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 msSignal 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 create your own curves for a range of tissues, try the Ernst Angle generator for Gradient Echo.
Learn more about gradient echo in chapters 4 and 13 of MRI from Picture to Proton.