By Charles Nicolson
The feature article in the January 2020 issue of RACA Journal introduced the lower regions of very cold and even colder – from a now generally accepted cryogenic upper limit of -50°C down to absolute zero temperature of -273.15°C.
A section of a hand X-ray plate showing the bones and knuckle joints.A section of a hand X-ray plate showing the bones and knuckle joints. Image credit: Pexels
The article went on to describe the first widely-used cryogenic product – dry ice or solid carbon dioxide – which conveniently sublimes or changes directly from solid to vapour phase at -78.15°C at normal atmospheric pressure. The fact that dry ice is still in common use since the 1920s is a good example of how substances which have to be made and stored for use at cryogenic temperatures continue to provide useful benefits. Many other useful products have been created in controlled cryogenic conditions including those which become electrically superconducting below specific temperatures.
The electrical resistance of ordinary metallic conductors decreases gradually as temperatures are lowered down to levels only a few degrees above absolute zero. However, certain other conducting materials which can acquire an ability known as ‘superconductivity’ suddenly lose all of their electrical resistance when reducing temperatures reach specific levels. Electric currents in superconducting circuits can persist indefinitely with no power source. Promising future applications of superconductivity include magnetic refrigeration at potentially higher coefficient of performance (COP) levels and far more efficient refrigeration with added benefits of greatly reduced carbon emissions.
History of superconductivity
The discovery of superconductivity was made in 1911 by Kamerlingh Onnes using solid mercury as the conductor, supercooled by liquid helium down to close to -270°C. Subsequently, Onnes attempted to make electromagnets with various types of superconducting material windings but found that even relatively low magnetic fields destroyed the superconductivity within the materials he was investigating.
Further progress of research during the first half of the 20th century into this new and complex field of superconducting tended to be drawn out by practical difficulties of running experiments at temperatures below -200°C. In addition, two world wars relegated most of the purely scientific research programmes to back burners although some specific metallurgical research concerning machining of metals at cryogenic temperatures was done in the USA. During the 1950s the physicist G B Yntema determined that superconducting wire made from niobium-based alloys could be used for windings which generated stable magnetic fields. Further developments incorporating niobium/tin combinations showed that these alloys could carry currents up to 100 000 amperes per square centimetre (A/cm2) in superconducting conditions controlled within a 5°C approach to absolute zero.
Despite being brittle and difficult to fabricate, niobium-tin coils have proved to be practical for generating magnetic fields as high as 20 Tesla. However, early in 1962 alloys of niobium and titanium which were far more ductile and easier to work with than the niobium-tin alloys were found to be suitable for producing magnetic field strengths up to 10 Tesla which has since proven to be high enough for many industrial and other practical applications.
In 1963, commercial production of niobium-titanium ‘supermagnet wire’ began at Westinghouse Electric Corporation and at Wah Chang Corporation, an American company located in Albany, Oregon. Although niobium-titanium has lower superconducting properties than those of niobium-tin, niobium-titanium has, nevertheless, become the most widely used supermagnet material on account of having high ductility and ease of fabrication. Both niobium-tin and niobium-titanium coil circuits are extensively used as particle beam bending and focusing magnets in high-energy-particle accelerators, MRI imaging machines and a host of other applications. A European superconductivity consortium, Conectus, estimated that in 2014, global economic activity for which superconductivity was indispensable amounted to about five billion euros, with Magnetic Resonance Imaging (MRI) systems (mostly medical units) accounting for about 80% of the total.
Head CT scan machine.Head CT scan machine. Image credit: Pixabay
X-rays and MRIs
Recognition of the value of the research and development work done in this new area of superconductivity is reflected in the number of Nobel physics prizes awarded. One went to Kamerlingh Onnes in 1913 and up to 2003, five further Nobel prizes have been shared amongst eleven scientists and researchers.
For the great majority of people MRI is one of the machines which, similarly to X-rays, produces detailed images of internal organs and bones in their bodies. As noted earlier, most MRI machines are used for medical scans but from a technical point of view, the only similarity between X-ray and MRI scans is that both require high levels of computer control and computer power to process electronic signals into high quality images. X-rays have been known experimentally and applied in practical ways for far longer than magnetic resonance radiation although the name ‘X-rays’ came into being only in 1895 when the German scientist, Wilhelm Röentgen, named it X-radiation to signify an unknown type of radiation which he could not sufficiently identify. The name caught on and has remained unchanged.
Röentgen is also credited with discovering their medical use when he made a picture of his wife’s hand on a photographic plate which was the first photograph of a human body part using X-rays.
In recognition of his pioneering work and research concerning X-rays, Röentgen was awarded the first ever Nobel Prize for physics in 2001. Modern scanning using X-rays, referred to as radiography for medical applications, is now called CT scanning which stands for ‘computerised tomography’.
A CT scan works by taking multiple X-rays at various angles and then utilises those X-rays to form a three-dimensional image of whatever organs or organ systems are being examined. A computer examines all of the various X-rays taken at different angles and processes the images to form a three-dimensional computer model.
MRIs use and send radiofrequency waves into the body or body parts positioned inside intense magnetic fields produced by high amperage, supercooled coil windings. The magnetic fields line up internal organ atoms either in a north or south position with a few atoms that are unmatched (keep spinning in a normal fashion). When radiofrequency radiation is then applied the unmatched atoms spin in an opposite direction, and when the radiofrequency is turned off, those unmatched atoms return to the normal position which involves energy emissions. These emissions send signals to a computer programmed to transform the signals into an image.
MRI scanners are particularly well suited to image the non-bony parts or soft tissues of the body. The brain, spinal cord and nerves, as well as muscles, ligaments, and tendons are seen much more clearly with MRI than with regular X-rays and CT, and for this reason MRI is often used to image knee and shoulder injuries.
In the brain, MRI can differentiate between white matter and grey matter and can also be used to diagnose aneurysms and tumours. Because MRI does not use X-rays or other ionising radiation, it is usually the choice when frequent imaging is required for diagnosis or therapy.
One kind of specialised MRI, functional Magnetic Resonance Imaging (fMRI) is used to observe brain structures and determine which areas of the brain ‘activate’ (consume more oxygen) during various cognitive tasks. It is used to advance the understanding of brain organisation and offers a potential new standard in assessing neurological status and neurosurgical risks.
Although as mentioned, MRI does not emit the ionising radiation that is found in X-ray and CT imaging, it does employ a strong magnetic field. The magnetic field extends beyond the machine and exerts very powerful forces on objects of iron, magnetic steels, and other magnetisable objects. In one instance it has been reported as ‘strong enough to fling a wheelchair across the room’! Patients must notify their physicians of any form of medical or other implants prior to undergoing an MRI scan.
X-rays carry enough energy to ionise atoms and disrupt molecular bonds. This makes it a type of ionising radiation which is potentially harmful to living tissue. High radiation doses over a short period of time can cause radiation sickness, while lower doses can increase risks of radiation-induced cancer. In medical imaging this increased cancer risk is generally greatly outweighed by the benefits of the examination. Also, the ionising capability of X-rays can be beneficially utilised in cancer treatment to kill malignant cells using radiation therapy.
One of the continually growing areas of X-ray technology, Phase-contrast X-ray imaging, refers to a variety of techniques that use phase information of an X-ray beam to image soft tissues. It has become an important method for visualising organic structures over a wide range of biological and medical studies. These methods provide higher contrasts making it possible to see smaller details. However, a disadvantage is that these methods require more sophisticated equipment such as high-resolution X-ray detectors.
In addition to medical uses, both MRI and X-rays are widely used across commercial, industrial and manufacturing processes to image the inside of visually opaque objects. The most often seen applications are airport security scanners, but many similar scanners have become important particularly for quality control on highly automated production lines. Compared to X-ray radiation, MRI is still a relatively new technology but is finding increasing uses for routine analysis of chemicals, measuring the ratio between water and fat in foods, monitoring flows of corrosive fluids in pipes and studying complex molecular structures such as catalysts.
A larger range and number of technical methods have been developed for X-ray radiation mainly due X-ray applications starting almost half a century before MRI. Examples are X-ray crystallography in which the pattern produced by diffraction of X-rays through a closely spaced lattice of atoms in a crystal is recorded and then analysed to reveal the nature of the lattice. Fibre diffraction; a technique which was used by Rosalind Franklin to discover the double helical structure of DNA. X-ray fluorescence, a technique in which X-rays are generated within a sample and detected and outgoing energies of the X-rays can be used to identify the composition of the sample. Industrial radiography in which X-rays are increasingly used for inspection of industrial parts, particularly welds.
X-ray technology is regarded as relatively mature while cryogenic cooling needed for newer technologies including MRI is still proceeding through developmental stages. Only nine countries in the world in addition to the US produce the core product for cryogenics, liquid helium. However, according to a recent Reuters report in The Star newspaper another country plans to bring liquid helium production on-line by 2021; South Africa, where deposits of liquid natural gas (LNG) containing sufficient helium have been found in the Virginia region of the Free State province. According to the report, the Virginia Gas Project is planned for daily production of 654.3tons of LNG and 350kg of helium making South Africa the only African country to have these facilities.