School of Chemistry, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel.
NMR Biomed. 2010 Aug;23(7):757-80. doi: 10.1002/nbm.1550.
One of the hallmarks of diffusion NMR and MRI is its ability to utilize restricted diffusion to probe compartments much smaller than the excited volume or the MRI voxel, respectively, and to extract microstructural information from them. Single-pulsed field gradient (s-PFG) MR methodologies have been employed with great success to probe microstructures in various disciplines, ranging from chemistry to neuroscience. However, s-PFG MR also suffers from inherent shortcomings, especially when specimens are characterized by orientation or size distributions: in such cases, the microstructural information available from s-PFG experiments is limited or lost. Double-pulsed field gradient (d-PFG) MR methodology, an extension of s-PFG MR, has attracted attention owing to recent theoretical studies predicting that it can overcome certain inherent limitations of s-PFG MR. In this review, we survey the microstructural features that can be obtained from conventional s-PFG methods in the different q regimes, and highlight its limitations. The experimental aspects of d-PFG methodology are then presented, together with an overview of its theoretical underpinnings and a general framework for relating the MR signal decay and material microstructure, affording new microstructural parameters. We then discuss recent studies that have validated the theory using phantoms in which the ground truth is well known a priori, a crucial step prior to the application of d-PFG methodology in neuronal tissue. The experimental findings are in excellent agreement with the theoretical predictions and reveal, inter alia, zero-crossings of the signal decay, robustness towards size distributions and angular dependences of the signal decay from which accurate microstructural parameters, such as compartment size and even shape, can be extracted. Finally, we show some initial findings in d-PFG MR imaging. This review lays the foundation for future studies, in which accurate and novel microstructural information could be extracted from complex biological specimens, eventually leading to new forms of contrast in MRI.
扩散 NMR 和 MRI 的一个特点是，它能够利用受限扩散来探测分别小于激发体积或 MRI 体素的小室，并从中提取微观结构信息。单脉冲场梯度 (s-PFG) MR 方法已被成功应用于化学、神经科学等各个领域的微观结构探测。然而，s-PFG MR 也存在固有缺陷，特别是在样本具有取向或尺寸分布的情况下：在这种情况下，s-PFG 实验提供的微观结构信息有限或丢失。双脉冲场梯度 (d-PFG) MR 方法是 s-PFG MR 的扩展，由于最近的理论研究预测它可以克服 s-PFG MR 的某些固有局限性，因此引起了人们的关注。在这篇综述中，我们调查了在不同 q 区域中可以从常规 s-PFG 方法获得的微观结构特征，并强调了其局限性。然后介绍了 d-PFG 方法的实验方面，以及其理论基础概述和将 MR 信号衰减与材料微观结构联系起来的通用框架，从而提供了新的微观结构参数。然后我们讨论了使用事先已知真实情况的模型进行的验证理论的最新研究，这是在神经元组织中应用 d-PFG 方法之前的关键步骤。实验结果与理论预测非常吻合，特别揭示了信号衰减的零交叉、对尺寸分布和信号衰减的角度依赖性的稳健性，从而可以从这些信号中提取准确的微观结构参数，例如小室大小，甚至形状。最后，我们展示了一些 d-PFG MR 成像的初步结果。这篇综述为未来的研究奠定了基础，未来可以从复杂的生物样本中提取准确和新颖的微观结构信息，最终导致 MRI 中的新对比形式。
