Magnetic resonance imaging uses the resonance of the protons to generate images. Protons are excited by a radio frequency pulse at an appropriate frequency (Larmor frequency) and then the excess energy is released in the form of a minuscule amount of heat to the surroundings as the spins return to their thermal equilibrium. The magnetization of the proton ensemble goes back to its equilibrium value with an exponential curve characterized by a time constant T (see Relaxation (NMR)).
T weighted images can be obtained by setting short repetition time (TR) such as  while setting TE values to less than 15 ms.
T is significantly different between grey matter and white matter and is used when undertaking brain scans. A strong T contrast is present between fluid and more solid anatomical structures, making T contrast suitable for morphological assessment of the normal or pathological anatomy, e.g., for musculoskeletal applications.