Magnetic resonance imaging

A magnetic resonance (MR) scanner produces an image of a cross-section of a patient by using magnetic fields and radio-frequency (RF) signals. The signals come from hydrogen nuclei, or protons, in the patient’s body.

Protons in the magnetic field

A superconducting magnet produces a strong magnetic field. In this field, more proton spins align parallel to the field than in the opposite direction. The spins also precess: their directions move around the magnetic field lines rather than remaining fixed in one direction.

Selecting regions and detecting signals

Gradient field coils make the magnetic field vary with position. Because the field affects the frequency at which protons respond to RF signals, the scanner can use a gradient and a suitably chosen RF pulse to select a region. Gradients also allow the positions from which signals come to be identified.

Short RF pulses excite protons in selected regions, changing their spin states. Successive regions of the cross-section can be selected and excited. As the protons de-excite and return towards their original alignment, they emit RF signals. The scanner detects these signals, and a computer uses them and their position information to construct the cross-sectional image.