Gradient (recalled) echo sequence


 

Gradient echo sequence

 

Spin echoes 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.

  1. RF pulse applied
  2. Slice-select gradient applied
  3. Phase-encoding gradient applied
  4. Frequency-encoding gradient applied
    1. A negative GFE is applied. The spins dephase, some faster than others.
    2. The positive GFE is applied. The spins start to rephase until they are again in phase and a signal is created - the Gradient Echo

 

Flip angle in 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 is not as high as if a 90° flip angle was used.

 

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Weighting using GRE sequences

 

Large and small flip angle

 

  PD T1w T2*w T2
Flip angle Small angle Large angle Small angle (minimise T1w) Can't achieve T2w as no 180° RF pulses to cancer T2* effect
TE Short Short Long  
TR Short Short Short  

Σ  Summary
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

Next page: Inversion recovery sequences


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