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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
Flip angleSmall angleLarge angleSmall angle (minimise T1w)Can’t achieve T2w as no 180° RF pulses to cancel T2* effect

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