It was a colleague’s MR images that gave David Brunner, a PhD student from the Institute of Biomedical Engineering of the ETH Zurich and the University of Zurich, the idea to use propagating waves for MRI. His colleague had taken images of a hand and captured so-called fold-over artifacts that seemed to originate from outside the detector. Clearly, signals were recorded not only from the target region but also from a considerable distance – although the detector was supposed to be sensitive only to its immediate surroundings. "This is only possible if the signals travel, that is, if they propagate as waves," explains Brunner.

The perfect wave

Klaas Prüssmann, Professor at the same Institute and David Brunner began to look for the optimal conditions for propagating waves together with other researchers from ETH Zurich and the University of Zurich. For efficient signal transmission they required a suitable waveguide with a sufficient diameter to support wave propagation. Here, the researchers were helped by a stroke of luck as their strongest magnet with a field strength of 7 Tesla (T), was just wide enough (58 cm) for unhindered propagation at the resonance frequency of 300 MHz. Lined with a conductive material, the magnet’s bore effectively acts as a waveguide. An antenna was used to generate propagating waves that penetrated the imaging sample and passed through the entire bore virtually without loss. Resonance signals, again in the form of propagating waves, were recorded by the same antenna, yielding MR images of greater coverage than previously possible at such high fields.

Antenna as a detector

Traditionally, MRI is based upon so-called near-field coupling, for which the detector is placed as close as possible to the body. Stationary radiofrequency fields are used to excite magnetic resonance in hydrogen nuclei (see box). According to the textbook, a good MRI detector is a resonator with optimal near-field coupling to the sample. However, at 7 T the signal wavelength in tissue is so short, about 10 cm, that the stationary radiofrequency fields form node regions, from which no image information can be obtained.

As a consequence, structures that are larger than the wavelength – such as the human head – can no longer be fully covered with the traditional concept. The scientists have now solved this problem by abandoning the stationary approach and rather adopting traveling radiofrequency waves, which do not form field nodes. In their study they show that the new method permits covering large parts of the body more uniformly, while receiving the underlying signals across distances in the meter range.

Current clinical MRI systems typically operate at a field strength of 1.5 T, corresponding to a signal frequency of 64 MHz. At this relatively low frequency the traditional, stationary detection principle does not limit coverage because the signal wavelength is much larger. However, compared to higher fields, the current clinical systems are less sensitive, yielding lower spatial resolution. Furthermore, close-range transmitters and detectors are often perceived as uncomfortable by patients and require special precautions to rule out exposure to excessive short-range electric fields in the event of malfunction.

Potential for hospitals and research

Initial imaging results from a volunteer’s lower leg and foot demonstrate more extensive coverage than possible with a traditional detector. «And the fact that MRI signals can be received with an antenna and across such large distances is remarkable; it’s a paradigm shift», says Klaas Prüssmann. The translation into clinical settings will take time notwithstanding. «Unfortunately, the cost of the strong magnets is still substantial and the clinical benefits of very high fields first need to be proven in extensive studies», he says. Yet traveling-wave MR detection from large volumes holds promise not only for medical imaging but could also facilitate very different applications. For instance, it may be used to examine large numbers of material samples or small animals in parallel for high-throughput screening.

Magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) is the imaging variant of nuclear magnetic resonance (NMR). It is based on an effect that occurs when hydrogen atoms are brought into a strong, static magnetic field and additionally exposed to an oscillating magnetic field in the radiofrequency range. At a certain resonance frequency the nuclei of the hydrogen atoms respond by oscillating and emitting weak radiofrequency signals themselves. MRI amounts to detecting these signals and rendering them in the form of images. In such depictions the contrast between different tissues reflects different hydrogen content and differences in the nuclei’s microscopic surroundings. The foundations of MRI were laid over 60 years ago by the American Edward M. Purcell and ETH Zurich graduate Felix Bloch through their discovery of NMR.

In recognition of this achievement the two researchers jointly received the Nobel Prize in Physics in 1952. NMR went on to revolutionize chemistry and structural biology, including key developments for which the Zurich-based scientists Richard R. Ernst and Kurt Wüthrich received Nobel Prizes in Chemistry. Still today, research at ETH Zurich and the University of Zurich is at the forefront of MRI development, including efforts to boost the speed, versatility and accuracy of the technique.

References:

Brunner DO, De Zanche N, Fröhlich J, Paska J & Pruessmann KP: Travelling-wave nuclear magnetic resonance, Nature (2009), 457, 944-998, doi:10.1038/nature07752.

Note: If no author is given, the source is cited instead.

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