The Russell H. Morgan Department of Radiology and Radiological Science, Division of MR Research, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Magn Reson Med. 2013 Apr;69(4):1003-13. doi: 10.1002/mrm.24334. Epub 2012 May 14.
Vascular-space-occupancy (VASO) MRI exploits the difference between blood and tissue T1 to null blood signal and measure cerebral blood volume changes using the residual tissue signal. VASO imaging is more difficult at higher field because of sensitivity loss due to the convergence of tissue and blood T1 values and increased contamination from blood-oxygenation-level-dependent (BOLD) effects. In addition, compared to 3T, 7T MRI suffers from increased geometrical distortions, e.g., when using echo-planar-imaging, and from increased power deposition, the latter especially problematic for the spin-echo-train sequences commonly used for VASO MRI. Third, non-steady-state blood spin effects become substantial at 7T when only a head coil is available for radiofrequency transmit. In this study, the magnetization-transfer-enhanced-VASO approach was applied to maximize tissue-blood signal difference, which boosted signal-to-noise ratio by 149% ± 13% (n = 7) compared to VASO. Second, a 3D fast gradient-echo sequence with low flip-angle (7°) and short echo-time (1.8 ms) was used to minimize the BOLD effect and to reduce image distortion and power deposition. Finally, a magnetization-reset technique was combined with a motion-sensitized-driven-equilibrium approach to suppress three types of non-steady-state spins. Our initial functional MRI results in normal human brains at 7T with this optimized VASO sequence showed better signal-to-noise ratio than at 3T.
血管空间占据（VASO）MRI 利用血液和组织 T1 之间的差异来使血液信号置零，并使用残留的组织信号来测量脑血容量变化。由于组织和血液 T1 值的收敛以及来自血氧水平依赖（BOLD）效应的污染增加，因此在较高的场强下，VASO 成像更加困难。此外，与 3T 相比，7T MRI 受到更多的几何失真的影响，例如在使用回波平面成像时，并且由于自旋回波序列中增加的功率沉积，后者对于 VASO MRI 中常用的自旋回波序列尤其成问题。第三，当仅使用头部线圈进行射频发射时，非稳态血液自旋效应在 7T 时变得非常显著。在这项研究中，应用了磁化转移增强的 VASO 方法来最大化组织-血液信号差异，与 VASO 相比，信号噪声比提高了 149%±13%（n=7）。其次，使用具有低翻转角（7°）和短回波时间（1.8ms）的 3D 快速梯度回波序列来最小化 BOLD 效应，并减少图像失真和功率沉积。最后，将磁化复位技术与运动敏感驱动平衡方法相结合，以抑制三种类型的非稳态自旋。使用这种优化的 VASO 序列，我们在 7T 下对正常人类大脑进行的初步功能 MRI 结果显示出比 3T 更高的信噪比。
