Department of Medical Radiation Physics, Lund, Lund University, Lund, Sweden.
Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA.
NMR Biomed. 2023 Jan;36(1):e4827. doi: 10.1002/nbm.4827. Epub 2022 Sep 27.
Monitoring time dependence with diffusion MRI yields observables sensitive to compartment sizes (restricted diffusion) and membrane permeability (water exchange). However, restricted diffusion and exchange have opposite effects on the diffusion-weighted signal, which can lead to errors in parameter estimates. In this work, we propose a signal representation that incorporates the effects of both restricted diffusion and exchange up to second order in b-value and is compatible with gradient waveforms of arbitrary shape. The representation features mappings from a gradient waveform to two scalars that separately control the sensitivity to restriction and exchange. We demonstrate that these scalars span a two-dimensional space that can be used to choose waveforms that selectively probe restricted diffusion or exchange, eliminating the correlation between the two phenomena. We found that waveforms with specific but unconventional shapes provide an advantage over conventional pulsed and oscillating gradient acquisitions. We also show that parametrization of waveforms into a two-dimensional space can be used to understand protocols from other approaches that probe restricted diffusion and exchange. For example, we found that the variation of mixing time in filter-exchange imaging corresponds to variation of our exchange-weighting scalar at a fixed value of the restriction-weighting scalar. The proposed signal representation was evaluated using Monte Carlo simulations in identical parallel cylinders with hexagonal and random packing as well as parallel cylinders with gamma-distributed radii. Results showed that the approach is sensitive to sizes in the interval 4-12 and exchange rates in the simulated range of 0 to 20 , but also that there is a sensitivity to the extracellular geometry. The presented theory constitutes a simple and intuitive description of how restricted diffusion and exchange influence the signal as well as a guide to protocol design capable of separating the two effects.
利用扩散 MRI 监测时间依赖性可得到对隔室大小（受限扩散）和膜通透性（水交换）敏感的可观察量。然而，受限扩散和交换对扩散加权信号有相反的影响，这可能导致参数估计中的误差。在这项工作中，我们提出了一种信号表示方法，它将受限扩散和交换的影响纳入到 b 值的二阶，并且与任意形状的梯度波形兼容。该表示法的特点是从梯度波形到两个标量的映射，这两个标量分别控制对限制和交换的敏感性。我们证明了这些标量跨越二维空间，可以用于选择选择性探测受限扩散或交换的波形，消除两种现象之间的相关性。我们发现具有特定但非常规形状的波形比传统的脉冲和振荡梯度采集具有优势。我们还表明，将波形参数化到二维空间中可以用于理解探测受限扩散和交换的其他方法的协议。例如，我们发现在滤子交换成像中混合时间的变化对应于在固定的限制加权标量值下我们的交换加权标量的变化。使用在具有六边形和随机堆积以及具有伽马分布半径的平行圆柱体中的蒙特卡罗模拟对所提出的信号表示法进行了评估。结果表明，该方法对 4-12 范围内的大小以及模拟范围内的 0 到 20 的交换率敏感，但对外细胞几何形状也敏感。所提出的理论构成了对受限扩散和交换如何影响信号的简单直观描述，以及能够分离两种效应的协议设计指南。
