spinlatticerelaksaatio

spinlatticerelaksaatio (spin-lattice relaxation) is the process by which the net magnetization of nuclear spins returns to thermodynamic equilibrium along the external magnetic field after perturbation by a radiofrequency pulse. It is quantified by the longitudinal relaxation time, T1, which characterizes the exponential recovery of magnetization toward its equilibrium value M0.

The microscopic origin of spinlatticerelaksaatio depends on the environment and the spin system. In liquids, rapid molecular motion couples spins to lattice vibrations (phonons), and the relaxation rate is governed by the spectral density of fluctuations at the Larmor frequency. In solids, dipolar couplings, chemical shift anisotropy, and sometimes paramagnetic impurities dominate. The rate 1/T1 generally depends on temperature, molecular mobility, magnetic field strength, and the presence of relaxants; it may increase or decrease with field depending on the dominant mechanism.

In measurement and applications, T1 is probed with inversion recovery, saturation recovery, or Look-Locker methods in NMR and MRI. In MRI, T1 weighting arises from choice of repetition time and recovery dynamics, and T1 mapping provides quantitative tissue properties useful for characterization and diagnostics.

Typical values vary widely. At clinical MRI field strengths, tissue T1 times range from hundreds of milliseconds to several seconds. Water often has longer T1 than fat, but exact numbers depend on field strength, temperature, and composition.