Elimination Of Motion Artifacts In Magnetic Resonance Imaging

Magnetic Resonance Imaging(MRI) is a new medical imaging method emerging in the recent years.It makes images based on nuclear magnetic resonance signals in the static magnetic field.It has many features such as high image resolution,multiimaging parameters,non-invasive and arbitrary slice imaging,etc.Now MRI is one of the most advanced medical imaging methods,and is more and more widely used in the clinical applications.Patient's rigid motion causes artifacts during his magnetic resonance imaging examination,and it affects the diagnosis of doctors seriously.Due to the uncertainty of motion,the prior motion knowledge can't be acquired,so it's very difficult to eliminate the motion artifacts,which will affect and hinder the development step of magnetic resonance imaging technology.Thus the study of MRI motion artifacts, which causes the wide concern of the domestic and foreign scientific research institutions and scholars,is a hot topic in the research area of medical imaging.Any research progress in this field will play an active role in the development and application of magnetic resonance imaging technology.After mastering the principle of magnetic resonance imaging,we do an in-depth study on motion artifacts and analyze the impacts of them on the magnetic resonance data acquisition and imaging.Several reconstruction and artifact correction algorithms are proposed,and they can suppress MRI motion artifacts efficiently.The research in this dissertation starts from an intensive study of PROPELLER method,which is a new proposed MR data acquisition and imaging method with the function of motion artifacts elimination.After the programming implementation of PROPELLER method, a circular gridding algorithm is proposed to improve the speed of the method.The gridding process is improved in PROPELLER to obtain higher executing speed and image quality.In the images postprocessing,several algorithms,which are valuable in practical applications,are also proposed to correct the translational and rotational artifacts and improve the quality of final images.To sum up,our meaningful and detailed research works are organized as follows:1.A new circle gridding algorithm is proposed to improve the PROPELLER MRI method.PROPELLER MRI is a new method in magnetic resonance imaging technology.It changes the way of data acquisition and imaging reconstruction,puts the correction steps into the reconstruction process,and is able to eliminate the motion artifacts in the MR images.Based on the analysis of the basic principles of PROPELLER method,we use mathematical algorithms to achieve each step of the method.And then to decrease the time consuming of the rotational motion correction method,a new circle gridding algorithm is proposed.It reduces the interpolation and gridding computing times,and also improves the accuracy of parameter estimation on the rotation angles and the executing speed of the whole method.Our improved PROPELLER method can eliminate the motion artifact effectively during the scanning process.It is fast,and always obtains high-quality MR images as well.2.Two improved PROPELLER gridding algorithms are proposed.Gridding is an important step in PROPELLER MRI,and it plays a decisive role for the image quality.Therefore,the improvement for the gridding method will contribute to imaging speed and quality markedly.The first algorithm takes the Voronoi area as the gridding density compensation function and puts forward a method of establishing closures,and improves the corresponding gridding method.The second algorithm proposes a large-matrix gridding method.It improves the gridding quality and uses matrix computation to speed up the process of the entire operation.3.Two MRI translational motion artifact correction algorithms are proposed. The first one is a new algorithm based on the data of artifact area.Constraint equations of translational motion parameters are established by the data of artifact area.Then it uses differential evolution algorithm to optimize the value function,gets the translation motion parameter,and corrects the translational motion artifacts in images.The second algorithm deals with translational motion artifacts on x and y directions respectively based on the non-relevance of movement.It uses the Fourier projection algorithm to get translational pixel motion parameters and corrects the motion artifact in the x direction.Then,it uses the genetic algorithm to correct the motion artifact in y direction and the subpixel motion artifact in x direction by the polynomial fitting.Test results demonstrate that the two methods can suppress the translational motion artifacts efficiently.4.A MRI rotational motion artifact correction algorithm is proposed for the rapid scanning sequence.The following new ideas are contained in the algorithm.It puts forward the view of stripping the uniform rectangular K-space data,and suggests a new similarity measure to estimate the rotation motion parameters.It proposes a Hermitian conjugate compensation algorithm to fill the undersampled data in the K-space.It designs a fast gridding algorithm to speed up the computations.The algorithm can correct rotational motion artifacts effectively with little effect of signal-to-noise ratio and strong robustness.