Gradient echo (GE, GRE, FLASH, SPGR, T1-FFE, FE) is one of the hardest elements of MRI to understand. Image signal and contrast in spoiled gradient echo depends upon three user-selected parameters:
- Flip angle <90º
- TR – repetition time, typically > 10 ms
- TE – echo time, typically > 2 ms
Signal and contrast also depend upon the magnetic field uniformity: a complex combination of scanner characteristics (ΔB – principally through the image slice) and tissue properties. Unlike spin echo, the signal behaviour cannot be summarised in a single figure- the graph opposite is for one particular combination of TR and TE.
For any given tissue T1 and scanner TR there is a flip angle, the Ernst angle, that gives the maximum signal for that tissue. The Ernst angle is
To aid your understanding of GRE and to estimate image contrast in different situations, try the Gradient Echo Signal and Contrast Calculator.
Learn more about gradient echo in chapters 4 and 13 of MRI from Picture to Proton.