Jiang Baiyan, Chen Weitian
Department of Imaging and Interventional Radiology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong SAR, China.
NMR Biomed. 2018 Jul;31(7):e3928. doi: 10.1002/nbm.3928. Epub 2018 Apr 25.
Spin-lock MRI is a valuable diagnostic imaging technology, as it can be used to probe the macromolecule environment of tissues. Quantitative T imaging is one application of spin-lock MRI that is reported to be promising for a number of clinical applications. Spin-lock is often performed with a continuous RF wave at a constant RF amplitude either on resonance or off resonance. However, both on- and off-resonance spin-lock approaches are susceptible to B and B inhomogeneities, which results in image artifacts and quantification errors. In this work, we report a continuous wave constant amplitude spin-lock approach that can achieve negligible image artifacts in the presence of B and B inhomogeneities for both on- and off-resonance spin-lock. Under the adiabatic condition, by setting the maximum B amplitude of the adiabatic pulses equal to the B amplitude of spin-lock RF pulse, the spins are ensured to align along the effective field throughout the spin-lock process. We show that this results in simultaneous compensation of B and B inhomogeneities for both on- and off-resonance spin-lock. The relaxation effect during the entire adiabatic half passage (AHP) and reverse AHP, and the stationary solution of the Bloch-McConnell equation present at off-resonance frequency offset, are considered in the revised relaxation model. We demonstrate that these factors create a direct current component to the conventional relaxation model. In contrast to the previously reported dual-acquisition method, the revised relaxation model just requires one acquisition to perform quantification. The simulation, phantom, and in vivo experiments demonstrate that the proposed approach achieves superior image quality compared with the existing methods, and the revised relaxation model can perform T quantification with one acquisition instead of two.
自旋锁定磁共振成像（Spin-lock MRI）是一种有价值的诊断成像技术，因为它可用于探测组织的大分子环境。定量T成像就是自旋锁定磁共振成像的一种应用，据报道在许多临床应用中很有前景。自旋锁定通常是在共振或非共振状态下，以恒定的射频幅度施加连续射频波来进行的。然而，共振和非共振自旋锁定方法都容易受到B和B不均匀性的影响，这会导致图像伪影和定量误差。在这项工作中，我们报告了一种连续波恒定幅度自旋锁定方法，对于共振和非共振自旋锁定，在存在B和B不均匀性的情况下，该方法能实现可忽略不计的图像伪影。在绝热条件下，通过将绝热脉冲的最大B幅度设置为自旋锁定射频脉冲的B幅度，可确保自旋在整个自旋锁定过程中沿着有效场排列。我们表明，这会同时补偿共振和非共振自旋锁定的B和B不均匀性。在修正的弛豫模型中考虑了整个绝热半通道（AHP）和反向AHP期间的弛豫效应，以及在非共振频率偏移处存在的布洛赫 - 麦康奈尔方程的稳态解。我们证明这些因素为传统弛豫模型创建了一个直流分量。与先前报道的双采集方法不同，修正的弛豫模型只需要一次采集就能进行定量。模拟、体模和体内实验表明，与现有方法相比，所提出的方法实现了更高的图像质量，并且修正的弛豫模型可以通过一次采集而不是两次来进行T定量。
