relaxation time, (in magnetic resonance imaging) the characteristic time it takes for a sample of atoms, whose nuclei have first been aligned along a static magnetic field and then excited to a higher-energy state by a radiofrequency (rf) signal, to return to a lower-energy equilibrium state. Two time parameters are used to describe the return, or relaxation, to the equilibrium state once the rf signal is turned off: T1 describes the relaxation of the system of spins into a condition of thermal equilibrium with its surroundings. T2 describes the relaxation of the energy that is traded within the system itself. Maps of the values of T1 or T2 as a function of position in the cross-sectional view constitute magnetic resonance images. Also called transverse relaxation time.
