THE BASIC SEQUENCE
A second RF pulse brings recoverable phase spread back together.
In the classic spin-echo sequence, a 90° RF pulse creates transverse magnetization. The spins begin to dephase because of true T2 interactions and differences in the local magnetic field.
At TE/2, a 180° RF pulse flips the phase arrangement. Spins affected by stable field differences continue precessing at their same relative rates, but the flipped arrangement allows faster spins to catch slower ones. They refocus and form an echo at TE.
FOLLOW THE PHASE
Fan out, flip, come back together.
A 90° pulse creates transverse magnetization with phase coherence.
Some spins advance faster and others slower, so the vectors spread.
The 180° pulse flips the arrangement at TE/2.
Reversible dephasing converges at TE and signal peaks.
The 180° pulse does not reverse true T2 decay. Signal permanently lost through microscopic spin-spin interactions cannot be fully recovered.
T2 INSTEAD OF T2 STAR
RF refocusing reduces sensitivity to stable field differences.
The 180° pulse can compensate for reversible dephasing that remains stable during the echo.
Irreversible microscopic loss of phase coherence continues throughout the sequence.
TR and TE still control conventional spin-echo weighting. Short TR and short TE can emphasize T1 differences; long TR and long TE can emphasize T2 differences; long TR and short TE can emphasize proton density.
Spin echo is less sensitive to many susceptibility and field-inhomogeneity effects than gradient echo, but the 180° RF pulse adds RF energy. Sequence timing, refocusing trains, field strength, anatomy, and patient conditions all affect SAR and scan strategy.
THE FLIPPED-FAN ANALOGY
Flip the spread so it can reconverge.
Imagine clock hands fanning apart because some move faster than others. Flip the entire fan across a line without changing each hand’s speed. The hands that were behind are now positioned ahead, allowing the faster hands to catch up and meet again.
Remember: 90 excites. 180 refocuses. The echo returns at TE.
CHECK YOUR UNDERSTANDING
Know what the refocusing pulse can, and cannot, recover.
When is the 180° RF pulse applied in a basic spin echo?
At TE/2, halfway between the 90° excitation and the center of the echo.
When does the spin echo peak?
At TE, after a second TE/2 interval following the 180° refocusing pulse.
Does the 180° pulse reverse true T2 decay?
No. It can refocus reversible dephasing from stable field differences, but it cannot restore coherence permanently lost through true T2 interactions.
LESSON 13 COMPLETE
You understand how a spin echo forms.Educational references
This lesson presents the classic spin-echo model. Fast spin echo, stimulated echoes, imperfect refocusing, flow, diffusion, and motion introduce additional behavior.