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How does an MRI scanner work? © Getty Images

How does an MRI scanner work?

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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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