Relaxation (NMR)
In nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) the term relaxation describes how signals change with time. In general signals deteriorate with time, becoming weaker and broader. The deterioration reflects the fact that the NMR signal, which results from nuclear magnetization, arises from the over-population of an excited state. Relaxation is the conversion of this non-equilibrium population to a normal population. In other words, relaxation describes how quickly spins "forget" the direction in which they are oriented. The rates of this spin relaxation can be measured in both spectroscopy and imaging applications.
The deterioration of an NMR signal is analyzed in terms of two separate processes, each with their own time constants. One process, associated with T1, is responsible for the loss of signal intensity. The other process, associated with T2, is responsible for the broadening of the signal. Stated more formally, T1 is the time constant for the physical processes responsible for the relaxation of the components of the nuclear spin magnetization vector M parallel to the external magnetic field, B0 (which is conventionally oriented along the z axis). T2 relaxation affects the components of M perpendicular to B0. In conventional NMR spectroscopy T1 determines the recycle time, the rate at which an NMR spectrum can be acquired. Values of T1 range from milliseconds to several seconds.
The longitudinal (or spin-lattice) relaxation time T1 is the decay constant for the recovery of the z component of the nuclear spin magnetization, Mz, towards its thermal equilibrium value, . In general,
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