Each pair of RF-pulses generates a spin echo (SE), that because of regular spacing of the RF-pulse train, forms and rises to a maximum exactly a the time of the next RF-pulse. Each set of three RF-pulses also generate stimulated echoes (STEs) that coincide with the spin echoes. The "Echo" portion of the GRE sequence is mostly T2-weighted with fewer T2* effects.
An interesting phenomenon occurs within an RF-pulse train if TR is made much shorter than T2. Here the "FID" following each RF-pulse does not completely die out before the "Echo" component begins to form. In this situation the transverse signal never completely disappears and the result is called a steady-state free precession (SSFP).
Either of the signals generated by the RF-pulse train ("FIDs" or "Echoes") can be temporarily suppressed and then made to reappear at a chosen time (TE) by application of an external magnetic gradient field. The gradient is typically applied in two steps: 1) a dephase portion that forces spins out of phase, and 2) a rephase portion that brings them back into phase as the GRE.
- GRE with "FID" refocusing (FISP, GRASS, FFE)
- GRE with "Echo" refocusing (PSIF, SSFP, T2-FFE)
- GRE with combined "FID" and "Echo" refocusing (True FISP, FIESTA, Balanced FFE)
- Coherent GRE sequences: preservation of transverse coherences
- Spoiled GRE sequences: complete disruption of transverse coherences
Advanced Discussion (show/hide)»
Most GRE sequences are considered coherent even though they often have constant spoiler gradients to control resonant offset artifacts. True spoiled GRE sequences go far beyond this, employing gradients of randomly varying amplitude and semi-randomized phase changes in RF-pulses.
Chavhan GB, Babyn PS, Jankharia BG et al. Steady-state MR imaging sequences: physics, classification, and clinical applications. Radiographics 2008;28:1147-1160.
Elster AD. Gradient echo imaging: techniques and acronyms. Radiology 1993; 186:1-8. (My older review; still accurate, though some vendors have gone out of business. Gives a good history of the development of GRE sequences).
Hargreaves B. Rapid gradient-echo imaging. J Mag Reson Imaging 2012;36:1300-1313. (A great, but not overly technical, modern review).
It seems as if every manufacturer has adopted a different name for their gradient echo sequences. Why is this? Can you sort this out for me?
If a spin echo results from 2 pulses, and a stimulated echo from 3 pulses, what do you get from 4 pulses?