Henning Anke, Fuchs Alexander, Murdoch James B, Boesiger Peter
Institute for Biomedical Engineering, University and ETH Zurich, Switzerland.
NMR Biomed. 2009 Aug;22(7):683-96. doi: 10.1002/nbm.1366.
In comparison to 1.5 and 3 T, MR spectroscopic imaging at 7 T benefits from signal-to-noise ratio (SNR) gain and increased spectral resolution and should enable mapping of a large number of metabolites at high spatial resolutions. However, to take full advantage of the ultra-high field strength, severe technical challenges, e.g. related to very short T(2) relaxation times and strict limitations on the maximum achievable B(1) field strength, have to be resolved. The latter results in a considerable decrease in bandwidth for conventional amplitude modulated radio frequency pulses (RF-pulses) and thus to an undesirably large chemical-shift displacement artefact. Frequency-modulated RF-pulses can overcome this problem; but to achieve a sufficient bandwidth, long pulse durations are required that lead to undesirably long echo-times in the presence of short T(2) relaxation times. In this work, a new magnetic resonance spectroscopic imaging (MRSI) localization scheme (free induction decay acquisition localized by outer volume suppression, FIDLOVS) is introduced that enables MRSI data acquisition with minimal SNR loss due to T(2) relaxation and thus for the first time mapping of an extended neurochemical profile in the human brain at 7 T. To overcome the contradictory problems of short T(2) relaxation times and long pulse durations, the free induction decay (FID) is directly acquired after slice-selective excitation. Localization in the second and third dimension and skull lipid suppression are based on a T(1)- and B(1)-insensitive outer volume suppression (OVS) sequence. Broadband frequency-modulated excitation and saturation pulses enable a minimization of the chemical-shift displacement artefact in the presence of strict limits on the maximum B(1) field strength. The variable power RF pulses with optimized relaxation delays (VAPOR) water suppression scheme, which is interleaved with OVS pulses, eliminates modulation side bands and strong baseline distortions. Third order shimming is based on the accelerated projection-based automatic shimming routine (FASTERMAP) algorithm. The striking SNR and spectral resolution enable unambiguous quantification and mapping of 12 metabolites including glutamate (Glu), glutamine (Gln), N-acetyl-aspartatyl-glutamate (NAAG), gamma-aminobutyric acid (GABA) and glutathione (GSH). The high SNR is also the basis for highly spatially resolved metabolite mapping.
与1.5 T和3 T相比，7 T磁共振波谱成像受益于信噪比（SNR）的提高和光谱分辨率的增加，应该能够在高空间分辨率下对大量代谢物进行图谱绘制。然而，为了充分利用超高场强，必须解决一些严峻的技术挑战，例如与非常短的T(2)弛豫时间相关的问题以及对最大可实现B(1)场强的严格限制。后者导致传统调幅射频脉冲（RF脉冲）的带宽显著降低，从而产生不希望有的大化学位移伪影。调频RF脉冲可以克服这个问题；但为了获得足够的带宽，需要较长的脉冲持续时间，这在T(2)弛豫时间较短的情况下会导致不希望有的长回波时间。在这项工作中，引入了一种新的磁共振波谱成像（MRSI）定位方案（通过外体积抑制定位的自由感应衰减采集，FIDLOVS），该方案能够以最小的因T(2)弛豫导致的SNR损失进行MRSI数据采集，从而首次在7 T下绘制人类大脑中扩展的神经化学图谱。为了克服T(2)弛豫时间短和脉冲持续时间长这两个相互矛盾的问题，在切片选择性激发后直接采集自由感应衰减（FID）。第二维和第三维的定位以及颅骨脂质抑制基于一个对T(1)和B(1)不敏感的外体积抑制（OVS）序列。宽带调频激发和饱和脉冲能够在最大B(1)场强受到严格限制的情况下将化学位移伪影最小化。与OVS脉冲交错的具有优化弛豫延迟的可变功率RF脉冲（VAPOR）水抑制方案消除了调制边带和强烈的基线失真。三阶匀场基于加速投影自动匀场程序（FASTERMAP）算法。显著的SNR和光谱分辨率使得能够明确量化和绘制包括谷氨酸（Glu）、谷氨酰胺（Gln）、N-乙酰天冬氨酰谷氨酸（NAAG）、γ-氨基丁酸（GABA）和谷胱甘肽（GSH）在内的12种代谢物。高SNR也是高空间分辨率代谢物图谱绘制的基础。
