Travelling-wave nuclear magnetic resonance

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are widely used in the sciences and medicine. Although the implementation details differ from application to application, the underlying detection principle is the same: the need for intimate coupling (and hence usually close proximity) between nuclear magnetization in the sample and the detector. Brunner et al. show that it is possible to abandon this traditional detection principle, and that the nuclear magnetization signal can be excited and detected by long-range interaction using travelling radiofrequency waves sent and received by an antenna. This approach offers more uniform coverage of larger samples. And by freeing up space in the centre of the costly high-field magnets needed for MRI, it could potentially make the imaging experience more comfortable for human subjects. Magnetic resonance imaging is widely used in the sciences and medicine, with the same basic underlying detection principle: the need for intimate coupling between nuclear magnetization in the sample under investigation and the detector. This study now shows a new detection principle, where the nuclear magnetization signal can be excited (and detected) via a long-range interaction utilizing travelling radiofrequency waves in a suitably modified MRI system. This approach offers more uniform coverage of larger samples. Nuclear magnetic resonance1,2 (NMR) is one of the most versatile experimental methods in chemistry, physics and biology3, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant—magnetic resonance imaging4,5 (MRI)—is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction6 in one or multiple radio-frequency resonators7,8,9,10 that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides11, superconducting quantum interference devices12, atomic magnetometers13, Hall probes14 or magnetoresistive elements15 have been explored. However, a common feature of all NMR implementations until now is that they rely on close coupling between the detector and the object under investigation. Here we show that NMR can also be excited and detected by long-range interaction, relying on travelling radio-frequency waves sent and received by an antenna. One benefit of this approach is more uniform coverage of samples that are larger than the wavelength of the NMR signal—an important current issue in MRI of humans at very high magnetic fields. By allowing a significant distance between the probe and the sample, travelling-wave interaction also introduces new possibilities in the design of NMR experiments and systems.