Superconductors Open New Frontiers for MRI
The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2003
"for pioneering contributions to the theory of superconductors and superfluids" jointly, to: "Superconducting material is used, for example, in magnetic resonance imaging for medical examinations and particle accelerators in physics. Knowledge about superfluid liquids can give us a deeper insight into the ways in which matter behaves at its lowest and most ordered state," said the Royal Swedish Academy of Sciences in its news release from Stockholm. The year 2003 Nobel Prize in Physics was awarded to three physicists who have made an outstanding contribution by explaining the quantum physics phenomena, in particular the effects of superconductivity and superfluidity. Superconducting materials made it possible to design and clinically apply sophisticated equipment for magnetic resonance imaging (MRI). Obviously, the year 2003 Nobel Prizes bore the signature of MRI, since Paul C Lauterbur and Peter Mansfield, (who were also awarded the Nobel Prize in medicine or physiology in 2003) made pioneering contributions to that modality of medical imaging. At very low temperatures, only a few kelvin above the absolute zero, some metals allow the flow of the electric current without resistance. These metals are called superconductors. In a superconductor, electrons are attracted to one another and form pairs, and therefore there is no resistance to the electrical current. The jointly developed BCS theory, developed in 1957 by John Bardeen, Leon Cooper and John Schrieffer from the University of Illinois at that time, brought these scientists the Nobel Prize in Physics in 1972. Their theory explains superconductivity at temperatures close to absolute zero, but has difficulty accounting for the higher temperatures that were later achieved with copper-containing superconductors. The BCS-theory explains type - I superconductors, i.e. those that displace the magnetic flow completely. Their theory however couldn't explain the behaviour of technically important superconductive materials, also called superconductors type - II. They allow superconductivity in presence of high magnetic fields. Vitaly Ginzburg developed a theory for superconductors of type - I, which was applicable for the new types of materials as well. Alexei Abrikosov explained theoretically the characteristics of the superconductors type -II that are also superconductive at temperatures much higher than absolute zero, and at strong magnetic fields. Superconductors have no electrical resistance, and superfluids have no viscosity, so they can flow freely. Anthony Leggett developed a theory which explained the flow of normal and superfluid helium liquids and other strongly-coupled superfluids. The development of superconductors has improved MRI equipment, as the superconducting magnet can be smaller and more efficient than a conventional magnet. The resolution of the magnetic resonance scanner is partly dependent on the strength of the magnetic field. As such, the main advantage of MRI at high field is in the increase in signal-to-noise ratio. This can be used to improve both the anatomic and the temporal resolution, as well as to reduce the scan time while preserving image quality. Strong superconducting magnets used these days are all of type-II. MRI magnets require type-II superconductor alloys such as niobium or niobium-titanium, or the newly discovered high-temperature superconductors, which all work in the presence of magnetic fields. Forming Cooper pairs is not enough for applications in MRI. So the magnetic vortices induced by the magnetic field have to be "pinned" or stopped in type-II superconductors, in order not to destroy the property of superconductivity. At the time Abrikosov first published his paper on existence of the "superconductors of the second group", in 1952 in Russia, these materials were considered as exotic. Today, in contrary, those of type - I are rather more unusual since the other type has found many applications. What about the possibility to discover and produce superconductive materials at room-temperatures (RTSC)? In his Nobel speach, Grinzburg was optimistic: "The development of high-temperature superconductivity had been my dream for 22 years, even with no guarantee that the goal was at all attainable and, in particular, attainable in the foreseeable future. In my view, obtaining room-temperature superconductivity now occupies the same place." The superconducting coils produce a strong and uniform magnetic field within the body of the patient. The scanner has a large size due to the cooling system containing liquid helium. The magnets in use today in MRI range from 0.5-tesla to 3.0-tesla. The introduction of the 3T whole-body MRI scanners into the clinical environment enabled MR applications like anatomical brain imaging, functional MRI, as well as diffusion and brain MRS. The body coil in 3T scanners provides opportunities for a broad range of MR research including technological development like parallel imaging, pulse sequence development etc. measurements of flow using arterial spin labeling, multinuclear imaging, imaging of the liver and abdomen, imaging of the breast, imaging of the prostate, cardiac MR and MR characterization of artherosclerotic plaques. MR spectroscopy (MRS) benefits from the use of an ultra-high field too, through increased chemical shift resolution. Improvements in spectral resolution further enhance the possibility of spectral editing and speeding up spectroscopic imaging so that they last only a couple of minutes, similar to any other MR imaging method [P. Folkers and P. Boesiger: "A compact 3.0 T MR system for routine application", Medicamundi 45/4, 2001, pp. 2-10]. The National Institute of Health is already using the 4.7 Tesla Vertical Magnet System for fMRI in non-human primates. The MRI vertical bore magnets have been specially designed for fMRI investigations. Robust SE BOLD and perfusion fMRI were obtained from volunteers with a nominal in-plane resolution up to 0.5 x 0.5 mm2 at 7 and 4 Tesla, and were highly reproducible under repeated measures. This methodology enables high-resolution and high-specificity studies of functional topography in the millimeter to submillimeter spatial scales of the human brain. [T. Q. Duong et all.: "High-Resolution, Spin-Echo BOLD, and CBF fMRI at 4 and 7 T", Magn Reson Med 48, 2002, pp. 589-593]. Introducing the new, stronger magnets was also a good reason for reviewing MRI magnet safety. When using high field MRI, the RF-wavelength and the dimensions of the human body are making the development of MR coils very complicated. The absorption of RF power causes heating of the tissue. The specific absorption rate (SAR) induced temperature changes of the human body are the most important safety issue of high field MR scanners. Patient safety is one of the issues to be addressed at the World Health Assembly (WHA) to be held from May 17 to May 22, 2004 in Geneva, Switzerland. In 2002, the WHO accepted the Resolution WHA55.18 and its implementation will be covered by a report for the 2004 WHA. (Read more about the WHA in the July issue of the News.)
Ratko Magjarevic
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