Magnetic resonance imaging (MRI) uses magnetic fields and radio waves to produce an image that is dependent on the distribution of hydrogen in the body.

MRI image of brains

Hydrogen atoms are magnetic due to the intrinsic spin of their nuclei, a property that can be utilised by placing the patient at the centre of a superconducting electromagnet. 

Under the influence of the strong magnetic field slightly more of the hydrogen nuclei become aligned with the field than against it and the body acquires a net magnetisation. 

The alignment of the nuclei is then disturbed using radio pulses and as they re-align they produce a radio signal of their own. 

The signal is very sensitive to the local chemical environment and can be used for high contrast anatomical or functional mapping of organs such as the brain. Another distinguishing feature of MRI, especially when compared to techniques that use ionising radiation such as X-ray or gamma imaging is that extended scans can be carried out without exposing the patient to any additional risk.


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