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Spin Relaxation• Spins do not continue to precess forever• Longitudinal magnetization returns to equilibrium
due to spin-lattice interactions – T1 decay
• Transverse magnetization is reduced due to both spin-lattice energy loss and local, random, spin dephasing – T2 decay
• Additional dephasing is introduced by magnetic field inhomogeneities within a voxel – T2' decay. This can be reversible, unlike T2 decay
T1 decay – “spins back down”
Collective MagneticMoment of Protons
end
start
B0
signalwe “hear”
V
Time
T1 Recovery
MRSignal
Time
Typical T1 Graph
1 s1 s
T2 decay – separation (dephasing) of “collective magnetic moment”
sometime after RF excitationImmediately after RF excitation
=
collective magnecticmoment
individual spins
separation (dephasing)
a little time later
T2 Decay
MRSignal
Typical T2 Graph
Time50 ms50 ms
T2 Decay
MRSignal
T1 Recovery
MRSignal
50 ms50 ms 1 s1 s
Proton Density Contrast
TE – echo time TR – repeat time
T2 Decay
MRSignal
T1 Recovery
MRSignal
50 ms50 ms 1 s1 s
T1 Contrast
time time
TE – echo time TR – repeat time
T2 Decay
MRSignal
T1 Recovery
MRSignal
50 ms50 ms 1 s1 s
T2* and T2 Contrast
TE – echo time TR – repeat time
ProtonProtonDensityDensityWeightedWeightedImageImage
T1 T1 Weighted Weighted ImageImage
T2 T2 Weighted Weighted ImageImage