Propagation improves MRI

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Here's a novel thought, what if propagating electromagnetic waves could be used for magnetic resonance imaging, wouldn't that improve picture quality significantly? That's what graduate student David Brunner of ETH Zurich concluded and his idea could represent a paradigm shift in how MRI is carried out. Brunner and his colleagues in the Institute of Biomedical Engineering and at the University of Zurich have succeeded in exciting and imaging nuclear magnetic resonance in the human body.

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Inspiration came to Brunner when looking at MR images obtained by a colleague that showed a hand with so-called "fold-over" artefacts that eerily seemed to originate from outside the detector. Clearly, signals were recorded not only from the target region but also at a considerable distance from the target, explains Brunner. The MRI detector is only meant to be sensitive to its immediate surroundings, but if the signals are propagating beyond this, then these artefacts appear. Brunner suggested that this phenomenon might be exploited to improve MRI.

Working with his supervisor Klaas Prüssmann, Brunner began to investigate what conditions might produce optimal propagation. The most significant factor in producing efficient signal transmission is to have a suitable waveguide with a sufficient diameter to support the propagation of electromagnetic waves. Serendipitously, the strongest magnet available to the team, with a field strength of 7 Tesla, is just wide enough (580 mm) for unhindered propagation at the resonance frequency of 300 MHz. Moreover, because the bore of the magnet is lined with a conductive material, it effectively acts as a waveguide.

The team added an antenna to generate propagating waves that are able to penetrate the sample being imaged and thence pass through along the entire length of the magnetic bore with almost no energy loss. They recorded resonance signals, again in the form of propagating waves, using this same antenna. The results are MR images with much greater coverage than is possible even at such high fields.

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In conventional MRI, which operates at a field strength of 1.5 T, corresponding to a signal frequency of 64 MHz, near-field coupling requires the detector to be positioned as close as possible to the sample or body part. The stationary radiofrequency fields used to excite magnetic resonance in the hydrogen of the sample produce signals that are best recorded with a good MRI detector that is itself a resonator with optimal near-field coupling to the sample.

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However, at 7 T signal wavelength is just 100 mm in tissue and so stationary radiofrequency fields form blind spots, nodes, from which no image information can be obtained. At this magnetic field strength whole structures, such as the human head cannot be scanned using the conventional approach without modification.

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The Swiss researchers have effectively solved this problem by abandoning the stationary approach and rather adopting travelling radiofrequency waves. This precludes the formation of 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 metre range. The main advantage of the discovery as it will be applied to clinical systems is that it makes much higher magnetic field strengths available to radiographers, which in turn means much higher resolution imaging.

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The team has demonstrated proof of principle with images of a volunteer's lower leg and foot. The images are much clearer than those obtained with conventional MRI and demonstrate more extensive coverage than possible with a traditional detector.

"The fact that MRI signals can be received with an antenna and across such large distances is remarkable," says Prüssmann." There remains an obstacle to moving the discovery into the clinic, however. "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 adds. Nevertheless, the approach holds great promise for medical applications and in research studies where it should now be possible image whole organisms and large non-biological samples.