Kim Mina, Gillen Joseph, Landman Bennett A, Zhou Jinyuan, van Zijl Peter C M
Russell H. Morgan Department of Radiology and Radiological Sciences, Neurology Section, Division of MR Research, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Magn Reson Med. 2009 Jun;61(6):1441-50. doi: 10.1002/mrm.21873.
Chemical exchange saturation transfer (CEST) is a contrast mechanism that exploits exchange-based magnetization transfer (MT) between solute and water protons. CEST effects compete with direct water saturation and conventional MT processes, and generally can only be quantified through an asymmetry analysis of the water saturation spectrum (Z-spectrum) with respect to the water frequency, a process that is exquisitely sensitive to magnetic field inhomogeneities. Here it is shown that direct water saturation imaging allows measurement of the absolute water frequency in each voxel, allowing proper centering of Z-spectra on a voxel-by-voxel basis independently of spatial B(0) field variations. Optimal acquisition parameters for this "water saturation shift referencing" (WASSR) approach were estimated using Monte Carlo simulations and later confirmed experimentally. The optimal ratio of the WASSR sweep width to the linewidth of the direct saturation curve was found to be 3.3-4.0, requiring a sampling of 16-32 points. The frequency error was smaller than 1 Hz at signal-to-noise ratios of 40 or higher. The WASSR method was applied to study glycogen, where the chemical shift difference between the hydroxyl (OH) protons and bulk water protons at 3T is so small (0.75-1.25 ppm) that the CEST spectrum is inconclusive without proper referencing.
化学交换饱和转移（CEST）是一种对比机制，它利用溶质质子与水质子之间基于交换的磁化转移（MT）。CEST效应与直接水饱和及传统MT过程相互竞争，通常只能通过对水饱和谱（Z谱）相对于水频率进行不对称分析来量化，而这一过程对磁场不均匀性极为敏感。本文表明，直接水饱和成像可测量每个体素中的绝对水频率，从而能够在逐个体素的基础上对Z谱进行适当的中心定位，而与空间B(0)场变化无关。使用蒙特卡罗模拟估算了这种“水饱和位移参考”（WASSR）方法的最佳采集参数，随后通过实验得到了证实。发现WASSR扫描宽度与直接饱和曲线线宽的最佳比值为3.3 - 4.0，需要16 - 32个点的采样。在信噪比为40或更高时，频率误差小于1 Hz。WASSR方法被应用于研究糖原，在3T场强下，羟基（OH）质子与大量水质子之间的化学位移差异非常小（0.75 - 1.25 ppm），以至于在没有适当参考的情况下，CEST谱无法得出明确结论。
