(SFP or SSFP) Steady state free
precession is any field or
gradient echo sequence in which a non-zero steady state develops for both components of
magnetization (transverse and longitudinal) and also a condition where the TR is shorter than the
T1 and
T2 times of the tissue. If the RF pulses are close enough together, the MR
signal will never completely
decay, implying that the spins in the transverse plane never completely dephase.
The
flip angle and the TR maintain the steady state. The
flip angle should be 60-90° if the TR is 100 ms, if the TR is less than 100 ms, then the
flip angle for steady state should be 45-60°.
Steady state free
precession is also a method of MR
excitation in which strings of RF pulses are applied rapidly and repeatedly with interpulse intervals short compared to both
T1 and
T2. Alternating the phases of the RF pulses by 180° can be useful. The
signal reforms as an
echo immediately before each
RF pulse; immediately after the
RF pulse there is additional
signal from the
FID produced by the pulse.
The strength of the
FID will depend on the time between pulses (TR), the tissue and the
flip angle of the pulse; the strength of the
echo will additionally depend on the
T2 of the tissue.
With the use of appropriate
dephasing gradients, the
signal can be observed as a frequency-encoded
gradient echo either shortly before the
RF pulse or after it; the
signal immediately before the
RF pulse will be more highly
T2 weighted.
The
signal immediately after the
RF pulse (in a rapid series of RF pulses) will depend on
T2 as well as
T1, unless measures are taken to destroy
signal
refocusing and prevent the development of steady state free
precession.
To avoid setting up a state of SSFP when using rapidly repeated
excitation RF pulses, it may be necessary to spoil the
phase coherence between excitations, e.g. with varying
phase shifts or timing of the exciting RF pulses or varying spoiler
gradient pulses between the excitations.
Steady state free
precession imaging methods are quite sensitive to the
resonant
frequency of the material.
Fluctuating
equilibrium MR (see also
FIESTA and
DRIVE)and linear combination SSFP actually use this sensitivity for
fat suppression. Fat saturated SSFP (FS-SSFP) use a more complex
fat suppression scheme than FEMR or LCSSFP, but has a 40% lower
scan time.
A new family of steady state free
precession sequences use a
balanced gradient, a
gradient waveform, which will act on any stationary
spin on resonance between 2 consecutive RF pulses and return it to the same
phase it had before the gradients were applied.
This
sequences include, e.g.
Balanced Fast Field Echo - bFFE,
Balanced Turbo Field Echo -
bTFE,
Fast Imaging with Steady Precession -
TrueFISP and
Balanced SARGE - BASG.
See also
FIESTA.
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