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Why does an MRI cost so much ? and how does MRI work?

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 An MRI machine uses a combination of magnetic fields and radio waves to look at the hydrogen atoms in our bodies. The magnetic field causes the hydrogen atoms to act like small magnets and then radio waves are sent in, which bounce back, giving us a signal we can measure.

In MRI we’re really looking at hydrogen atoms in the body, and fortunately we have a lot of hydrogen atoms in our body, we’re made up of a lot of water and the way MRI works is with a combination of magnetic fields and sending in radio waves. So this is kind of different from x-ray imaging where you’re using very high energy x-rays, radio waves are much less damaging. We call them non-ionising radiation. These radio waves, the photons don’t have enough energy to cause any chemical changes, so that’s why we believe that MRI is completely safe. The MRI scan uses the magnetic field to align these hydrogen atoms so instead of them being just in a random arrangement, the magnetic field tries to pull them into alignment like a whole set of compass needles.



So what we do then is to send some energy into the body, and this is at radio frequencies, and then we listen for radio waves that are being emitted from these hydrogen atoms and that’s the signal that we measure, and by changing the magnetic field we can determine where the signal came from.

A very good analogy for MRI is with a gyroscope. It rotates about its axis and it rotates at a particular frequency. And the same is true of a hydrogen nucleus, the proton at the centre of a hydrogen atom, when you put that into a magnetic field it’ll tend to rotate, and that rotation speed, the frequency, depends on the magnetic field strength. So by changing the magnetic field strengths, we can make the hydrogen nuclei rotate at different frequencies, and we can measure those frequencies and determine where the hydrogen nucleus is.

Why does an MRI cost so much?


The main cost of the MRI scanner is in generating this very large, very uniform magnetic field. And the way that we do that is using super conducting wires. So super conducting materials have this unique property that they have no electrical resistance. It’s not nearly nothing, it’s absolutely nothing. So once you start a current flowing through these coils that current will continue to flow essentially forever. So the main cost of the MRI scanner is this coil of wire, and to keep that coil of wire super-conducting, to keep it with zero resistance we have to keep it very cold. And we keep it cold using liquid helium. Liquid helium is four degrees above absolute zero, absolute zero is minus 273 degrees celcius. And when you look at the MRI scanner itself, what you’re looking at is a big tub that contains several thousand litres of liquid helium.


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