In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B In recent years, the used frequency of radio frequency (RF) pulses in MRI has increased considerably as a consequence of the increase in magnetic field strengths. As an increased frequency leads to a shorter wavelength, the electromagnetic fields (EM-fields) induced by the RF pulse should more and more be regarded as waves (1,2) impinging onto a very dielectrically heterogeneous object: the human body. As a wave propagates in the human body, it is being affected by the local dielectric properties which are tissue dependent. The interaction between the applied EM-fields and the body leads to a variation of the EM-fields in amplitude (3,4) as well as in phase (5); this is the origin of B 1 field inhomogeneities. The variations in the B + 1 field can be separated in two categories: body-scale effects and local effects. A recent simulation study has shown that the B + 1 field variations depend predominantly on the global dielectric properties and the contour of the body (6). Small deviations were found; these were attributed to local variations in dielectric properties. Furthermore, it was found that these local B + 1 variations can amount up to 10-20% in magnitude of the average B + 1 field at 3 T (7). These findings indicate that the local B + 1 field variations contain information about the local dielectric environment; in this study we aim to exploit this information. As the B

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 ...

ZTE imaging in humans was demonstrated for the first time, enabled by dedicated, high-performing RF hardware.

The results are found to be consistent with the history of safe use in MR scanning, but not with current safety guidelines. For future safety concepts, we suggest to use thermal dose models instead of temperatures or SAR. Special safety concerns for patients with impaired thermoregulation (e.g., the elderly, diabetics) should be addressed.

A flexible and versatile monitoring system is presented, delivering camera-like access to otherwise hardly accessible field dynamics with nanotesla resolution. Its stand-alone nature enables field analysis even during unknown MR system states.

It has been demonstrated that artifacts due to physiologically induced dynamic field perturbations can be greatly reduced by retrospective image correction based on field monitoring. The necessity to perform such correction is greatest at high fields and for field-sensitive techniques such as T2*-weighted imaging.

Use of an expanded signal model largely overcomes the traditional vulnerability of spiral imaging with long readouts. It renders single-shot spirals competitive with echo-planar readouts and thus deploys shorter echo times and superior readout efficiency for diffusion imaging and further prospective applications. Magn Reson Med 77:83-91, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Feedback field control is an effective means of eliminating dynamic field distortions in MR systems. Third-order spatial control at an update time of 100 ms has proven sufficient to largely eliminate thermal and breathing effects in brain imaging at 7 Tesla.