Physics of Medical Scans
MRI In the Future
MRI-PET Hybrid Scans
Scans combining MRI technology with PET was first developed in 2010. It has an advantage of producing even more detailed soft tissue images, while minimising the amount of harmful radiation. [1]
However, at the moment, MRI-PET combination scans are still largely in development. Currently, there are a limited number of doctors trained in interpreting these images. [2] This problem will be eliminated in the future, as it is expected that MRI-PET scans are going to become increasingly common.
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High-Temperature Superconductors?
Researchers are currently in the search for room-temperature superconductivity. This would enable us to maintain the strong magnetic field while eliminating the use of helium, a finite resource that we may run out in 30 years.
In 2018, researchers from the Plank Institute for Chemistry in Germany reported the discovery of superconductors with a critical temperature of 203K. This temperature can be reached without the use of cryogenics. The material is sulphide hydride, H3S, which is formed under high pressures, between 155-160 GPa. [3]
Another promising candidate is Lanthanum Hydride, which has a critical temperature of 209K at a pressure of 170 GPa. [4]
7T Magnets?
Does increasing the strength of the magnetic field increase the resolution? The state-of-the-art 7T MRI scanners produce images of soft tissues with an even greater contrast. Research shows that the high resolution offered by these 7T MRI scanners allow structural changes of the brain caused by neurodegenerative diseases such as the Alzheimer’s disease to be seen, whereas before the emergence of 7T MRI, these changes could only be seen upon post-mortem tissue analysis. 7T MRI scanners are a promising neuro-imaging technique. [5]
However, a study by Laarder et al found that, although 7T MRI offers a higher spatial resolution, it suffered from a reduced image quality when it was used to scan the abdominal region. This is because 7T scanners are more susceptible to artefacts, as well as inhomogeneity caused by the gradient coils. (Recall: Gradient coils create variations in the magnetic field strength)
Artefacts such as the scanner hardware and the room shielding have a characteristic appearance in the produced images and can be picked up by experienced physicians.[6]
Zero Boil-Off Helium
Conventional liquid helium boils off and thus need to be refilled every month. Not only is this expensive, helium is also a finite resource. By 2000, new technology has reduced the helium boil-off rate to 0.03 litres per hour. Ideally, zero boil-off helium would allow us to reuse the helium over and over again under normal operation. [7]
Zero boil-off refrigeration systems employ a condenser to condense the boiled-off helium gas back into liquid. The condenser is surrounded by a vacuum vessel for insulation. [8]
The pressure within the condenser must be higher than that of the atmosphere. If this is not the case, contaminants from the surrounding can be drawn into the condenser, thus reducing its performance capacity. [9]
References:
[1] Eheman E et al, "PET/MRI: Where Might It Replace PET/CT?", Journal of Medical Research and Innovation, vol 46, Issue 5 (2017)
[2] Dementias Platform UK, "News: Dementias Platform UK trebles MRI-PET scanner resource for the UK", https://www.dementiasplatform.uk/news/dementias-platform-uk-trebles-mri-pet-scanner-resource-for-the-uk [Accessed: 21.02.19]
[3] Mozafari S et al, 2019, "Superconducting Phase-Diagram of H3S under High Magnetic Fields", arXiv:1901.11208
[4] Drozdov D et al, 2018, "Superconductivity at 215K in Lanthanum Hydride at High Pressures" arXiv:1808.07039
[5] Kerchner G et al, 2011, "Ultra-High Field 7T MRI: A New Tool for Studying Alzheimer's Disease", Journal of Alzheimer's Disease, vol26, no.3
[6] Laader A et al, 2017, "1.5 versus 3 versus 7 Tesla in abdominal MRI: A comparative study, PLOS One
[7] Wang. M et al, 2012, "Development of a Zero Boil-Off Cryostat for Superconducting Magnets", Journal of Physics: Conference Series, vol 400, no.5
[8] Eckels P W, 1998, "Pressure Control System for Zero Boil-Off Superconducting Magnet", patent no. 09/025,366, Retrieved from https://patentimages.storage.googleapis.com/0c/43/33/1e992b1c52a60d/US5936499.pdf
[9] Einziger W L, 1998, "Wide Multilayer Insulating Blankets for Zero Superconducting Magnets", patent no. 09/127,499, Retrieved from https://patentimages.storage.googleapis.com/f8/61/92/5b401898243c9f/US6038867.pdf