So how does MRI work?

Dr Richard Watts from the Department of Physics and Astronomy at the University of Canterbury talks about how an MRI (magnetic resonance imaging) machine works.

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.

Transcript

DR RICHARD WATTS 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.