Repeated measurements: combined T1 and T2* weighting

1
Steady-state magnetization

• Repeated measurements combined T1 and T2
  weighting
• Ernst angle

2
T1 and T2 re-cap
Equilibrium
Excitation
Relaxation
M0
M?(0) M0sin(?)
?
3
T2 vs.T2 recap

• T2
• intrinsic (unrecoverable) spin-spin relaxation
• describes transverse magnetization decay in spin
  echo pulse sequences
• T2
• dephasing of spin isochromats due to microscopic
  field inhomogeneities
• Free induction decay (FID)
• describes transverse magnetization decay in
  gradient echo pulse sequences

4
T2 vs.T2 recap
Refocusing pulse
Excitation pulse
Echo
Read-out
MT
T2
T2
S
5
But who has time to wait for equilibrium?
Excitation
Magnetization preparation
Read-out
Nrep 128 TR 6s Tacq 13 min.
RF
GSS
GPE
GRO
DAC
6
Experiments with TR lt T1
7
Ernst angle maximizing steady-state signal
Excerpt from BPhy 8147 notes
Setting derivative with respect to alpha equal to
zero finds the expression for the Ernst angle.
8
Ernst angle calculation

• Typical EPI parameters
• Gray matter T1 1100 ms
• Volume TR 1500 ms
• Flip angle (Matlab code) alpha_ernst
  acos(exp(-TR/T1))180/pi
  acos(exp(-1.36))180/pi 75º

9
Gray/white contrast at steady-state
1st image
Steady-state