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Relaxation 1Psy 8960, Fall ‘06
Relaxation
• Exponential time constants
• T1
– Longitudinal (spin-lattice) – Inversion recovery experiment
• T2
– Transverse (spin-spin) relaxation– Spin-echo experiment
• T2*
– T2 with field inhomogeneities
– Gradient echo experiment
TE = 15 ms
TE = 20 ms
TE = 30 ms
TE = 60 ms
Relaxation 2Psy 8960, Fall ‘06
Exponential decay
t
TT eMtM )0()(
= 30 ms
Relaxation 3Psy 8960, Fall ‘06
T2
= T2= 30 ms
T2 = 50 ms
TE = 15 ms
TE = 20 ms
TE = 30 ms
TE = 60 ms
Relaxation 4Psy 8960, Fall ‘06
Exponential recoveryInversion recovery
Saturation recovery
)21()( 1||||
Tt
eMtM
)1()( 1||||
Tt
eMtM
Relaxation 5Psy 8960, Fall ‘06
T1
Inversion recovery
TI = 50 ms 100 ms 800 ms400 ms 1600 ms 3200 ms 4800 ms
T1= 1100 ms T1 = 2000 ms
T1= 900 ms
Relaxation 6Psy 8960, Fall ‘06
Excitation
Pulse sequence diagrams
Nrep
RF
GSS
GPE
GRO
DAC
Magnetization preparation
Read-out
Relaxation 7Psy 8960, Fall ‘06
Spin Echo EPI pulse sequence (not to scale)
RF
GSS
GPE
GRO
DAC
Refocusing pulse
Excitation pulse
…
…
…
Relaxation 8Psy 8960, Fall ‘06
Spin Echo measures T2, not T2*Refocusing pulseExcitation pulse Echo
S
MT
T2*
T2
Read-out
Relaxation 9Psy 8960, Fall ‘06
Spin Echo EPI pulse sequence
RF
GSS
GPE
GRO
DAC
TE/2TE/2
Relaxation 10Psy 8960, Fall ‘06
Inversion Recovery Spin Echo EPI
RF
GSS
GPE
GRO
DAC
TE/2TE/2
Relaxation 11Psy 8960, Fall ‘06
Inversion Recovery FLASH
RF
GSS
GPE
GRO
DAC
NPE
NRO
Relaxation 12Psy 8960, Fall ‘06
Spin Echo: erasing magnetic field imperfections
Imaging signal comes from protons on water molecules.
Sensitive to macro- and microscopic variations in B0.
Hz
Frequency map, zoomed in on lateral temporal cortex
On resonance
100 Hz off resonance
250 Hz off resonance
Relaxation 13Psy 8960, Fall ‘06
Spin Echo: erasing magnetic field imperfections
Summing all spins (e.g. axial slice) creates rapid signal decay
Relaxation 14Psy 8960, Fall ‘06
Spin Echo: erasing magnetic field imperfections
Applying a 180 pulse at TE/2 refocuses the inhomogeneity-induced dephasing at TE
t = 0 ms t = TEt = TE/2
Relaxation 15Psy 8960, Fall ‘06
SE EPI: reduction of through-slice dephasing
Gradient echo Spin echo
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