of particle detection signal.The magnetic nanoparticle signal detection system is used to study the relationship between particle signal and particle size and particle concentration.The particles with particle size of 40 nm and concentration of3-5mol / L are determined as the tracer of the detection system.The particle concentration distribution in the central plane is reconstructed according to the second harmonic and third harmonic signals of magnetic particles,which verifies the feasibility of applying the magnetic nanoparticle signal detection method to the actual MPI system.In conclusion,the magnetic nanoparticle signal detection method based on second harmonic proposed in this paper provides a higher precision signal detection method for MPI system.