Department of Biomedical Engineering, Vanderbilt University, Nashville, Tennessee 37232-2310, USA.
Magn Reson Med. 2013 May;69(5):1357-66. doi: 10.1002/mrm.24379. Epub 2012 Jul 12.
Variations in local magnetic susceptibility may induce magnetic field gradients that affect the signals acquired for MR imaging. Under appropriate diffusion conditions, such fields produce effects similar to slow chemical exchange. These effects may also be found in combination with other chemical exchange processes at multiple time scales. We investigate these effects with simulations and measurements to determine their contributions to rotating frame (R1ρ ) relaxation in model systems. Simulations of diffusive and chemical exchange effects on R1ρ dispersion were performed using the Bloch equations. Additionally, R1ρ dispersion was measured in suspensions of Sephadex and latex beads with varying spin locking fields at 9.4 T. A novel analysis method was used to iteratively fit for apparent chemical and diffusive exchange rates with a model by Chopra et al. Single- and double-inflection points in R1ρ dispersion profiles were observed, respectively, in simulations of slow diffusive exchange alone and when combined with rapid chemical exchange. These simulations were consistent with measurements of R1ρ in latex bead suspensions and small-diameter Sephadex beads that showed single- and double-inflection points, respectively. These observations, along with measurements following changes in temperature and pH, are consistent with the combined effects of slow diffusion and rapid -OH exchange processes.
局部磁化率的变化可能会引起磁场梯度，从而影响磁共振成像采集的信号。在适当的扩散条件下，这些场会产生类似于缓慢化学交换的效果。这些效果也可能与其他化学交换过程在多个时间尺度上结合出现。我们通过模拟和测量来研究这些效果，以确定它们对模型系统中旋转框架（R1ρ）弛豫的贡献。使用布洛赫方程对扩散和化学交换对 R1ρ 弥散的影响进行了模拟。此外，还在 9.4 T 下用不同的自旋锁定场测量了葡聚糖和乳胶珠悬浮液中的 R1ρ 弥散。使用一种新的分析方法，通过 Chopra 等人的模型对表观化学和扩散交换率进行迭代拟合。在单独的缓慢扩散交换和与快速化学交换相结合的模拟中，分别观察到 R1ρ 弥散分布中的单拐点和双拐点。这些模拟与乳胶珠悬浮液和小直径葡聚糖珠的 R1ρ 测量结果一致，它们分别显示出单拐点和双拐点。这些观察结果以及对温度和 pH 值变化的测量结果与缓慢扩散和快速-OH 交换过程的综合影响一致。
