Resources/MRI fundamentals
Spin echo
A spin echo sequence pairs a 90° excitation pulse with a 180° refocusing pulse. The refocusing pulse rewinds the dephasing caused by magnetic field inhomogeneities, which is why spin echo images are robust and remain the workhorse of clinical MRI.
What actually happens
The 90° pulse tips magnetization into the transverse plane, where spins immediately begin to dephase — partly from tissue physics (true T2 decay) and partly from field imperfections (T2* effects). At time TE/2, the 180° pulse flips the spins so the fast ones now trail and the slow ones lead. By TE they re-converge: the echo. Field-imperfection dephasing is undone; only true T2 decay remains.
Why it matters at the console
- Robustness: spin echo tolerates metal, air interfaces and field inhomogeneity far better than gradient echo.
- True T2 contrast: because T2* effects are refocused, contrast follows tissue T2 values.
- The cost is time: one 180° pulse per echo makes conventional SE slow — which is why fast/turbo spin echo (multiple echoes per TR) dominates clinically.
Where you meet it
T1-weighted anatomy (short TR, short TE), T2-weighted pathology screening via fast spin echo (long TR, long TE), and any region where susceptibility would wreck a gradient echo image.
Try it in ScanFlo: Scan the brain case with T1 SE, then switch the same planning to a gradient echo sequence with matched timing. Compare the areas near the sinuses — the signal loss you see on GRE is the T2* dephasing that spin echo's 180° pulse rewinds.