Signal Loss and Echo Time in MRI

The realignment of spins toward a direction parallel to B0 naturally removes them from the plane transverse to B0, in which they precess to form the MR image signal. Therefore, the signal decays as the spins realign due to spin-lattice relaxation. An additional mechanism, however, dominates this transverse decay and can be explained by recalling that the frequency of spin precession is proportional to the local magnetic field. The molecular environment in which protons move has variations in the magnetic field (Fig. 3A). Thus, the precession of each spin is always speeding up and slowing down as it moves through these fields, slowly losing magnetic alignment with neighboring spins. This loss of spin coherence on a molecular scale is called spin-spin relaxation. The total loss of precessing (signal-producing) magnetization due to spin-lattice and spin-spin relaxation (the latter being the dominant mechanism) is characterized by the time constant T2, the time required for 63% of the precessing magnetization to decay.

The amount of signal decay depends on T2 and the time between the spin excitation and the measurement of the induced current in the coil, known as echo time (TE). As with the TR/T1 ratio, the TE/T2 ratio indicates the relative fraction of precessing magnetization (and therefore of the available signal) that remains at the time of measurement.