Spin echo sequences work fine for sequences of a long TR. If a short TR is needed (for example, in T1 weighted scans), we need to cut down the scan time. We do this by forgoing the 180° RF pulse and, instead, using a gradient to rephase the spins. This is a gradient echo sequence.
- RF pulse applied
- Slice-select gradient applied
- Phase-encoding gradient applied
- Frequency-encoding gradient applied
- A negative GFE is applied. The spins dephase, some faster than others.
- The positive GFE is applied. The spins start to rephase until they are again in phase and a signal is created – the Gradient Echo
The other aspect of a GRE sequence is that you don’t have to use a 90° RF pulse at the start of the cycle; an RF pulse of any flip angle can be used. If an RF pulse with a smaller flip angle is used, it will take less time for the spins to regain all their Mz as they are closer to 0°. However, this also means that the Mxy signal is not as high as if a 90° flip angle was used.
Weighting using GRE sequences
|Flip angle||Small angle||Large angle||Small angle (minimise T1w)||Can’t achieve T2w as no 180° RF pulses to cancel T2* effect|
|Spin echo||Gradient echo|
|RF pulse used to rephase||Gradient applied to rephase|
|Uses flip angle of 90°||Uses variable flip angle|
|Slow sequence||Fast sequence|
|True T2 weighting||T2* weighting – susceptible to magnetic field inhomogeneities|