Steady-state free precession (SSFP) imaging is a magnetic resonance imaging (MRI) technique which uses steady states of magnetizations. In general, SSFP MRI sequences are based on a (low flip angle) gradient-echo MRI sequence with a short repetition time which in its generic form has been described as the FLASH MRI technique. While spoiled gradient-echo sequences refer to a steady state of the longitudinal magnetization only, SSFP gradient-echo sequences include transverse coherences (magnetizations) from overlapping multi-order spin echoes and stimulated echoes. This is usually accomplished by refocusing the phase-encoding gradient in each repetition interval in order to keep the phase integral (or gradient moment) constant. Fully balanced SSFP MRI sequences achieve a phase of zero by refocusing all imaging gradients.
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Steady-state free precession (SSFP)
States of SSFP is a kind of steady states of magnetizations achieved by a series of radiofrequency (RF) irradiation and natural relaxation behaviors of spins. The influencing factors thus include: the flip angles of RF pulses, repetition time (TR) of pulse repeats, the relaxation time constants including longitudinal (T1) and transverse (T2) ones, plus if gradient moments (i.e. the integral of gradients with time) in one TR are zero, etc.
Gradient moments are zero or not
If, within one TR, either one of the gradient moments of magnetic gradients along three logical directions, including slice selection direction (Gss), phase encoding (Gpe) and readout (Gro), is not zero, then spins along such direction obtain different phases, making the signal intensity (SI) of a single voxel is a vector sum of magnetizations therein. It causes some inevitable loss of SI. Such situations belong to ordinary SSFP imaging, with its commercial names listed below.
Otherwise, if all gradient moments are zero within one TR, i.e. gradients of opposite polarities cancel out, then there are no additional effects on the phase from gradients; that is to say, SI of each voxels is the contributions of a series of RF pulses and relaxation phenomena.