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Relaxation

Relaxation. Exponential time constants T 1 Longitudinal (spin-lattice) Inversion recovery experiment T 2 Transverse (spin-spin) relaxation Spin-echo experiment T 2 * T 2 with field inhomogeneities Gradient echo experiment. TE = 15 ms. TE = 20 ms. TE = 30 ms. TE = 60 ms.

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Relaxation

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  1. Relaxation • Exponential time constants • T1 • 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  = T2= 30 ms T2= 50 ms TE = 30 ms TE = 60 ms Relaxation

  4. Exponential recovery Inversion recovery Saturation recovery Relaxation

  5. T1 Inversion recovery T1= 900 ms T1= 1100 ms T1= 2000 ms TI = 50 ms 100 ms 400 ms 800 ms 1600 ms 3200 ms 4800 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 NRO DAC 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

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