Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts, USA.
Nuffield Department of Clinical Neurosciences, Wellcome Centre for Integrative Neuroimaging, FMRIB Division, University of Oxford, Oxford, United Kingdom.
Magn Reson Med. 2023 Jan;89(1):299-307. doi: 10.1002/mrm.29444. Epub 2022 Sep 11.
Chemical exchange saturation transfer (CEST) MRI is promising for detecting dilute metabolites and microenvironment properties, which has been increasingly adopted in imaging disorders such as acute stroke and cancer. However, in vivo CEST MRI quantification remains challenging because routine asymmetry analysis (MTR ) or Lorentzian decoupling measures a combined effect of the labile proton concentration and its exchange rate. Therefore, our study aimed to quantify amide proton concentration and exchange rate independently in a cardiac arrest-induced global ischemia rat model.
The amide proton CEST (APT) effect was decoupled from tissue water, macromolecular magnetization transfer, nuclear Overhauser enhancement, guanidinium, and amine protons using the image downsampling expedited adaptive least-squares (IDEAL) fitting algorithm on Z-spectra obtained under multiple RF saturation power levels, before and after global ischemia. Omega plot analysis was applied to determine amide proton concentration and exchange rate simultaneously.
Global ischemia induces a significant APT signal drop from intact tissue. Using the modified omega plot analysis, we found that the amide proton exchange rate decreased from 29.6 ± 5.6 to 12.1 ± 1.3 s (P < 0.001), whereas the amide proton concentration showed little change (0.241 ± 0.035% vs. 0.202 ± 0.034%, P = 0.074) following global ischemia.
Our study determined the labile proton concentration and exchange rate underlying the in vivo APT MRI. The significant change in the exchange rate, but not the concentration of amide proton demonstrated that the pH effect dominates the APT contrast during tissue ischemia.
化学交换饱和传递（CEST）MRI 有望用于检测稀代谢物和微环境特性，已越来越多地应用于急性中风和癌症等疾病的成像中。然而，体内 CEST MRI 定量仍然具有挑战性，因为常规的不对称分析（MTR）或洛伦兹解耦测量的是易变质子浓度及其交换率的综合效应。因此，我们的研究旨在在心脏骤停诱导的全脑缺血大鼠模型中独立定量酰胺质子浓度和交换率。
在 Z 谱上使用图像下采样加速自适应最小二乘法（IDEAL）拟合算法，在多个 RF 饱和功率水平下，在全脑缺血前后，从组织水、大分子磁化传递、核奥弗豪瑟增强、胍基和胺质子中解耦酰胺质子 CEST（APT）效应。应用 omega 图分析同时确定酰胺质子浓度和交换率。
全脑缺血导致完整组织的 APT 信号显著下降。使用改良的 omega 图分析，我们发现酰胺质子交换率从 29.6±5.6 降至 12.1±1.3 s（P<0.001），而酰胺质子浓度变化不大（0.241±0.035% 与 0.202±0.034%，P=0.074）。
本研究确定了体内 APT MRI 下的易变质子浓度和交换率。交换率的显著变化而不是酰胺质子的浓度表明，在组织缺血期间，pH 值效应主导 APT 对比。
