text: Like spin-lattice relaxation, spin-spin relaxation can be studied using a molecular tumbling autocorrelation framework. The resulting signal decays exponentially as the echo time (TE), i.e., the time after excitation at which readout occurs, increases. In more complicated experiments, multiple echoes can be acquired simultaneously in order to quantitatively evaluate one or more superimposed T decay curves. The relaxation rate experienced by a spin, which is the inverse of T, is proportional to a spins tumbling energy at the frequency difference between one spin and another; in less mathematical terms, energy is transferred between two spins when they rotate at a similar frequency to their beat frequency,  in the figure at right. In that the beat frequency range is very small relative to the average rotation rate , spin-spin relaxation is not heavily dependent on magnetic field strength. This directly contrasts with spin-lattice relaxation, which occurs at tumbling frequencies equal to the Larmor frequency . Some frequency shifts, such as the NMR chemical shift, occur at frequencies proportional to the Larmor frequency, and the related but distinct parameter T' can be heavily dependent on field strength due to the difficulty of correcting for inhomogeneity in stronger magnet bores.
  Assuming isothermal conditions, spins tumbling faster through space will generally have a longer T. Since slower tumbling displaces the spectral energy at high tumbling frequencies to lower frequencies, the relatively low beat frequency will experience a monotonically increasing amount of energy as  increases, decreasing relaxation time. The figure at the left illustrates this relationship. It is worth noting again that fast tumbling spins, such as those in pure water, have similar T and T relaxation times, while slow tumbling spins, such as those in crystal lattices, have very distinct relaxation times.