Relaxation time

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 (nuclear magnetic resonance) state by a radiofrequency signal, to return to a lower energy equilibrium state. Two time parameters are used to describe the return (relaxation) to the equilibrium state once the rf source is turned off. The spin-lattice relaxation time (also called the longitudinal relaxation time), denoted by 71, characterizes the dying away of the NMR signal as the system returns to its lower energy state or, in other words, comes back into thermal equilibrium with its neighboring atoms (the “lattice”). Equivalently, T1 is the time constant for the return of the magnetization to its longitudinal alignment along the static field. The spin-spin relaxation time (also called the transverse relaxation time), denoted by T2, characterizes the exponential decay of the component of the magnetization transverse to the static field owing to the interactions among the system of nuclei after the rf excitation is removed. These interactions cause the nuclei to acquire a range of precessional frequencies and thus to fall out of phase with each other. Put another way, 7i describes the relaxation of the system of spins into a condition of thermal equilibrium with its surroundings, while T2 describes the relaxation of the energy that is traded within the system itself. Maps or “images” of the values of 71, or T2 as a function of position in the cross-sectional view can be made.


 


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