Spin–spin relaxation
Amorphous solids have T2 in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms.
When excited nuclear spins—i.e., those lying partially in the transverse plane—interact with each other by sampling local magnetic field inhomogeneities on the micro- and nanoscales, their respective accumulated phases deviate from expected values.
[4] While the slow- or non-varying component of this deviation is reversible, some net signal will inevitably be lost due to short-lived interactions such as collisions and random processes such as diffusion through heterogeneous space.
T2 decay does not occur due to the tilting of the magnetization vector away from the transverse plane.
In more complicated experiments, multiple echoes can be acquired simultaneously in order to quantitatively evaluate one or more superimposed T2 decay curves.
[6] In that the beat frequency range is very small relative to the average rotation rate
[4] Assuming isothermal conditions, spins tumbling faster through space will generally have a longer T2.
A spin echo experiment can be used to reverse time-invariant dephasing phenomena such as millimeter-scale magnetic inhomogeneities.
In more complicated experiments, multiple echoes can be acquired simultaneously in order to quantitatively evaluate one or more superimposed T2 decay curves.
[6] In MRI, T2-weighted images can be obtained by selecting an echo time on the order of the various tissues' T2s.
