Institute of Imaging Science, Vanderbilt University, Nashville, Tennessee 37232-2310, USA.
Magn Reson Med. 2011 Dec;66(6):1563-71. doi: 10.1002/mrm.22947. Epub 2011 May 16.
Variations in T(1ρ) with locking-field strength (T(1ρ) dispersion) may be used to estimate proton exchange rates. We developed a novel approach utilizing the second derivative of the dispersion curve to measure exchange in a model system of cross-linked polyacrylamide gels. These gels were varied in relative composition of comonomers, increasing stiffness, and in pH, modifying exchange rates. Magnetic resonance images were recorded with a spin-locking sequence as described by Sepponen et al. These measurements were fit to a mono-exponential decay function yielding values for T(1ρ) at each locking-field measured. These values were then fit to a model by Chopra et al. for estimating exchange rates. For low stiffness gels, the calculated exchange values increased by a factor of 4 as pH increased, consistent with chemical exchange being the dominant contributor to T(1ρ) dispersion. Interestingly, calculated chemical exchange rates also increased with stiffness, likely due to modified side-chain exchange kinetics as the composition varied. This article demonstrates a new method to assess the structural and chemical effects on T(1ρ) relaxation dispersion with a suitable model. These phenomena may be exploited in an imaging context to emphasize the presence of nuclei of specific exchange rates, rather than chemical shifts.
T1ρ 值随锁定场强度（T1ρ 弥散）的变化可用于估计质子交换速率。我们开发了一种新方法，利用弥散曲线的二阶导数来测量交联聚丙烯酰胺凝胶模型系统中的交换。这些凝胶在共聚单体的相对组成、增加的刚度和 pH 值方面存在差异，从而改变了交换速率。磁共振图像是通过 Sepponen 等人描述的自旋锁定序列记录的。这些测量值符合单指数衰减函数，可得出在每个测量锁定场下 T1ρ 的值。然后，这些值符合 Chopra 等人的模型，用于估计交换速率。对于低刚度凝胶，随着 pH 值的增加，计算出的交换值增加了 4 倍，这与化学交换是 T1ρ 弥散的主要贡献者一致。有趣的是，计算出的化学交换速率也随刚度增加而增加，这可能是由于组成变化时侧链交换动力学发生了改变。本文展示了一种新方法，通过合适的模型评估 T1ρ 弛豫弥散的结构和化学效应。这些现象可以在成像中加以利用，以强调具有特定交换速率的核的存在，而不是化学位移。
