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Steady-state magnetization

Steady-state magnetization. Repeated measurements: combined T 1 and T 2 * weighting Ernst angle. T 1 and T 2 re-cap. Equilibrium. Excitation. Relaxation. M 0. M  (0) = M 0 sin(  ). . T 2 vs.T 2 * recap. T 2 intrinsic (unrecoverable) spin-spin relaxation

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Steady-state magnetization

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

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

  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 Steady-state magnetization

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

  5. 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

  6. Experiments with TR < T1 Steady-state magnetization

  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. Steady-state magnetization

  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 Steady-state magnetization

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

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