| In recent years,the problems such as smoking,air pollution and population aging have become increasingly prominent,accelerating and exacerbating the development of major lung diseases,such as lung cancer,chronic obstructive pulmonary disease(COPD)and so on.Moreover,the rates of suffering,disability and death due to the major lung diseases are increasing year by year in China.Major lung diseases have become a serious public health problem in China,which greatly threatens the human survival and health.In the study and clinical diagnosis of major lung diseases,commonly used imaging techniques include chest X-ray,computed tomography(CT),positron emission tomography(PET),and so on.However,these imaging techniques are radioactive,and they are powerless to analyze the air-air exchange and gas-blood exchange functions of the lungs.Magnetic resonance imaging(MRI)is widely used for diagnostic and therapeutic purposes because of its non-radioactive,non-invasive properties.However,conventional MRI is to image the protons in water or fat,and the density of protons in the lungs is about 1000 times lower than that in normal tissues.Therefore,lung is the" blind zone" of conventional MRI.Recently developed hyperpolarized gas MRI technique caU enhance the polarization of inert gases(e.g.3He,129Xe)by more than four orders of magnitude using the spin-exchange optical pumping(SEOP),which makes the gas imaging of lung feasible.Accordingly,hyperpolarized gas MRI can detect structural and functional details of early pulmonary lesions,such as pulmonary ventilation,microstructures and gas-blood exchange function,which has great clinical application prospects in early detection,diagnosis and treatment of major pulmonary diseases.However,imaging speed is a major bottleneck restricting the development of hyperpolarized gas MRI.Firstly,the polarization of hyperpolarized gas is non-renewable in the imaging process(The polarization exponentially decays with imaging time and excitation times),which makes it difficult to obtain high resolution and high Signal-to-Noise Ratio(SNR)images in hyperpolarized gas MRI.Secondly,patients need to hold their breath for about 15 seconds after inhaling the hyperpolarized gases and keep still until the MRI scan is completed,which is particularly difficult for children,elderly and critical patients.Thirdly,dynamic hyperpolarized gas MRI can obtain comprehensive information of lung structure and function,but it is limited by the imaging speed and cannot acquire dynamic images with high temporal-spatial resolution and high quality.Therefore,accelerating the limaging speed of hyperpolarized gas MRI can promote the popularization and application of the new imaging technology of lung.Compressed sensins(CS)can recover signals and images accurately using significantly fewer measurements than the number required by traditional Nyquist sampling,and CS-MRI is a useful method to accelerate the acquisition speed of MRI by introducing CS to MRI.CS-MRI technology can save sampling time w ithout additional hardware,which provides a new idea for accelerating the imaging speed of hyperpolarized gas MRI.In this dissertation,we propose several pulmonary hyperpolarized 129Xe CS-MRI methods.After that,we apply them in dynamic imaging of lung and obtain high temporal-spatial resolution and hish quality hyperpolarized 129Xe dynamic MR images.On this basis,the dynamic characteristics of gas flow in the lung are studied,and the gas flow is visualized,which is helpful for the early diagnosis of major lung diseases.The main contents of this dissertation are as follows:Firstly,to realize fast dynamic hyperpolarized 1'9Xe MRI and obtain comprehensive information of lung structure and function,a dynamic CS-MRI method based on the low-rank plus sparse plus gas-inflow effects(L+S+G)for hyperpolarized 129Xe MRI of lung is proposed.Since the imaae quality of conventional CS in hyperpolarized gas dynamic MRI is limited by the gas-infloxw effects,we design an undersampling scheme with variable-flip-angle(VFA)considering the gas-inflow effects,which is able to compensate for the fast decay of the polarization of hyperpolarized gases.Moreover,a new objective function is constructed considering the low-rank(L),sparse(S)and gas-inflow effects(G)constraints for image reconstruction.The results show that the L+S+G method can greatly improve the speed(3 times,that is,only 33%of the fully sampled data is sampled)and maintain the quality of hyperpolarized gas MRI(The temporal and spatial resolutions are 445 ms/frame and 3 mm,respectively).Secondly,due to the tree structure and gas flow characteristics of the lung,the requirements for different imaging speed and details are different when gases flow through different positions of the lung(The number of branches increases rapidly with the branch level,while the diameter of trachea decreases rapidly with the branch level).However,the L+S+G method is based on the constant sampling rate,and it is difficult for the L+S+G method to take into account the different requirements in dynamic imaging process.Therefore,we propose a highly and adaptively undersampling pattern for pulmonary hyperpolarized 12gXe dynamic MRI(HUP).By adaptively adjusting the sampling rate according to the structural and gas-flow characteristics of the lung,a high undersampling rate can be achieved when the image sparsity is high,which can significantly increase the imag:ing speed,and a low undersampling rate is adopted to preserve image details when the image sparsity is low.Then,an objective function is constructed according to the dynamic change of image sparsity in transform domain.Low-rank,global sparsity,gas-inflow effects and joint sparsity constraints are used in the obj ective function to eliminate noise and artifacts.The results show that high quality dynamic hyperpolarized 129Xe MRI of lung can be acquired from highly undersampled data(accelerate factors are 3-10),and the temporal and spatial resolution are 202 ms/frame and 3 mm,respectively.Thirdly,after obtaining hyperpolarized 129Xe dynamic lung MR images,we depict the gas-flow pattern in the lung by analyzing the dynamic MR data,which can provide help for the research of pulmonary physiology and pathology.First,a gas-flow model of the lung is obtained according to the CT data and hydrodynamic model.Second,the velocity and direction of the gas flow(motion field)are calculated from the dynamic images of the lung,and they are corrected by the gas-flow model.The results show that the proposed method can effectively observe the direction and velocity of the hyperpolarized gas flow in the whole lung with good temporal-spatial resolution. |
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