1. Relaxation Exponential time constants T1 T2 T2*
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

2. Exponential decay
 = 30 ms
Relaxation

3. T2 TE = 15 ms TE = 20 ms TE = 30 ms TE = 60 ms  = T2= 30 ms
Relaxation

4. Exponential recovery
Inversion recovery
Saturation recovery
Relaxation

5. T1 Inversion recovery T1 = 2000 ms TI = 50 ms 100 ms 400 ms 800 ms
Relaxation

6. Pulse sequence diagrams
Magnetization preparation
Excitation
Read-out
Nrep RF
GSS
GPE
GRO
DAC
Relaxation

7. Spin Echo EPI pulse sequence (not to scale)
Excitation pulse
Refocusing pulse RF
GSS …
GPE …
GRO …
DAC
Relaxation

8. Spin Echo measures T2, not T2*
Excitation pulse
Refocusing pulse
Echo
Read-out MT T2
T2* S
Relaxation

9. Spin Echo EPI pulse sequence
TE/2
TE/2 RF
GSS
GPE
GRO
DAC
Relaxation

10. Inversion Recovery Spin Echo EPI
TE/2
TE/2 RF
GSS
GPE
GRO
DAC
Relaxation

11. Inversion Recovery FLASH
NPE RF
GSS
GPE
GRO
DAC
NRO
Relaxation

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
Relaxation

13. Spin Echo: erasing magnetic field imperfections
Summing all spins (e.g. axial slice) creates rapid signal decay
Relaxation

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/2
t = TE
Relaxation

15. SE EPI: reduction of through-slice dephasing
Gradient echo
Spin echo
Relaxation