Matson Gerald B
Department of Veterans Affairs Medical Center, San Francisco, CA 94121, United States; Department of Pharmaceutical Chemistry, University of California, San Francisco, CA, United States.
Magn Reson Imaging. 2017 Dec;44:146-156. doi: 10.1016/j.mri.2017.09.006. Epub 2017 Sep 7.
Over the years, a variety of MRI methods have been developed for visualizing or measuring blood flow without the use of contrast agents. One particular class of methods uses flow-encoding gradients associated with an RF pulse sequence to distinguish spins in flowing blood from stationary spins. While a strength of these particular methods is that, in general, they can be tailored to capture a desired range of blood flow, such sequences either do not provide a sharp transition from stationary spins to flowing spins, or else are long, generating relaxation losses and undesirable SAR, and have limited immunity to resonance offsets and to RF inhomogeneity. This article provides design methods for improving these longer RF pulse sequences, especially to provide improved immunity to RF inhomogeneity, and also to improve immunity to resonance offsets, as well as to minimize RF sequence length. These design methods retain the flexibility to capture a desired range of blood flow, with sharp transitions between stationary spins and flowing blood. These improvement strategies are demonstrated through Bloch equations simulations of examples of these new sequences in the presence of blood flow. Examples of improved sequences that should prove suitable for use at 3.0Tesla are presented.
多年来，已经开发出多种磁共振成像（MRI）方法，用于在不使用造影剂的情况下可视化或测量血流。一类特定的方法使用与射频（RF）脉冲序列相关的流动编码梯度，以区分流动血液中的自旋与静止自旋。虽然这些特定方法的一个优点是，一般来说，它们可以进行调整以捕获所需的血流范围，但此类序列要么无法提供从静止自旋到流动自旋的清晰过渡，要么序列很长，会产生弛豫损耗和不理想的比吸收率（SAR），并且对共振偏移和RF不均匀性的抵抗力有限。本文提供了改进这些较长RF脉冲序列的设计方法，特别是为了提高对RF不均匀性的抵抗力，同时提高对共振偏移的抵抗力，并使RF序列长度最小化。这些设计方法保留了捕获所需血流范围的灵活性，在静止自旋和流动血液之间实现清晰过渡。通过在存在血流的情况下对这些新序列示例进行布洛赫方程模拟，展示了这些改进策略。还给出了应适用于3.0特斯拉的改进序列示例。
