Magnetic Particle Imaging Technique For On-line Monitoring Of Vascular Morphology

Magnetic Particle Imaging(MPI)is an emerging tracer-based tomographic Imaging technology,which shows high spatial resolution and high sensitivity in small animals.It uses Magnetic nanoparticles(MNPs)which composed of Superparamagnetic iron oxide(SPIO)nonlinear magnetization response to the time-varying excitation field,and then imaging MNPs spatial distribution to medical diagnosis.Vascular imaging as one of the medical imaging,which requests background noise as small as possible during imaging,and the MPI only detect magnetic nanoparticles,human tissue do not generate magnetic signal,so it is very suitable for vascular imaging.In this paper,we study the structure,superparamagnetism,particle magnetization,particle distribution with different particle sizes,and relaxation effect of magnetic nanoparticle based on the principle of magnetic nanoparticle imaging.Then,we show the magnetic particle imaging principle: we use the gradient field generate a free field point(FFP),and within the FFP,MNPs respond the excitation field and generate the nonlinear magnetization signal,without the FFP,MNPs do not respond the excitation field.and then use the receiving coil detect the response induced voltage,which amplitude is proportional to the number of MNPs in FFP,so we can measuring the induction voltage amplitude to obtain the particle concentration distribution,and then drive FFP imaging though whole Field Of View(FOV)by the driving field.Then design one-dimensional and two-dimensional MPI platform based on the principle of magnetic particle imaging,and show the detail hardware implementation of the two-dimensional MPI platform,which mainly include Maxwell coil that used to generate direct current gradient field,solenoid which used to generate the dynamic magnetic excitation field,receive coil which used to receive magnetization signal and cancelation coil which used to remove the excitation signal.In our experiment,two kinds different particle diameter sizes samples were used to simulate the induced voltage of the receiving coil under different particle diameter sizes,and further simulated the induced voltage of the receiving coil under different gradient strength.And the result showed that a larger particle diameter size and a higher gradient field strength would obtain a higher inductive voltage of the receiving coil.Finally,we study the MPI image reconstruction algorithm and the main indicators of MPI.There are two main MPI image reconstruction algorithms: system matrix algorithm and X-space algorithm.This article use the ART System matrix algorithm for reconstructing a ideal two-dimensional FFP phantom,and analyzes the reconstruction performance under different iterations and Signal-to-Noise Ratio(SNR).The results show that higher iteration times and SNR can produce better image reconstruction performance.Then study the main indicators of MPI,spatial resolution,receiver coil sensitivity and receiver coil signal-to-noise ratio.The analysis shows that high gradient magnetic field strength and large particle diameter can improve the spatial resolution of MPI,higher driving field current can improve the sensitivity of the receiving coil,and higher particle concentration can make the SNR of the receiving coil higher.