About 80% of the body is made up of water (H2O). Magnetic resonance imaging relies on the use of a strong magnetic field to align these hydrogen ions (2 protons) in the direction of the field. A radiofrequency (RF) pulse is then applied to disrupt alignment. When the RF pulse is turned off, these ions will attempt to realign with the magnetic field again and release a signal that is interpreted by a computer at different times (more about that below). The strength of this signal varies depending on the type of tissue and the content of hydrogen ions inside (fat, muscle, water).
The time taken for the protons to fully relax is measured in two ways. The first is the time taken for the magnetic vector to return to its resting state (longitudinal relaxation) and the second is the time needed for the axial spin to return to its resting state (transverse relaxation). The first is called T1 relaxation, the second is called T2 relaxation.
All tissues have different relaxation times in the longitudinal and transverse position and these correlate with the different images that are acquired as a result.