| Magnetic particle imaging(MPI)is crucial for biomedical applications such as magnetic labeling metabolic imaging,targeted transport and release of magnetic nanomedicine,and magnetic hyperthermia of tumor,etc.In MPI,magnetic nanomarkers are the only source of signal,and the spatial distribution of magnetic nanomarkers can be obtained by field-free point movement and weak magnetic measurement.However,magnetic nanoparticles outside the zero-field region can produce a non-negligible magnetization response,which has been a key bottleneck to further improve the imaging quality.In the dissertation,a magnetic particle imaging method based on the imaginary part of complex magnetic susceptibility is explored to solve this problem.The response of magnetic nanoparticles outside the region is suppressed to improve the spatial resolution and contrast of magnetic particle imaging.Aiming at the systematic crosstalk from spatial positioning magnetic field to concentration measurement,which is unique in imaginary part-based magnetic particle imaging,a data processing method under special excitation magnetic field is studied to reduce the imaging error.The main work of the dissertation is as follows:Aiming at the problems of low resolution and low image contrast in magnetic particle imaging,an MPI method based on imaginary part information was established to improve the resolution and contrast of MPI.Simulations and experiments show that the full width at half maximum(FWHM)of the point spread function(PSF)based on the imaginary part is narrower than that of the PSF based on the modulus and real part,and the tail of the PSF converges faster.If MPI is realized based on the imaginary part of magnetic susceptibility,image resolution can break through the resolution limit,which equals the FWHM of the Langevin function derivative in the classical MPI method,and image contrast is enhanced.Aiming at the unique systematic crosstalk in imaginary part-based magnetic particle imaging,which is transmitted from spatial positioning magnetic field to concentration measurement,a segmented averaging method under cosine excitation magnetic field is studied,and the false phase response in imaginary part-based magnetic particle imaging is successfully solved.This method builds a segmented averaging model based on the magnetization signal under forward and reverse scanning,and eliminates the false phase response introduced by the ramping focus field.The segmented averaging method can suppress the imaginary part-based imaging error caused by the false phase and improve the imaging quality.On this basis,the parameters of the focus field are optimized,and the imaging rate of imaginary part-based one-dimensional magnetic particle imaging is increased from 2 fps to 80 fps.Aiming at the problem of low signal-to-noise ratio(SNR)and low resolution in the implementation of the instrument system,a signal processing scheme with high SNR is proposed by combining the resonant filter circuit,the filter based on digital phase-sensitive detection and the segmented magnetic susceptibility extraction algorithm with variable duration.It is found that this method can filter noise and interference out of the signal frequency components,overcome the potential risk of clipping distortion in magnetic susceptibility extraction,improve the SNR,and avoid the reduction of spatial resolution.The point spread function with high SNR can be obtained by combining the gradient-free method with the above-mentioned signal processing method.On this basis,image deconvolution is studied under different gradient strength,and the results show that image deconvolution can suppress image noise and improve spatial resolution.In order to verify the resolution of the magnetic particle imaging device,a performance test experiment was designed.The test results of the MPI device based on the imaginary part of magnetic susceptibility show that the x-direction resolution of the imaging device is2.5 to 3 mm,and the z-direction resolution is about 2 mm.The x-direction resolution can be increased to 1 mm after using the deconvolution algorithm.In addition,the dissertation also verifies that the complex susceptibility-based MPI device can distinguish magnetic nanoparticles with different particle sizes and simultaneously realize the concentration imaging of different particles,which has the potential to realize color magnetic particle imaging. |
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