Steady-state magnetization

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Presentation transcript:

Steady-state magnetization Repeated measurements: combined T1 and T2* weighting Ernst angle Steady-state magnetization

Steady-state magnetization T1 and T2 re-cap Equilibrium Excitation Relaxation M0 M(0) = M0sin()  Steady-state magnetization

Steady-state magnetization 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 Steady-state magnetization

Steady-state magnetization T2 vs.T2* recap Excitation pulse Refocusing pulse Echo Read-out MT T2 T2* S Steady-state magnetization

But who has time to wait for equilibrium? Magnetization preparation Excitation Read-out Nrep = 128 TR = 6s Tacq = 13 min. RF GSS GPE GRO DAC Steady-state magnetization

Experiments with TR < T1 Steady-state magnetization

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. Steady-state magnetization

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 Steady-state magnetization

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