Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Gloriastrasse 35, 8092 Zurich, Switzerland.
Neuroimage. 2021 Feb 1;226:117286. doi: 10.1016/j.neuroimage.2020.117286. Epub 2020 Sep 28.
T2*-weighted gradient-echo sequences count among the most widely used techniques in neuroimaging and offer rich magnitude and phase contrast. The susceptibility effects underlying this contrast scale with B, making T2*-weighted imaging particularly interesting at high field. High field also benefits baseline sensitivity and thus facilitates high-resolution studies. However, enhanced susceptibility effects and high target resolution come with inherent challenges. Relying on long echo times, T2*-weighted imaging not only benefits from enhanced local susceptibility effects but also suffers from increased field fluctuations due to moving body parts and breathing. High resolution, in turn, renders neuroimaging particularly vulnerable to motion of the head. This work reports the implementation and characterization of a system that aims to jointly address these issues. It is based on the simultaneous operation of two control loops, one for field stabilization and one for motion correction. The key challenge with this approach is that the two loops both operate on the magnetic field in the imaging volume and are thus prone to mutual interference and potential instability. This issue is addressed at the levels of sensing, timing, and control parameters. Performance assessment shows the resulting system to be stable and exhibit adequate loop decoupling, precision, and bandwidth. Simultaneous field and motion control is then demonstrated in examples of T2*-weighted in vivo imaging at 7T.
T2*-weighted 梯度回波序列是神经影像学中应用最广泛的技术之一，提供丰富的幅度和相位对比。这种对比的磁化率效应与 B 相关，因此在高磁场下，T2*-weighted 成像特别有趣。高磁场还受益于基线灵敏度，从而促进了高分辨率研究。然而，增强的磁化率效应和高目标分辨率带来了固有挑战。依赖于长回波时间，T2*-weighted 成像不仅受益于增强的局部磁化率效应，而且还受到由于身体运动和呼吸而引起的场波动的影响。反过来，高分辨率使神经影像学特别容易受到头部运动的影响。本工作报告了一个旨在共同解决这些问题的系统的实现和特性。它基于两个控制回路的同时操作，一个用于磁场稳定，一个用于运动校正。这种方法的关键挑战是，这两个回路都在成像体积中的磁场上运行，因此容易相互干扰和潜在的不稳定性。在传感、定时和控制参数方面解决了这个问题。性能评估表明，所得到的系统稳定，并且具有足够的环路解耦、精度和带宽。然后在 7T 下进行的体内 T2*-weighted 成像示例中展示了同时的磁场和运动控制。
