Volegov P L, Mosher J C, Espy M A, Kraus R H
Los Alamos National Laboratory, Biological and Quantum Physics Group, Los Alamos, NM 87545, USA.
J Magn Reson. 2005 Jul;175(1):103-13. doi: 10.1016/j.jmr.2005.03.015.
Growing interest in magnetic resonance imaging (MRI) at ultra-low magnetic fields (ULF, approximately muT fields) has been motivated by several advantages over its counterparts at higher magnetic fields. These include narrow line widths, the possibility of novel imaging schemes, reduced imaging artifacts from susceptibility variations within a sample, and reduced system cost and complexity. In addition, ULF NMR/MRI with superconducting quantum interference devices is compatible with simultaneous measurements of biomagnetic signals, a capability conventional systems cannot offer. Acquisition of MRI at ULF must, however, account for concomitant gradients that would otherwise result in severe image distortions. In this paper, we introduce the general theoretical framework that describes concomitant gradients, explain why such gradients are more problematic at low field, and present possible approaches to correct for these unavoidable gradients in the context of a non-slice-selective MRI protocol.
超低磁场（ULF，约为微特斯拉场）下磁共振成像（MRI）的关注度日益增长，这是由其相对于更高磁场下的同类技术的若干优势所推动的。这些优势包括线宽窄、可采用新颖的成像方案、样本内由于磁化率变化产生的成像伪影减少，以及系统成本和复杂性降低。此外，带有超导量子干涉器件的ULF NMR/MRI与生物磁信号的同步测量兼容，这是传统系统所不具备的能力。然而，在ULF下进行MRI采集时，必须考虑伴随梯度，否则会导致严重的图像失真。在本文中，我们介绍了描述伴随梯度的一般理论框架，解释了为何此类梯度在低场下更成问题，并提出了在非切片选择MRI协议背景下校正这些不可避免梯度的可能方法。
