Study On The Design Of Gradient Coils For Open Permanent Magnet MRI System

Today magnetic resonance imaging (MRI) has been extensively used in clinical applications and it requires for high imaging accuracy and imaging speed. The enhancement of the accuracy of imaging depends on an increase in the strength of the main magnetic field as well as the field uniformity; the enhancement of the speed of imaging depends on the improvements of sequence design associated with highly switched gradient field. However, High-strength main magnetic field and rapid switched gradient field consequently raise concerns in the safety in MRI scanning suites. On the other hand, for the continually improvement in function and low investment, low-field open magnetic resonance imaging systems are increasingly subject to medical attention, and a lot of functions of high, middle strength field system have been transferred to low-field system. The new system design needs to take these into consideration.The research work of this thesis focuses on the gradient coil—an extremely important part of the MRI system. Specifically, it will explore novel methods in the design of gradient coils, correction of gradient-nonlinearity distortion, and consideration of the biological effects caused by gradient field switching.Gradient coil designFrom the perspective of real space, a deformation-space method for the design of gradient coils has been proposed. This new algorithm can adapt to the complex structure of the coil, and are free to be applied to a variety of coil shape. Using this novel method, we have designed unconventional 'fingerprint' bi-planar transversal gradient coil for open MRI system, as well as the bi-planar longitudinal gradient coil based on the Maxwell pairs and the spiral longitudinal gradient coil.A finite difference-based has also been proposed for gradient coil design. With a combination of the regularization technique, a bi-planar transversal gradient coil has been designed. As this approach does not constrained with any kind of special stream function, it is ready for complex three-dimensional gradient coil structure. In addition, a unique modulation function method has been used to solve the dense return path problem. This function is quite different from the traditional apodization approach.These two gradient coil design methods are very convenient to take into consideration of the actual engineering constraints, thus easy for practical applications. They can also be applied to free space, with no need of constrained expansion of the current density in those traditional methods.Correction of gradient-nonlinearity distortionA gradient nonlinearty distortion correction method has been proposed dedicated for open MRI systems. The correction is an image-based method, and it has been performed/tested on a permanent MRI system. Preliminary experiments have shown that the method has significantly improved of obtained MRI images.Biological effect of the gradient fieldBased on the realistic human body model and quasi-static impedance method, the influence of eddy currents induced by the rapid switching gradient field in the human body, are discussed. The Human body model was obtained from the U.S. Air Force research laboratory. The safety is evaluated by computing the electric filed and eddy current induced in the human body, under the excitation of conventional cylindrical longitudinal gradient coil and bi-planar longitudinal gradient coil. The study is of great significance in the future development of a new generation of high-performance gradient coils.