Vanderbilt University Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, USA; Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA.
Magn Reson Imaging. 2014 Jan;32(1):28-40. doi: 10.1016/j.mri.2013.08.002. Epub 2013 Nov 13.
Magnetic resonance images of biological media based on chemical exchange saturation transfer (CEST) show contrast that depends on chemical exchange between water and other protons. In addition, spin-lattice relaxation rates in the rotating frame (R1ρ) are also affected by exchange, especially at high fields, and can be exploited to provide novel, exchange-dependent contrast. Here, we evaluate and compare the factors that modulate the exchange contrast for these methods using simulations and experiments on simple, biologically relevant samples.
Simulations and experimental measurements at 9.4 T of rotating frame relaxation rate dispersion and CEST contrast were performed on solutions of macromolecules containing amide and hydroxyl exchanging protons.
The simulations and experimental measurements confirm that both CEST and R1ρ measurements depend on similar exchange parameters, but they manifest themselves differently in their effects on contrast. CEST contrast may be larger in the slow and intermediate exchange regimes for protons with large resonant frequency offsets (e.g. >2 ppm). Spin-locking techniques can produce larger contrast enhancement when resonant frequency offsets are small (<2 ppm) and exchange is in the intermediate-to-fast regime. The image contrasts scale differently with field strength, exchange rate and concentration.
CEST and R1ρ measurements provide different and somewhat complementary information about exchange in tissues. Whereas CEST can depict exchange of protons with specific chemical shifts, appropriate R1ρ-dependent acquisitions can be employed to selectively portray protons of specific exchange rates.
基于化学交换饱和传递（CEST）的生物介质磁共振图像显示的对比度取决于水和其他质子之间的化学交换。此外，在旋转框架中的自旋晶格弛豫率（R1ρ）也受到交换的影响，尤其是在高场中，并且可以被利用来提供新的、依赖于交换的对比度。在这里，我们使用模拟和对简单的、与生物学相关的样本的实验，评估和比较了这些方法调制交换对比度的因素。
在 9.4 T 下，对含有酰胺和羟基交换质子的大分子溶液进行了旋转框架弛豫率弥散和 CEST 对比度的模拟和实验测量。
模拟和实验测量证实，CEST 和 R1ρ 测量都依赖于类似的交换参数，但它们在对比度的影响方面表现不同。对于具有较大共振频率偏移（例如 >2 ppm）的质子，在慢和中间交换区，CEST 对比度可能更大。当共振频率偏移较小时（<2 ppm）且交换处于中间到快速区时，自旋锁定技术可以产生更大的对比度增强。图像对比度与场强、交换率和浓度的不同方式缩放。
CEST 和 R1ρ 测量提供了关于组织中交换的不同且有些互补的信息。虽然 CEST 可以描绘具有特定化学位移的质子的交换，但适当的 R1ρ 相关采集可以用于选择性地描绘具有特定交换率的质子。
