Exponential%20time%20constants

1
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
2
Exponential decay
? 30 ms
3
T2
TE 15 ms
TE 20 ms
? T2 30 ms
T2 50 ms
TE 30 ms
TE 60 ms
4
Exponential recovery
Inversion recovery
Saturation recovery
5
T1
Inversion recovery
T1 900 ms
T1 1100 ms
T1 2000 ms
TI 50 ms
100 ms
800 ms
400 ms
1600 ms
3200 ms
4800 ms
6
Pulse sequence diagrams
Excitation
Magnetization preparation
Read-out
Nrep
RF
GSS
GPE
GRO
DAC
7
Spin Echo EPI pulse sequence (not to scale)
Excitation pulse
Refocusing pulse
RF
GSS

GPE

GRO

DAC
8
Spin Echo measures T2, not T2
Refocusing pulse
Excitation pulse
Echo
Read-out
MT
T2
T2
S
9
Spin Echo EPI pulse sequence
TE/2
TE/2
RF
GSS
GPE
GRO
DAC
10
Inversion Recovery Spin Echo EPI
TE/2
TE/2
RF
GSS
GPE
GRO
DAC
11
Inversion Recovery FLASH
NPE
RF
GSS
GPE
GRO
NRO
DAC
12
Spin Echo erasing magnetic field imperfections
Imaging signal comes from protons on water
molecules.
Frequency map, zoomed in on lateral temporal
cortex
Hz
On resonance
Sensitive to macro- and microscopic variations in
B0.
100 Hz off resonance
250 Hz off resonance
13
Spin Echo erasing magnetic field imperfections
Summing all spins (e.g. axial slice) creates
rapid signal decay
14
Spin Echo erasing magnetic field imperfections
Applying a 180 pulse at TE/2 refocuses the
inhomogeneity-induced dephasing at TE
t 0 ms
t TE
t TE/2
15
SE EPI reduction of through-slice dephasing
Gradient echo
Spin echo