Research On Rapid T₁ Mapping Imaging Pulse Sequence And Image Reconstruction Method Based On Magnetic Resonance Spectroscopy

To obtain the longitudinal relaxation parameter T₁ images based on Magnetic Resonance Spectroscopy is an important research direction in Magnetic Resonance Imaging（MRI）,which is becoming increasingly popular in different applications e.g.quantitative tracer kinetics,radiation dosimetry,and the response of tumors to therapy.Traditionally,T₁ maps based on Magnetic Resonance Spectroscopy can be generated by using the inversion recovery spin echo（IRSE）pulse sequence.The scanning time of this method is more than dozens of minutes,which has great limitations in practical application.Although many different methods have been proposed to accelerate T₁ mapping acquisition,such as multi-echo acquisition or excitation at variable flip angles,multiple weighted images still need to be acquired to fit and reconstruct T₁ maps.Herein,a novel T₁ mapping scheme is proposed in this dissertation,in order to break through the limitations of current methods.There is no need to collect multiple echo images during the experiment which effectively improves the scanning speed of T₁ mapping.The main work of this dissertation is as follows:Firstly,according to T₁ measurement and inversion based on Magnetic Resonance Spectroscopy,a high-multiple under-sampling MRI pulse sequence is proposed and designed for rapid T₁ mapping.The high-multiple under-sampling K-space data of target region is obtained via the reverse recovery pulse sequence based on FLASH gradient echo in the process of signal acquisition.Then,combined with the characteristics of longitudinal relaxation signal recovery,the image reconstruction method based on low rank constraint is used to process the k-space data,and a series of echo images with different T₁ weighted information at different recovery time are obtained.Finally,T₁ map is obtained by data fitting.Simulation result shows that the new method has excellent accuracy and applicability,which can obtain accurate T₁ maps of the brain.In order to verify the effectiveness and practicability of the new method,based on the abovementioned research,the proposed T₁ mapping scheme is experimentally verified and compared.Phantom was used for scanning test,and the sequence sampling template was optimized according to the reconstruction results.At the same time,combined with the k-space center oversampling,the T₁ quantitative results were further improved.The scanning time of the optimized experimental scheme is only 2.7 s.Compared with the T₁ mapping scheme widely used in clinical practice,the scanning time of presented scheme is accelerated nearly 79 times,and the error of experimental quantitative results is less than5%.Further experimental analysis was carried out on living mice.The results show that T₁mapping scheme based on low rank constraint can effectively improve the scanning speed on the premise of ensuring the accuracy of quantitative results,which has a good application prospect in clinical practice.