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Gradient (recalled) echo sequence

Gradient echo sequence
Gradient echo sequence

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.

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

Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.

Weighting using GRE sequences

Lage and small flip angle
Lage and small flip angle
 PDT1wT2*wT2
Flip angleSmall angleLarge angleSmall angle (minimise T1w)Can’t achieve T2w as no 180° RF pulses to cancel T2* effect
TEShortShortLong 
TRLongShortShort 

Σ  Summary

Spin echoGradient echo
RF pulse used to rephaseGradient applied to rephase
Uses flip angle of 90°Uses variable flip angle
Slow sequenceFast sequence
True T2 weightingT2* weighting – susceptible to magnetic field inhomogeneities
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