Zu Zhongliang, Afzal Aqeela, Li Hua, Xie Jingping, Gore John C
Vanderbilt University Institute of Imaging Science, Vanderbilt University, Nashville, TN, USA.
Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, TN, USA.
NMR Biomed. 2018 Apr;31(4):e3893. doi: 10.1002/nbm.3893. Epub 2018 Feb 9.
We have previously reported that the dispersion of spin-lattice relaxation rates in the rotating frame (R ) of tissue water protons at high field can be dominated by chemical exchange contributions. Ischemia in brain causes changes in tissue pH, which in turn may affect proton exchange rates. Amide proton transfer (APT, a form of chemical exchange saturation transfer) has been shown to be sensitive to chemical exchange rates and able to detect pH changes non-invasively following ischemic stroke. However, the specificity of APT to pH changes is decreased because of the influence of several other factors that affect magnetization transfer. R is less influenced by such confounding factors and thus may be more specific for detecting variations in pH. Here, we applied a spin-locking sequence to detect ischemic stroke in animal models. Although R images acquired with a single spin-locking amplitude (ω ) have previously been used to assess stroke, here we use ΔR , which is the difference in R values acquired with two different locking fields to emphasize selectively the contribution of chemical exchange effects. Numerical simulations with different exchange rates and measurements of tissue homogenates with different pH were performed to evaluate the specificity of ΔR to detect tissue acidosis. Spin-lock and APT data were acquired on five rat brains after ischemic strokes induced via middle cerebral artery occlusions. Correlations between these data were analyzed at different time points after the onset of stroke. The results show that ΔR (but not R acquired with a single ω ) was significantly correlated with APT metrics consistent with ΔR varying with pH.
我们之前曾报道，在高场下组织水质子旋转框架（R）中的自旋 - 晶格弛豫率分散可能主要由化学交换贡献主导。脑缺血会导致组织pH值变化，进而可能影响质子交换率。酰胺质子转移（APT，一种化学交换饱和转移形式）已被证明对化学交换率敏感，并且能够在缺血性中风后无创地检测pH值变化。然而，由于影响磁化转移的其他几个因素的影响，APT对pH值变化的特异性降低。R受此类混杂因素的影响较小，因此可能对检测pH值变化更具特异性。在这里，我们应用自旋锁定序列来检测动物模型中的缺血性中风。尽管之前已使用具有单个自旋锁定幅度（ω）获取的R图像来评估中风，但在这里我们使用ΔR，它是用两个不同锁定场获取的R值之差，以选择性地强调化学交换效应的贡献。进行了不同交换率的数值模拟以及不同pH值的组织匀浆测量，以评估ΔR检测组织酸中毒的特异性。在通过大脑中动脉闭塞诱导缺血性中风后的五只大鼠大脑上采集了自旋锁定和APT数据。在中风发作后的不同时间点分析了这些数据之间的相关性。结果表明，ΔR（而非用单个ω获取的R）与APT指标显著相关，这与ΔR随pH值变化一致。
