Coherent gradient echo sequences can measure the
free induction decay (
FID), generated just after each
excitation pulse or the
echo formed prior to the next pulse.
Coherent gradient echo sequences are very sensitive to
magnetic field inhomogeneity.
An alternative to spoiling is to incorporate residual
transverse magnetization directly into the longitudinal steady state.
These GRE
sequences use a
refocusing gradient in the
phase encoding direction during the end module to maximize remaining transverse (xy)
magnetization at the time when the next
excitation is due, while the other two gradients are, in any case, balanced.
When the next
excitation pulse is sent into the system with an opposed
phase, it tilts the
magnetization in the -
a direction. As a result the z-magnetization is again partly tilted into the xy-plane, while the remaining xy-magnetization is tilted partly into the z-direction.
A fully refocused sequence with a properly selected and uniform
f would yield higher signal, especially for tissues with long
T2 relaxation times (high water content) so it is used in angiographic,
myelographic or arthrographic examinations and is used for
T2* weighting.
The
repetition time for this sequence has to be short. With short TR,
coherent GE is also useable for breath hold and 3D technique. If the
repetition time is about 200 msec there's no difference between
spoiled or un
spoiled GE. T1 weighting is better with
spoiled techniques.
The common types include
GRASS,
FISP, FAST, and FFE.
The
T2* component decreases with long TR and short TE. The
T1 time is controlled by
flip angle. The common TR is less than 50 ms and the common TE less than 15 ms
Other types have stronger T2 dependence but lower
SNR. They include SSFP, CE-FAST,
PSIF, and CE-FFE-T2.
Examples of fully refocused
FID sequences are
TrueFISP, bFFE and
bTFE.
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