Spin–lattice relaxation in the rotating frame is the mechanism by which M, the transverse component of the magnetization vector, exponentially decays towards its equilibrium value of zero, under the influence of a radio frequency (RF) field in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).   It is characterized by the spin–lattice relaxation time constant in the rotating frame, T. It is named in contrast to T, the spin-lattice relaxation time.
T MRI is an alternative to conventional T and T MRI by its use of a long-duration, low-power radio frequency referred to as spin-lock (SL) pulse applied to the magnetization in the transverse plane.  The magnetization is effectively spin-locked around an effective B field created by the vector sum of the applied B and any off-resonant component.  The spin-locked magnetization will relax with a time constant T, which is the time it takes for the magnetic resonance signal to reach 37% (1/e) of its initial value, .  Hence the relation:
, where t is the duration of the RF field.