Boesch C, Kreis R
Department of Clinical Research, University Bern, Switzerland.
Ann N Y Acad Sci. 2000 May;904:25-31.
Magnetic resonance (MR) methods are increasingly being used to investigate the physiology of human muscle. Although MR imaging (MRI) reveals the morphology of muscles in great detail, for example, for determining their volume and fiber orientation, MR spectroscopy (MRS) provides information on the chemical composition of the tissue. Depending on the observed nucleus, MRS allows the observation of high-energy phosphates (31P-MRS), glycogen (13C-MRS), or intramyocellular lipids (1H-MRS), to give only a few examples. 1H-MRS of human skeletal muscle requires special techniques because 1H nuclei in water or adipose tissue are far more concentrated than in any other metabolite of human tissue. The strong signal from water can be suppressed by special prepulses, whereas large signals from fat in adipose tissue can be reduced by carefully selecting the region of interest. Until recently, it was presumed that only a few metabolites would be visible underneath the large resonances of water and subcutaneous fat. Meanwhile, it was clear that 1H-MR spectra of human muscle reveal much metabolic and structural information. The determination of intramyocellular lipids (IMCL) by 1H-MRS was initiated by the observation of two compartments of triacylglycerols with a resonance-frequency shift of approximately 0.2 ppm. The two resonances can be attributed to CH2 protons of lipids in fat cells, and to lipids inside muscle cells (IMCL). 1H-MRS examinations are noninvasive and, therefore, can be repeated many times and with a high temporal resolution. MRS has the potential to replace biopsy to follow-up IMCL levels; however, biopsy still has the advantage that other methods, such as molecular biology, can be applied to the sample. It can be shown that IMCL levels (expressed in mMol/kg wet weight and volume %) are muscle specific and vary with diet and physical activity. In addition, it has been reported that IMCL levels are correlated with insulin sensitivity. A comparison of different methods for assessing IMCL levels, including MRS, chemical analysis, and morphometry, revealed a satisfactory correlation among them and a superior correlation of MRS with the average of the three methods. The observation of IMCL levels by means of 1H-MRS is extremely promising, but several methodological limitations and pitfalls need to be considered.
磁共振（MR）方法越来越多地用于研究人体肌肉的生理学。例如，尽管磁共振成像（MRI）能非常详细地显示肌肉的形态，用于确定其体积和纤维方向，但磁共振波谱（MRS）可提供有关组织化学成分的信息。根据所观察的原子核，MRS可以观察高能磷酸盐（31P - MRS）、糖原（13C - MRS）或细胞内脂质（1H - MRS）等，这里仅举几个例子。人体骨骼肌的1H - MRS需要特殊技术，因为水或脂肪组织中的1H原子核比人体组织的任何其他代谢物都要集中得多。水的强信号可以通过特殊的预脉冲抑制，而脂肪组织中来自脂肪 的大信号可以通过仔细选择感兴趣区域来降低。直到最近，人们还认为在水和皮下脂肪的大共振之下只能看到少数代谢物。与此同时，很明显人体肌肉的1H - MR波谱揭示了许多代谢和结构信息。通过1H - MRS测定细胞内脂质（IMCL）是由观察到三酰甘油的两个区室具有约0.2 ppm的共振频率偏移而启动的。这两个共振可归因于脂肪细胞中脂质的CH2质子以及肌肉细胞内的脂质（IMCL）。1H - MRS检查是非侵入性的，因此可以多次重复且具有高时间分辨率。MRS有可能取代活检来跟踪IMCL水平；然而，活检仍然具有可以对样本应用其他方法（如分子生物学）的优势。可以证明，IMCL水平（以毫摩尔/千克湿重和体积百分比表示）具有肌肉特异性，并随饮食和身体活动而变化。此外，据报道IMCL水平与胰岛素敏感性相关。对包括MRS、化学分析和形态测量在内的评估IMCL水平的不同方法进行比较，结果显示它们之间具有良好的相关性，且MRS与这三种方法的平均值具有更好的相关性。通过1H - MRS观察IMCL水平非常有前景，但需要考虑几个方法学上的局限性和陷阱。
