开放式永磁MRI系统梯度线圈设计方法研究
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摘要
磁共振成像的应用日益广泛，对系统的成像精度和速度要求也越来越高。成像精度的提升有赖于主磁场强的增高和场均匀性的改善，而成像速度的提升有赖于伴随高梯度切换速率的序列设计的改进；高场强和梯度场的高速切换又引起了人们对临床检测安全性的关注。同时，低场强的开放式磁共振成像系统以其不断完善的功能和低投资成本而日益受医学界重视，高、中场的功能不断被移植到低场系统。这些都对MRI系统提出了新的设计要求。在此背景下本文围绕磁共振系统中极为重要的组成部分——梯度线圈，探索了相关方面的内容，主要是从梯度线圈的设计新方法探索，梯度场非线性引起的几何失真校正，梯度场切换在人体引起的生物效应这三个方面展开。
(1)关于梯度线圈设计
从实空间法的角度提出了形变空间的设计思想，并设计了指纹式的适用于开放式MRI的横向双平面型梯度线圈和基于Maxwell线圈对的纵向双平面型梯度线圈，还设计了连续螺旋形的纵向梯度线圈。这种新方法可以适应复杂线圈结构的要求，且可以自由推广应用到各种空间形状。
从流函数空间法的角度提出了一种基于有限差分流函数法的梯度线圈设计新方法，结合正则化方法，设计了双平面横向梯度线圈。该方法不依赖于任何特殊的流函数假设，适用于三维的复杂几何结构。此外还提出了通过构造调制函数的方法来解决边缘回线密集的问题，该方法简单易行，有别于传统的切趾滤波，也不属于普通的后处理方式。
上述两种梯度线圈设计新方法都非常易于加入实际的工程约束，易于实现工程的合理设计，并且可以应用于自由空间，不需要传统的复杂的电流密度解析展开过程。
(2)关于梯度非线性失真校正
基于开放式MRI系统，进行了由梯度场非线性引起的几何失真校正的研究。应用图像分析的方法，构造校正模型，对永磁开放式MRI系统下的失真图像进行了校正，初步的结果显示了良好的校正效果。该研究对提高MRI图像质量以及进一步扩展MRI应用领域具有重要意义。
(3)关于梯度场生物效应
基于美国空军实验室人体模型，研究探讨了人体组织在梯度线圈中的生物效应。应用阻抗法计算了传统圆柱形Z线圈和双平面型Z线圈中人体的感应电场和涡流效应。该研究对今后发展新一代高性能高安全性的梯度线圈具有重要意义。
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 design
From 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 distortion
A 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 field
Based 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.
(1)关于梯度线圈设计
从实空间法的角度提出了形变空间的设计思想，并设计了指纹式的适用于开放式MRI的横向双平面型梯度线圈和基于Maxwell线圈对的纵向双平面型梯度线圈，还设计了连续螺旋形的纵向梯度线圈。这种新方法可以适应复杂线圈结构的要求，且可以自由推广应用到各种空间形状。
从流函数空间法的角度提出了一种基于有限差分流函数法的梯度线圈设计新方法，结合正则化方法，设计了双平面横向梯度线圈。该方法不依赖于任何特殊的流函数假设，适用于三维的复杂几何结构。此外还提出了通过构造调制函数的方法来解决边缘回线密集的问题，该方法简单易行，有别于传统的切趾滤波，也不属于普通的后处理方式。
上述两种梯度线圈设计新方法都非常易于加入实际的工程约束，易于实现工程的合理设计，并且可以应用于自由空间，不需要传统的复杂的电流密度解析展开过程。
(2)关于梯度非线性失真校正
基于开放式MRI系统，进行了由梯度场非线性引起的几何失真校正的研究。应用图像分析的方法，构造校正模型，对永磁开放式MRI系统下的失真图像进行了校正，初步的结果显示了良好的校正效果。该研究对提高MRI图像质量以及进一步扩展MRI应用领域具有重要意义。
(3)关于梯度场生物效应
基于美国空军实验室人体模型，研究探讨了人体组织在梯度线圈中的生物效应。应用阻抗法计算了传统圆柱形Z线圈和双平面型Z线圈中人体的感应电场和涡流效应。该研究对今后发展新一代高性能高安全性的梯度线圈具有重要意义。
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 design
From 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 distortion
A 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 field
Based 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.
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