Steady-state free precession imaging

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.

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.

Commercial names for gradient-echo imaging sequences

Gradient Echo Pulse Sequences
Academic Classification Spoiled Gradient Echo Steady-State Free Precession (SSFP) Balanced Steady-State Free Precession (bSSFP)
Ordinary type Turbo type
(Magnetization preparation,
extremely low angle shot, short TR)		 FID-like Echo-like
Siemens FLASH
Fast Imaging using Low Angle Shot 		TurboFLASH
Turbo FLASH 		FISP
Fast Imaging with Steady-state Precession 		PSIF
Reversed FISP 		TrueFISP
True FISP
GE SPGR
Spoiled GRASS 		FastSPGR
Fast SPGR 		GRASS
Gradient Recall Acquisition using Steady States 		SSFP
Steady State Free Precession 		FIESTA
Fast Imaging Employing Steady-state Acquisition
Philips T1 FFE
T1-weighted Fast Field Echo 		TFE
Turbo Field Echo 		FFE
Fast Field Echo 		T2-FFE
T2-weighted Fast Field Echo 		b-FFE
Balanced Fast Field Echo

See also