Uses of Superconductivity in Medical Science

Brief History of Superconductivity

The beginnings of superconductivity can be traced back to the research done by scientist Kammerling Onnes in the year 1911. Onnes, while serving a professorship of physics at the University of Leiden (in the Netherlands), was studying the electrical properties of pure metals at low temperatures (near zero degrees Kelvin) using another recently discovered phenomenon, liquefied helieum. He found that when he immersed the metals (mercury, for historical purposes) the electrical resistivity for the metal suddenly dropped to zero. This was not what was expected, neither by Onnes nor his peers at the time, and was thus the discovery of the phenomenon known as superconductivity.

Jump forward to the year 1933, and into the research of Walther Meissner and Robert Ochsenfeld. The two were conducting research in the newly maturing area of metallic properties at low temperatures when they discovered an important characteristic for superconductors (Type I only), the Meissner Effect. The researchers noticed that once their metallic sample had been cooled to a temperature in which it could make its transition into a superconducting state, it would repel any applied magnetic fields that were placed in the sample’s presence. Building upon the work of Meissner and Ochsenfeld, researchers Fritz and Heinz London produced some theoretical groundwork that was useful for explaining the experimental results that had been observed thus far, but little else. Another attempt to explain superconductivity was made in 1950, by scientists L. D. Landau and Vitaly Ginzburg. Although this theory too did little to fully explain the nature of superconductivity, it did predict the splitting of superconducting materials into two categories, Type I and Type II superconductors.

With a growing list of theories that could only explain the experimental results that were found in many laboratories and little else, it was appearing as though superconductivity at the microscopic level would remain a mystery. However, in 1957, researchers John Bardeen, Leon Cooper, and John Schrieffer put forth their work, called the BSC Theory of superconductivity, which provided theoretical groundwork that did more than just explain experiments.

In the last few decades there have been fewer advances to the area of superconductivity. Many hours and many researchers have sought to find so–called high temperature superconductors, but functioning superconductors at room temperature are still in the horizon. With the dearth of understanding of superconductivity mounting, researchers have turned to applications of superconductivity as a means to advance the field.

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Applications to Medicine

MRI (Magnetics Resonance Imaging)

Origins

MRI, or Magnetic Resonance Imaging, is probably the most well known application of superconductivity in medicine. It was discovered in 1937 by scientist Isidor Isaac Rabi, who had been studying the effect of magnetic fields on a beam of lithium chloride molecules, a method called molecular beam resonance. Rabi had postulated that with precisely the right perturbation, the magnetic dipole moments of the particles in an atom could be induced to flip their orientation, and when radio waves were passed over the lithium chloride beam, his hypothesis was confirmed. And thus the precursor to the MRI was born, molecular beam magnetic resonance.

The next step towards the modern MRI came from research groups led by Edward Purcell and Felix Bloch at Harvard and Stanford, respectively. Both researchers were studying the nuclear resonance that Rabi had pioneered, but were focusing on hydrogen due to its large magnetic dipole. Each group was able to reproduce the results from Rabi’s experiments using different sources of hydrogen in 1945. The work of Purcell and Bloch is significant to the MRI of today because it showed the practicality of tracking hydrogen using nuclear magnetic resonance. Since hydrogen is so prevalent in the human body, many chemical processes and actions are able to be mapped, and consequently studied.

The final key step in the evolution of the modern day MRI was made back in the late 1940s by two independent researchers, Henry Torrey and Erwin Hahn. Both scientists were able to gather much more complete data on the relaxation time (the time between flips of a particle’s spin) when they switched from a continuous supply of radio waves to a few short pulses of radio waves. These pulses allowed for magnetic resonance imaging to be possible, albeit after a more sophisticated pulse method was developed by Russle Varian, called Fourier transform NMR (Nuclear Magnetic Resonance).

How it Works