MRI – magnetic resonance imaging

This week I went to a Physics talk at my school where a teacher spoke about MRI scans, their uses and the images produced.

Scans are mainly used in order to monitor or diagnose neurological, brain or muscular conditions. At this talk I noted the interdisciplinary skills used to create and use an MRI scanner including physicists for the mechanism, electricians for the servicing, mathematicians for quantitive analysis, radiologists for running the machine and doctors for overall analysis of pictures, before patients receive their results and prognosis.

I noted that MRI scans show up areas that are ‘soft’ as light grey and ‘hard’ materials in black eg. the brain looks light grey whilst the skull looks black. This directly contrasts images of X-Rays and CT scans which are lighter shades where ‘hard’ material is found in the body. The reason for MRI showing lighter colours in ‘soft’ areas is due to it highlighting areas of the body containing water and fat. The image is made by shooting low frequency and long wavelength radio waves at the cells – those containing water (H2O) absorb some energy due to the proton in each nuclei of H-atoms, and then reemit the waves at the same wavelength.

The MRI scanner looks like a large donut with a bed which slides in and out of the central hole. Along the bed is a very strong uniform magnetic field, like that of a coil, which must overcome the thermal limit of the body to magnetise the body (it is never turned off). The current flowing through the machine is very high, in order to significantly increase the magnetic field and create a ‘superconductor’ (a metal which does not have any electrical resistance), so liquid helium is used to ensure the scanner does not overheat.

The loud sound during an MRI is due to the magnetic field and fast flowing current making a loudspeaker. Ear muffs must be worn to protect eardrums from permanent damage. Magnetic objects must never be bought near to the machine due to the extremely strong magnetic field of around 1.5 teslas (on earth the average background magnetic field strength is 50 microteslas), which would attract them into the machine, where the person is lying, at a very high speed.

Current research using MRIs includes its potential use in monitoring neurological conditions associated with losing muscle function due to fat coating the muscles. Researchers are looking for ways to monitor this condition using numbers (quantitive analysis) to get a more accurate representation of improvement or deterioration, as opposed to making a judgement using the less informative black and white pictures (qualitative data). They are looking at finding a way to count protons in the nuclei of fat molecules by monitoring the amount of radio waves reemitted from the body.


Emily Buchanan


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