Novel Technologies for Global Optimization of Magnetic Field Homogeneity

描述(申请人提供)：功能磁共振成像、弥散磁共振成像和磁共振波谱在脑部疾病和损伤的研究和诊断以及指导手术治疗方面具有巨大的潜力。所有这些方法都大大得益于高场强磁铁(3T及以上)增加的灵敏度和对比度。然而，由于空气和组织的磁化率差异引起的磁场不均匀性(MFI)的混杂效应，强磁场的优势还没有完全实现。MFI在MRI中会导致信号丢失和空间失真，在MRS中会导致光谱分辨率和灵敏度的损失。这些伪影导致的可靠性损失是这些技术没有在临床上广泛应用的主要原因。目前的磁场均质化方法(即垫片)在小体积上效果很好，但在整个人脑中却不够充分。在这里，我们提出了一些新的技术，旨在对活体中的人和动物大脑中的磁场进行全局优化。(1)动态垫片更新将全局3D问题分成多个切片，在这些切片上可以获得足够的磁场均匀度。与多切片MRI序列同步地动态更新预定切片垫片确保了所有切片的最佳同质性。(2)由强介质和顺磁性材料制成的局部无源垫片将被构造来提供高度局域化的不均匀性补偿。(3)基于小鼠大脑体内磁场分布的梯度/垫片线圈组件的开发将导致更好地利用垫片电流，从而改善均质性。(4)尽管B0的空间和时间变化与MFI没有直接关系，但它们可以以类似的方式影响MRI和MRS结果。在这里，我们建议刻画B0场变化，并建立一个数字补偿单元。在R21阶段，我们将实施DSU，并在体内提供MFI的完整表征。R33阶段主要是被动垫片、优化的垫片/梯度线圈和B0补偿单元的构建和测试。一旦所有技术都得到充分发展，预计人类和动物大脑中的MFI将降低到不会对大多数MRI/MRS应用构成限制的水平。由于MFI影响到活体核磁共振的许多方面，所提出的技术将对MRI和MRS的几乎所有方面产生重大影响。

DESCRIPTION (provided by applicant): Functional MRI, diffusion MRI and MRS have great potential for the study and diagnosis of brain disease and injury, and guiding surgical therapy. All of these methods benefit greatly from the added sensitivity and contrast from high field strength magnets (3T and above). However, the advantages of higher magnetic fields have not been fully realized due to the increasingly confounding effects of magnetic field inhomogeneity (MFI) caused by magnetic susceptibility differences between air and tissue. MFI leads to signal loss and spatial distortion in MRI and loss in spectral resolution and sensitivity in MRS. The loss of reliability due to these artifacts is a major reason why these techniques have not seen wide use in clinical applications. Current methods of magnetic field homogenization (i.e. shimming) work well on small volumes but are inadequate over the entire human brain. Here we propose a number of novel techniques that are aimed at the global optimization of magnetic fields in human and animal brain in vivo. (1) Dynamic shim updating divides a global 3D problem into a number of slices over which adequate magnetic field homogeneity can be achieved. Dynamically updating the pre-determined slice shims in sync with the multi-slice MRI sequence ensures optimal homogeneity for all slices. (2) Local passive shims made from strong dia-and paramagnetic materials will be constructed to provide highly localized compensation of inhomogeneities. (3) The development of a gradient/shim coil assembly based on in vivo magnetic field distributions in the mouse brain will lead to a better use of shim currents and hence an improved homogeneity. (4) While spatial and temporal B0 variations are not directly related to MFI, they can affect MRI and MRS results in a similar manner. Here we propose to characterize B0 field variations and build a digital compensation unit. During the R21 phase, we will implement DSU and provide a complete characterization of MFI in vivo. The R33 phase is dominated by the construction and testing of passive shims, optimized shim/gradient coils and a B0 compensation unit. Once all the techniques are fully developed it is anticipated that MFI in the human and animal brain are reduced to levels in which they do not pose a limitation to the majority of MRI/MRS applications. Since MFI affects many facet's of in vivo NMR, the proposed techniques will have major impacts on almost all aspects of MRI and MRS.