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re·lax·a·tion time (τ),
the time required for the substrate in an enzymatic or chemical reaction to fall to 1/e of its initial value.
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.
relaxation timeAn MRI term for the post-excitation return of spins to their equilibrium distribution, in which there is no transverse magnetisation and the longitudinal magnetisation is at its maximum value and oriented in the direction of the static magnetic field. Transverse magnetisation decays toward zero after excitation at a characteristic time constant T2; longitudinal magnetisation returns toward equilibrium with a characteristic time constant T1.
re·lax·a·tion time(τ) (rē'lak-sā'shŭn tīm)
The time required for the substrate in an enzymatic or chemical reaction to fall to 1/e of its initial value.