Magnetic resonance imaging (MRI) (also known as nuclear spin tomography) is an imaging technique that uses magnetic fields and radio waves to create detailed cross-sectional images of the human body.

The procedure is based on the physical principle that atomic nuclei, which contain an odd number of protons or neutrons, have an intrinsic angular momentum (or nuclear spin) in their ground state.

Clinical MRI usually utilises hydrogen atoms in this procedure, as these are present in large numbers in water molecules, a major component of the human body. The person is first placed in a scanner within a strong external magnetic field. This strong magnetic field (usually with magnetic field strengths of 1.5 to 3 Tesla in everyday clinical practice) excites the atoms in the molecules of the body tissue in such a way that they initially align themselves in the direction of this magnetic field. Additional alternating magnetic fields now deflect the hydrogen atoms, which are all orientated in one direction, away from this main direction. After switching off the alternating fields, the atoms oscillate back along the main magnetic field (relaxation). The different changes in orientation are registered by corresponding detectors in the MR tomograph and visualised as a representation of the tissue in different shades of grey through further processing steps. The image contrast in the MRI images of the human body is therefore due to the different reaction of the various types of tissue – depending on the composition of liquid, fat or solid components – to the magnetic excitation.

One advantage of magnetic resonance imaging is that - in contrast to other imaging techniques – no harmful X-rays are used for high-resolution images and, according to current knowledge, no long-term physical damage is caused by the effects of the magnetic fields.

A special measurement method, functional MRI (fMRI), is used to investigate brain functions. The fMRI is based on the fact that active tissue consumes oxygen and the resulting changes in the magnetic properties of the red blood pigment haemoglobin can be measured.