Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
J Magn Reson. 2013 Apr;229:127-41. doi: 10.1016/j.jmr.2013.02.009. Epub 2013 Feb 15.
MRI remains the premier method for non-invasive imaging of soft-tissue. Since the first demonstration of ULF MRI the trend has been towards ever higher magnetic fields. This is because the signal, and efficiency of Faraday detectors, increases with ever higher magnetic fields and corresponding Larmor frequencies. Nevertheless, there are many compelling reasons to continue to explore MRI at much weaker magnetic fields, the so-called ultra-low field or (ULF) regime. In the past decade many excellent proof-of-concept demonstrations of ULF MRI have been made. These include combined MRI and magnetoencephalography, imaging in the presence of metal, unique tissue contrast, and implementation in situations where a high magnetic field is simply impractical. These demonstrations have routinely used pulsed pre-polarization (at magnetic fields from ~10 to 100 mT) followed by read-out in a much weaker (1-100 μT) magnetic fields using the ultra-sensitive Superconducting Quantum Interference Device (SQUID) sensor. Even with pre-polarization and SQUID detection, ULF MRI suffers from many challenges associated with lower magnetization (i.e. signal) and inherently long acquisition times compared to conventional >1 T MRI. These are fundamental limitations imposed by the low measurement and gradient fields used. In this review article we discuss some of the techniques, potential applications, and inherent challenges of ULF MRI.
MRI 仍然是软组织无创成像的首选方法。自首次展示 ULF MRI 以来，趋势一直是朝着更高的磁场发展。这是因为信号和法拉第探测器的效率随着磁场和相应的拉莫尔频率的增加而增加。然而，有许多令人信服的理由继续在弱得多的磁场下探索 MRI，即所谓的超低场或（ULF）范围。在过去的十年中，已经进行了许多出色的 ULF MRI 概念验证演示。这些包括 MRI 和脑磁图的组合、金属存在下的成像、独特的组织对比度，以及在高磁场根本不切实际的情况下的实现。这些演示通常使用脉冲预极化（磁场为~10 至 100 mT），然后在弱得多的磁场（1-100 μT）中使用超灵敏超导量子干涉仪（SQUID）传感器进行读出。即使使用预极化和 SQUID 检测，与传统的>1 T MRI 相比，ULF MRI 仍然存在许多与较低磁化（即信号）和固有较长采集时间相关的挑战。这些是由使用的低测量和梯度场造成的基本限制。在这篇综述文章中，我们讨论了 ULF MRI 的一些技术、潜在应用和固有挑战。
