MR experiments frequently rely on signal encoding by the application of magnetic fields that vary in both space and time. The accurate interpretation of the resulting signals often requires knowledge of the exact spatiotemporal field evolution during the experiment. To better fulfill this need, a new approach is presented that enables measuring the field evolution concurrently with any MR sequence. Miniature NMR probes are used to monitor the MR phase evolution around the object under investigation. Based on these data, a global phase model is calculated that can then be used as a basis for processing the actual image or spectroscopic data. The new method is demonstrated by MRI of a phantom, using spin-warp, spiral, and EPI trajectories. Throughout, the monitoring results enabled highly accurate image reconstruction, even in the presence of massive gradient imperfections. Magn Reson Med 60:187-197, 2008.

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Nuclear magnetic resonance (NMR) is one of the most versatile experimental methods in chemistry, physics and biology, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant-magnetic resonance imaging (MRI)-is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction in one or multiple radio-frequency resonators that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides, superconducting quantum interference devices, atomic magnetometers, Hall probes or magnetoresistive elements 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. E-mail: paska@ifh.ee.ethz.ch, Telephone: +41 44 6320430, 5 Institute for Biomedical Engineering, University and ETH Zürich, Gloriastrasse 35, 8092 Zürich, Telephone: +41 44 632 66 96, Fax: +41 44 632 11 93 In this work we introduce a novel concept of signal excitation and detection to NMR and MRI. We propose to abandon the long-standing principle of near-field inductive coupling between nuclear magnetization and the detector, commonly an RF resonator, by far-range travelling-wave interaction with an antenna probe. Along with the feasibility of this approach we demonstrate that it addresses a key obstacle to high-field MRI in large samples, particularly in humans. We believe that the transition to travelling-wave excitation and detection is significant both from a fundamental, physical point of view and with respect to the numerous applications that NMR and MRI have in the sciences and medicine. Uniform spatial coverage in NMR and MRI is traditionally achieved by tailoring the reactive near field of resonant Faraday probes 7-10 . This approach is valid when the RF wavelength at the Larmor frequency is substantially larger than the target volume, which 3 does not hold for recent wide-bore high-field systems. At the currently highest field strength that is used for human studies, 9.4 tesla 16,17 , the resonance frequency of hydrogen nuclei reaches 400 MHz, corresponding to a wavelength in tissue on the order of 10 cm. At such short wavelengths head or body resonators form standing-wave field patterns, which degrade MRI results by causing regional signal losses and perturbing the contrast between different types of tissue. Travelling-Wave ...