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首页> 外文期刊>Journal of magnetic resonance >Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields
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Combined passive and active shimming for in vivo MR spectroscopy at high magnetic fields

机译：被动和主动匀场相结合，用于高磁场下的体内MR光谱

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摘要

The use of high magnetic fields increases the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS) of brain metabolites. Practical limitations arise, however, from susceptibility-induced field distortions, which are increased at higher magnetic field strengths. Solutions to this problem include optimized shimming, provided that active, i.e., electronic, shimming can operate over a sufficient range. To meet our shim requirements, which were an order of magnitude greater than the active shim capacity of our 7 T MR system, we developed a combined passive and active shim approach. Simple geometries of ferromagnetic shim elements were derived and numerically optimized to generate a complete set of second-order spherical harmonic shim functions in a modular manner. The major goals of the shim design were maximization of shim field accuracy and ease of practical implementation. The theoretically optimized ferro-shim geometries were mounted on a cylindrical surface and placed inside the magnet bore, surrounding the subject's head and the RF coil. Passive shimming generated very strong shim fields and eliminated the worst of the field distortions, after which the field was further optimized by flexible and highly accurate active shimming. Here, the passive-shimming procedure was first evaluated theoretically, then applied in phantom studies and subsequently validated for in vivo H-1 MRS in the macaque visual cortex. No artifacts due to the passive shim setup were observed; adjustments were reproducible between sessions. The modularity and the reduction to two pieces per shim term in this study is an important simplification that makes the method applicable also for passive shimming within single sessions. The feasibility of very strong, flexible and high-quality shimming via a combined approach of passive and active shimming is of great practical relevance for MR imaging and spectroscopy at high field strengths where shim power is limited or where shimming of specific anatomical regions inherently requires strong shim fields. (c) 2006 Elsevier Inc. All rights reserved.
机译：高磁场的使用增加了脑代谢物的磁共振波谱（MRS）的灵敏度和光谱分散度。但是，实际的局限性是由磁化率引起的磁场畸变引起的，这种畸变在较高的磁场强度下会增加。解决该问题的方法包括优化匀场，前提是有源匀场（即电子匀场）可以在足够的范围内工作。为了满足我们的匀场要求，该要求比我们的7 T MR系统的主动匀场能力要大一个数量级，我们开发了被动和主动匀场相结合的方法。推导了简单的铁磁垫片元件的几何形状，并对其进行了数值优化，以模块化方式生成了一套完整的二阶球谐垫片函数。匀场片设计的主要目标是最大化匀场精度和易于实施。将经过理论优化的铁垫片的几何形状安装在圆柱表面上，并放置在磁体孔内，围绕对象的头部和RF线圈。被动匀场产生非常强的匀场，并消除了最坏的场失真，此后，通过灵活，高精度的主动匀场进一步优化了场。在此，首先对被动匀场程序进行了理论评估，然后将其应用于幻像研究，随后针对猕猴视皮层中的体内H-1 MRS进行了验证。没有观察到由于被动垫片设置引起的伪影;会话之间的调整是可重复的。在这项研究中，模块化和每垫片项减少到两件是一个重要的简化，这使得该方法也适用于单节内的被动匀场。通过无源和有源匀场相结合的方法进行非常强，灵活和高质量的匀场的可行性，对于匀场功率有限或固有的特定解剖区域匀场需要高场强的高场强的MR成像和光谱学，具有重大的现实意义。匀场。 （c）2006 Elsevier Inc.保留所有权利。

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