Scientists Image Material That Could Improve MRI Technologies

The research, led by William Halperin, professor of physics and astronomy, in collaboration with scientists from Northwestern, Argonne National Laboratory and the National High Magnetic Field Laboratory (NHMFL), was the highest magnetic field imaging experiment ever conducted.

The findings, including the direct evidence of an electronic Doppler effect, will be published Oct. 4 in the journal Nature.

"Currently, hospitals use low-temperature superconductors in MRI, but high-temperature superconductors — a relatively new discovery — may be a better material, with the bonus of requiring less cooling, thus reducing costs," said Halperin.

To advance its potential application in future technologies, the researchers first need to understand the physical properties of these materials and especially how they behave in the presence of very large magnetic fields.

"The goal in the medical world is to get better resolution in MRI technology in order to improve diagnoses — the better the resolution the more detail for analysis," said Halperin. "Our imaging method is a major technical advance in the study of superconductors and one that has basic implications for magnetic imaging."

For the first time, researchers were able to peer into the cores of vortices — tiny electrical tornadoes swirling around in the copper oxide compound, YBa2Cu3O7, the classic high-temperature superconducting material. (The vortices result from magnetic fields trapped inside the material.) The core of the vortex in the superconductor is very much like the eye of a hurricane except that it is so small it is hard to investigate. Each core measures only three nanometers across (the width of about 10 atoms strung together). Extremely high resolution — made possible by a large magnetic field — was required to look inside the vortex core.

Halperin’s team took advantage of the recently commissioned hybrid magnet in the Tallahassee Laboratory of the NHMFL, which provided them with the highest steady magnetic field in the world — a million times stronger than the Earth’s. This allowed the scientists to distinguish the core from what is around it.

All superconductors have two essential properties. First, a superconductor, when cooled to its appropriate temperature, conducts electricity without any resistance. In other words, an electrical charge can flow without generating any heat, which means no loss of energy. Second, a superconductor expels some of the magnetic field inside it. In order to keep the desirable zero resistance state, the magnetic fields remaining in the material (in the form of vortices) need to be understood and controlled.

The most important component of the MRI technology used in hospitals is the main magnet, which supplies the steady magnetic field critical for high-quality imaging. Superconducting magnets are the most widely used because they require less electricity, thus reducing the cost of operation. Also important are the gradient magnets that generate magnetic fields that vary in space. These non-uniform fields make it possible to distinguish one region of the body from another, generating three-dimensional images from any angle and direction, in a non-invasive fashion.

In order to study superconducting vortices in tiny crystals of YBa2Cu3O7, Halperin and his team relied on a similar MRI technique. In this case, the hybrid magnet at NHMFL — weighing 34 tons and standing 22 feet tall — provided the steady magnetic field, and the whirling electrical tornadoes — the vortices in the material — provided the non-uniform magnetic fields.

The extremely high spatial resolution of their method revealed new electrical and physical properties that the researchers did not expect to find. They learned that the electrons behave very differently inside the vortex’s core than outside. The core is no longer superconducting due to the intense magnetic fields generated by the vortex’s swirling "electronic winds."

Of particular interest was the observation that the circulating electrical currents surrounding the vortex’s core create an electronic Doppler effect. The acoustic version of this effect is well known. For example, sound waves from the whistle of an approaching train have a higher pitch than when the train is receding. Similarly, an electron moving in the current circulating around the vortex has its energy shifted upwards if it moves with the current and downwards if it is moving against the current. The electronic Doppler effect is key to understanding many properties of high-temperature superconductors.

In addition to Halperin, other authors on the paper are Vesna Mitrovic (principal author), Eric Sigmund and Nathan Bachman, from Northwestern; Mathias Eschrig, from Argonne National Laboratory; and William Moulton, Philip Kuhns and Arneil Reyes, from the National High Magnetic Field Laboratory.

The research was supported by the National Science Foundation.