WHEN RF STOPS
Relaxation begins immediately.
After the RF pulse ends, the excited spin system moves back toward equilibrium. Two different processes occur at the same time: longitudinal magnetization recovers and transverse magnetization decays.
Longitudinal recovery
Mz grows back toward its equilibrium value along B0.
RECOVERS ↑Transverse decay
Mxy becomes smaller as phase coherence is lost in the transverse plane.
DECAYS ↓READ THE CURVES
One curve rises while the other falls.
After one T1 time, about 63% of longitudinal magnetization has recovered.
After one T2 time, about 37% of transverse magnetization remains.
T1 and T2 are time constants, not the moment when a process is fully complete. Different tissues have different relaxation times.
T2 AND T2 STAR
The observed FID fades faster than pure T2 alone.
True T2 decay reflects microscopic interactions that cause irreversible loss of phase coherence. T2 star also includes additional dephasing from magnetic field variations and susceptibility differences. Therefore, T2 star is shorter than or equal to T2.
This common relationship applies to ordinary biological tissues under typical MRI conditions.
Pulse sequences sample recovering and decaying magnetization at selected times. That timing allows tissue relaxation differences to contribute to image contrast.
THE STANDING AND CLAPPING ANALOGY
Two changes happen together.
Imagine a crowd that was tipped sideways while clapping together. After the push ends, people begin standing upright again while their clapping gradually falls out of sync. Standing upright represents T1 recovery. Losing the shared rhythm represents T2 decay.
Remember: T1 stands back up. T2 falls out of step.
CHECK YOUR UNDERSTANDING
Keep recovery and decay separate.
What does T1 describe?
It describes longitudinal recovery toward equilibrium along B0.
What does T2 describe?
It describes the decay of transverse magnetization caused by loss of phase coherence from microscopic interactions.
Why is T2 star shorter than or equal to T2?
T2 star includes both true T2 effects and additional dephasing from magnetic field variations and susceptibility.
LESSON 06 COMPLETE
You understand T1 recovery and T2 decay.Educational references
This lesson introduces relaxation using simplified models. Later lessons connect T1, T2, and T2 star to sequence timing and image weighting.