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Optimization of static magnetic field homogeneity in the human and animal brain in vivo
Progress in Nuclear Magnetic Resonance Spectroscopy ( IF 6.1 ) Pub Date : 2009-02-01 , DOI: 10.1016/j.pnmrs.2008.04.001
Kevin M Koch 1 , Douglas L Rothman , Robin A de Graaf
Affiliation  

Samples studied with conventional magnetic resonance (MR) techniques are subjected to a polarizing static magnetic field B(r)=B0z^. Ideally, this magnetic field is assumed to be spatially homogeneous over the sample volume. However, the presence of any object of finite magnetic susceptibility (i.e. a sample) will inevitably perturb this magnetic field and provide the Larmor frequencies of MR-sensitive spins with an unwanted spatial dependence. These susceptibility-induced B0 perturbations, which are a fundamental result of Maxwell’s electromagnetic field theory, scale roughly linearly in magnitude with applied B0 field strengths. B0 inhomogeneity degrades the signal-to-noise ratio (SNR) of all MR measurements. While Hahn spin-echoes can be used to refocus B0 inhomogeneity at one instantaneous temporal point, the remainder of a spin-echo is tempered by B0 inhomogeneity-induced signal relaxation (commonly referred to as T2∗). Furthermore, many techniques cannot utilize spin-echo procedures, and are thus susceptible to the full evolution of T2∗ relaxation. The spatial variation of Larmor frequencies within a spectro-scopic voxel will broaden and distort spectral lineshapes. Standard spectroscopic techniques such as frequency-selective resonance suppression and spectral editing are easily degraded by these effects. MR images are susceptible to the same B0-induced SNR deterioration as spectroscopic acquisitions. Additionally, they can further suffer from spatial distortion in regions of high B0 inhomogeneity. In particular, commonly utilized rapid imaging strategies such as steady-state free precession (SSFP), spiral, and echo-planar imaging (EPI) are often compromised by B0 inhomogeneity. Broadly speaking, B0 shimming refers to the optimized application of external magnetic fields to compensate unwanted inhomogeneity of the B0 magnetic field. For applications that require high-degrees of B0 homogeneity, fine-tuned shimming has historically been accomplished using sets of dedicated electromagnets. These electromagnet coils, or ‘active shims’, can be adjusted on a subject-specific basis. This review is focused on high-field B0 shimming of the brain, specifically within small rodents and humans. B0 field perturbations within the brain are particularly prominent near the air-tissue interfaces at the sinus and auditory cavities. The recent increases of B0 field strengths used in both clinical and research MR systems necessitates maximal utility of conventional shim technology. For some applications, this conventional technology cannot adequately homogenize designated shim volumes (particularly larger volumes). Therefore, along with the development of automated optimization protocols for conventional active shim systems, recent investigations have explored alternative approaches to shim hardware design. A theoretical treatment of B0 inhomogeneity and its effects on magnetic resonance acquisitions are presented in the next section. Section 3 then presents the techniques utilized in MR-based mapping of static magnetic fields, which is now a standard tool in automated shimming methods. The hardware and methods utilized in room-temperature shimming (also commonly referred to as ‘active’ shimming) are developed in Section 4. The challenges of optimizing B0 homogeneity over extended volumes are introduced in Section 5, followed by a discussion of the methods and capabilities of Dynamic Shim Updating (DSU) of room-temperature shims. Finally, recent novel approaches to shimming that deviate from any previous technological methodologies, such as local active shimming and subject-specific passive shimming, are presented.

中文翻译:

人和动物大脑体内静磁场均匀性的优化

使用传统磁共振 (MR) 技术研究的样品会受到极化静磁场 B(r)=B0z^ 的影响。理想情况下,假设该磁场在样本体积上是空间均匀的。然而,任何具有有限磁化率的物体(即样品)的存在将不可避免地扰乱该磁场,并为 MR 敏感自旋的拉莫尔频率提供不希望的空间依赖性。这些磁化率引起的 B0 扰动是麦克斯韦电磁场理论的基本结果,其大小与所施加的 B0 场强度大致呈线性关系。B0 不均匀性会降低所有 MR 测量的信噪比 (SNR)。虽然哈恩自旋回波可用于在一个瞬时时间点重新聚焦 B0 不均匀性,但自旋回波的其余部分会通过 B0 不均匀性引起的信号弛豫(通常称为 T2*)来缓和。此外,许多技术无法利用自旋回波程序,因此容易受到 T2* 弛豫完全演化的影响。光谱体素内拉莫尔频率的空间变化将使光谱线形变宽和扭曲。标准光谱技术(例如频率选择性共振抑制和光谱编辑)很容易因这些效应而降级。与光谱采集一样,MR 图像也容易受到 B0 引起的 SNR 恶化的影响。此外,它们还可能在 B0 不均匀性较高的区域遭受空间扭曲。特别是,常用的快速成像策略,例如稳态自由进动 (SSFP)、螺旋和平面回波成像 (EPI),通常会受到 B0 不均匀性的影响。从广义上讲,B0 匀场是指优化应用外部磁场来补偿 B0 磁场的不均匀性。对于需要高度 B0 均匀性的应用,微调匀场历来是使用专用电磁体组来完成的。这些电磁线圈或“主动垫片”可以根据具体对象进行调整。这篇综述的重点是大脑的高场 B0 匀场,特别是小型啮齿动物和人类的大脑。大脑内的 B0 场扰动在窦和听觉腔的空气组织界面附近尤其突出。最近临床和研究 MR 系统中使用的 B0 场强不断增加,因此需要最大限度地利用传统匀场技术。对于某些应用,这种传统技术无法充分均匀化指定的垫片体积(特别是较大的体积)。因此,随着传统有源匀场系统的自动优化协议的发展,最近的研究探索了匀场硬件设计的替代方法。下一节将介绍 B0 不均匀性的理论处理及其对磁共振采集的影响。第 3 节介绍了基于 MR 的静态磁场映射所使用的技术,该技术现已成为自动匀场方法中的标准工具。第 4 节开发了室温匀场(通常也称为“主动”匀场)中使用的硬件和方法。第 5 节介绍了在扩展体积上优化 B0 均匀性的挑战,然后讨论了方法和方法。室温垫片的动态垫片更新 (DSU) 功能。最后,提出了偏离任何先前技术方法的最新匀场新方法,例如局部主动匀场和特定于主题的被动匀场。
更新日期:2009-02-01
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