psy 8960, fall ‘06 relaxation1 exponential time constants t 1 –longitudinal (spin-lattice)...
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![Page 1: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/1.jpg)
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
![Page 2: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/2.jpg)
Relaxation 2Psy 8960, Fall ‘06
Exponential decay
t
TT eMtM )0()(
= 30 ms
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Relaxation 3Psy 8960, Fall ‘06
T2
= T2= 30 ms
T2 = 50 ms
TE = 15 ms
TE = 20 ms
TE = 30 ms
TE = 60 ms
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Relaxation 4Psy 8960, Fall ‘06
Exponential recoveryInversion recovery
Saturation recovery
)21()( 1||||
Tt
eMtM
)1()( 1||||
Tt
eMtM
![Page 5: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/5.jpg)
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
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Relaxation 6Psy 8960, Fall ‘06
Excitation
Pulse sequence diagrams
Nrep
RF
GSS
GPE
GRO
DAC
Magnetization preparation
Read-out
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Relaxation 7Psy 8960, Fall ‘06
Spin Echo EPI pulse sequence (not to scale)
RF
GSS
GPE
GRO
DAC
Refocusing pulse
Excitation pulse
…
…
…
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Relaxation 8Psy 8960, Fall ‘06
Spin Echo measures T2, not T2*Refocusing pulseExcitation pulse Echo
S
MT
T2*
T2
Read-out
![Page 9: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/9.jpg)
Relaxation 9Psy 8960, Fall ‘06
Spin Echo EPI pulse sequence
RF
GSS
GPE
GRO
DAC
TE/2TE/2
![Page 10: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/10.jpg)
Relaxation 10Psy 8960, Fall ‘06
Inversion Recovery Spin Echo EPI
RF
GSS
GPE
GRO
DAC
TE/2TE/2
![Page 11: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/11.jpg)
Relaxation 11Psy 8960, Fall ‘06
Inversion Recovery FLASH
RF
GSS
GPE
GRO
DAC
NPE
NRO
![Page 12: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/12.jpg)
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
![Page 13: Psy 8960, Fall ‘06 Relaxation1 Exponential time constants T 1 –Longitudinal (spin-lattice) –Inversion recovery experiment T 2 –Transverse (spin-spin) relaxation](https://reader036.vdocument.in/reader036/viewer/2022062516/56649d375503460f94a0fc2c/html5/thumbnails/13.jpg)
Relaxation 13Psy 8960, Fall ‘06
Spin Echo: erasing magnetic field imperfections
Summing all spins (e.g. axial slice) creates rapid signal decay
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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
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Relaxation 15Psy 8960, Fall ‘06
SE EPI: reduction of through-slice dephasing
Gradient echo Spin echo