How does an MRI scanner work? © Getty Images

How does an MRI scanner work?

This ingenious device uses the science of magnetism to safely see inside your body.

Magnetic Resonance Imaging (MRI) takes advantage of the fact that the nucleus of a hydrogen atom (a single proton) behaves like a weak compass needle. In the presence of a strong magnetic field, the hydrogen atoms will align themselves, but a radio signal of the correct resonant frequency will cause them to deflect slightly. When the signal is removed, the atoms return to their equilibrium state and emit a radio signal of their own. An MRI scanner can detect these signals and use them to map the distribution of molecules with lots of hydrogen atoms – ie, water and fat. In this way, it can create detailed images of the inside of the body.


Scanning table: The patient can only be scanned from inside the magnetic coil, so a motorised table slides them in and out.

RF system: An antenna produces a radio signal to ‘nudge’ the hydrogen nuclei and listen to the answering radio wave they emit.

Liquid helium: Liquid helium is pumped through an enclosing jacket to cool the superconducting magnets almost to absolute zero.

Main magnet: Superconducting magnetic coils produce a magnetic field of 1.5 teslas – that’s about 300 times stronger than a fridge magnet.

Patient: The high magnetic fields mean that patients with cochlear implants, pacemakers or embedded shrapnel usually can’t be scanned.

Gradient system: A second coil distorts the main magnetic field so that the resonant frequency of the protons varies according to position.

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