Since magnetization precesses about the primary magnetic field (B0), the MRI scanner can only measure magnetization perpendicular to B0.
The protons continue to precess along B0 in the same direction, but they now rephase to the same spot after a 180-degree radiofrequency pulse flips their direction. The gradient echo is formed by a gradient of a negative magnetic field.
Gradient echo sequences do not create a spin echo using a 180-degree pulse; instead, they employ the free induction decay signal from the original excitation pulse. As a result, rather than T2, T2* determines when the dephasing happens.
To acquire numerous spin echoes from each excitation, the fast spin echo employs multiple 180-degree pulses after a single 90-degree pulse. In echo planar imaging, successive frequency-encoding gradients are used.
In addition to the FID signal, balanced SSFP sequences maintain transverse magnetization, which creates spin echoes following successive excitation pulses. Spin echo sequences use 180-degree pulses; standard GRE uses spoiler gradients; and echo planar imaging uses alternating gradients.
T2 is a tissue-specific characteristic that denotes dephasing associated with changing molecular field inhomogeneities, which are connected to the molecular motion and vibration speed (spin-spin relaxation).
Because multiple echoes are obtained at late echo times, FSE sequences are more T2 weighted; susceptibility is reduced due to a shorter dephasing time between the subsequent 180-degree pulses.
T2* features show that proton dephasing occurs initially in the xy plane, while T1 characteristics show that longitudinal recovery along B0 is substantially slower.
Because molecular motion constantly disrupts their orientation along the main magnetic field, protons will only have a small preference. However, they will precess—or wobble—around the axis of the magnetic field. However, since the random precession of all the protons will cancel out, the net sum of the proton magnetization will point along the main magnetic field rather than appear to precess.
T2* dephasing forces GRE sequences to use substantially shorter TEs. Consequently, substantially shorter TRs can be used in GRE sequences.
