Contrast enhanced GRE
sequences provide T2
contrast but have a relatively poor
SNR. Repetitive RF pulses with small flip angles together with appropriate
gradient profiles lead to the superposition of two
resonance signals.
The first signal is due to the
free induction decay FID observed after the first and all ensuing RF excitations.
The
second is a
resonance signal obtained as a result of a
spin echo generated by the
second and all addicted RF-pulses.
Hence it is absent after the first
excitation, it is a result of the
free induction decay of the
second to last RF-excitation and has a TE, which is almost 2TR.
For this
echo to occur the gradients have to be completely symmetrical relative to the half time between two RF-pulses, a condition that makes it difficult to integrate this
pulse sequence into a
multiple slice imaging technique.
The
second signal not only contains
echo contributions from
free induction decay, but obviously weakened by T2-decay.
Since the
echo is generated by a RF-pulse, it is truly T2 rather than
T2* weighted. Correspondingly it is also less sensitive to
susceptibility changes and field inhomogeneities.
Companies use different acronyms to describe certain techniques.
Different terms (see also acronyms) for these gradient echo pulse sequences:
CE-FAST Contrast Enhanced Fourier Acquired Steady State,
CE-FFE Contrast Enhanced Fast Field Echo,
CE-GRE Contrast Enhanced Gradient-Echo,
DE-FGR Driven Equilibrium FGR,
FADE FASE Acquisition Double Echo,
PSIF Reverse Fast Imaging with Steady State Precession,
SSFP Steady State Free Precession,
T2 FFE Contrast Enhanced Fast Field Echo (T2 weighted).
In this context, 'contrast enhanced' refers to the
pulse sequence, it does not mean enhancement with a
contrast agent.
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