The technical limitations of MRI machines are caused by all sorts of laws of nature. One reason, for example, why machines over 3 Tesla are not manufactured for clinical purposes, is that the frequency of the radio signal sent through the body does not lead to quality images. Researchers from the Institute of Biomedical Technology at ETH Zurich and the University of Zurich seem to have in one brilliant move overcame a number of physical constraints.
From the Swiss Federal Institute of Technology Zurich:
It was a colleague’s MRI images that gave David Brunner, a PhD student from the Institute of Biomedical Technology at ETH Zurich and the University of Zurich, the idea to use diffusing waves for the MRI. He had taken pictures of hands and captured so-called deconvolution artifacts in the process that came from objects outside the detector. Clearly, not only were signals recorded from close quarters but also from a considerable distance from the detector – although the detector is only supposed to be sensitive in its immediate surroundings. “This is only possible if the signals travel, meaning the waves propagate”, explains Brunner.
Klaas Prüssmann, Professor at the Institute for Biomedical Technology and his PhD student David Brunner therefore began to look for the ideal conditions for propagating waves together with other researchers from ETH Zurich and the University of Zurich in order to be able to use them. For this to work, they required a suitable waveguide with a big enough diameter to allow the desired wave propagation. Here, the researchers were helped by a stroke of luck as, with a field strength of 7 Tesla (T), the resonance signals reach a frequency of 300 MHz in the 35-ton magnetic tube in the MRI research plant of ETH Zurich and Uni Zurich. Thanks to the electrically conductive layer that lines the tube, the short ten-centimeter wavelengths generated at these high frequencies can use the magnetic tube as a waveguide and their diameter of 58 centimeters is just big enough to enable the waves to spread out. Consequently, it was possible to generate propagating waves that penetrated the object being examined and passed through the entire tube practically without loss. The waves were then recorded by a special antenna and transformed into high-resolution, unusually well-illuminated MRI images. The results of the study were published today in the current issue of the science journal “Nature”.
Traditionally, the MRI is based upon so-called near-field coupling, where the detector is placed as close to the body as possible. Stationary waves are generated by stimulating hydrogen nuclei in an organism (see box). According to the textbook, a good MRI detector is a resonator with optimal near-field coupling. For a 7 T field strength, however, at about 10 centimeters the wavelength is so short that nodes develop every five centimeters, upon which no image information can be obtained. Structures that are larger than the wavelength – such as the human head – can therefore no longer be fully illuminated using the usual approach at high field strengths. Within this mode of thought, the problem of illumination at high field strengths appeared unsolvable. The scientists have now solved the problem with their study, however: unlike stationary waves, the propagating waves they generated did not display any nodes.
The concept of using an antenna as a detector has created a new margin. The propagating waves make it possible to illuminate the body in a uniform and high-contrast manner, even with high field strengths, and receive the signals of the oscillating nuclei up to a distance of three meters.
For the field strengths of 1.5 Tesla and a frequency of 64 megahertz (MHz) used today in infirmaries, the stationary waves are not a problem due to larger wavelengths. However, compared to high field strengths, the images are low-contrast and have a poorer resolution. Then there is the fact that placing the detectors close by is often perceived as unpleasant and, in the event of electronic defects there is always the danger that the body will be heated excessively by the electric fields.
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Images: Top: Structure of the NMR/MRI experiments with propagating waves: a) The magnetic AC field (B) of a propagating wave is able to stimulate and detect nuclear magnetization (M) over large distances from the antenna. However, for this to take place, the frequency of the wave has to be high enough and the diameter of the tube large enough. b) Large objects, like an entire human body for instance, can also be illuminated within the wave guide, even in the highest field strengths. In the process, the antenna can even be placed some distance away from the body. Side: Comparison of two MRI images with 7 Tesla field strength: a) Was taken with the aid of propagating waves, whereby the detector to initiate and receive the signals was 70 centimeters from the test person. b) Image with conventional detector.