Study On Fast Magnetic Susceptibility Imaging

Magnetic susceptibility imaging is a magnetic resonance imaging(MRI)technology based on phase information,which has become one of the hotspots in the field of MRI.Susceptibility weighted imaging(SWI)and quantitative susceptibility mapping(QSM)developed in recent years are representative magnetic susceptibility imaging techniques.SWI obtains relevant structural information through phase information(or magnetic susceptibility),which can be applied to the evaluation of iron deposition,traumatic brain injury imaging,and vascular malformation imaging.To quantitatively analyze the magnetic susceptibility of magnetically sensitive substances,QSM was further proposed based on SWI.QSM obtains the susceptibility distribution from the acquired original phase by solving the field-to-source inverse problem,which can be used to analyze the changes of the magnetic sensitive components in tissues.QSM is a powerful tool for detecting diseases,especially for some brain diseases,including Alzheimer's disease,Thalassemia and Parkinson's disease.Multi-shot gradient echo sequence is usually used for SWI and QSM,which takes long scan time.Traditionally QSM reconstruction needs multiple steps.The errors generated in previous steps are easily amplified in the subsequent processing,resulting in significant error in the final susceptibility result.This thesis focuses on single-step QSM reconstruction method and fast SWI method.The main contents include the following three parts:1.The basic concepts,differences and clinical applications of SWI and QSM technologies are systematically introduced.The basic principles and methods of SWI and QSM technologies are described in detail.The post-processing flow of SWI and the implement processes multi-step QSM reconstruction are described,including the representative methods used in field map fitting,phase unwrapping,background field removal,and susceptibility inversion.2.A single-step fast quantitative susceptibility reconstruction method based on weighted Lo norm(Fast TFI)is proposed.The expression for the relationship between the total field and the wrapped phase is introduced into the optimization problem of dipole inversion,the Lo norm is used to constrain the gradient domain,and the susceptibility distribution image is directly reconstructed by the wrapped phase.At the same time,the prior matrix based on R2*,map is introduced to make the estimated susceptibility close to the reference susceptibility,effectively maintaining the details of the tissue structure and suppressing the occurrence of susceptibility artifacts.The reconstruction results of the gadolinium phantom,human healthy brain and pathological brain show that Fast TFI method can improve the reconstruction speed while providing a high quality susceptibility map.3.A fast SWI method based on single-shot spatiotemporal encoding is proposed,in which SWI technique is combined with single-shot segmented spatiotemporally encoded(SeSPEN)sequence,and the images are reconstructed by deconvolution super-resolution algorithm.The experimental results of gadolinium phantom and in vivo rat brain prove that,compared with traditional SWI under gradient echo sequence and SWI uder echo planar imaging sequence,SeSPEN SWI can not only effectively reduce the scan time,but also have better resistance to the influence of chemical shift and inhomogeneous field.