Transcript Introduction to MRI: NMR - University of Minnesota

Steady-state magnetization
• Repeated
measurements:
combined T1 and T2*
weighting
• Ernst angle
Psy 8960, Fall ‘06
Steady-state magnetization
1
T1 and T2 re-cap
Equilibrium
Excitation
Relaxation
M || ( t )  M 0 cos(  ) e
 t
T1
 M 0 (1  e
 t
T1
)
M0
M(0) = M0sin()
M  ( t )  M 0 sin(  ) e
 t
T2
(*)

Psy 8960, Fall ‘06
Steady-state magnetization
2
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
Psy 8960, Fall ‘06
Steady-state magnetization
3
T2 vs.T2* recap
Excitation pulse
Refocusing pulse
Echo
Read-out
MT
T2
T 2*
S
Psy 8960, Fall ‘06
Steady-state magnetization
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But who has time to wait for equilibrium?
Magnetization
preparation
Excitation
RF
Read-out
Nrep = 128
TR = 6s
Tacq = 13 min.
GSS
GPE
GRO
DAC
Psy 8960, Fall ‘06
Steady-state magnetization
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Experiments with TR < T1
Psy 8960, Fall ‘06
Steady-state magnetization
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Ernst angle: maximizing steady-state signal
Excerpt from BPhy 8147 notes …
cos(  )  e
 TR
T1
Setting derivative with respect to alpha equal to zero finds the
expression for the Ernst angle.
Psy 8960, Fall ‘06
Steady-state magnetization
7
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
Psy 8960, Fall ‘06
Steady-state magnetization
8
Gray/white contrast at steady-state
1st image
Psy 8960, Fall ‘06
Steady-state
Steady-state magnetization
9