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Echoes, gradients and dephasing

Echoes, gradients and dephasing. Gradient echo. Spin echo. Imaging gradients: gradient echo. If the sample is a long pencil …. A gradient (on the long axis) produces a distribution of frequencies ( macroscopic ). Imaging gradients: gradient echo. If the sample is a long pencil ….

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Echoes, gradients and dephasing

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  1. Echoes, gradients and dephasing Gradient echo Spin echo Gradient and Spin Echoes

  2. Imaging gradients: gradient echo If the sample is a long pencil … A gradient (on the long axis) produces a distribution of frequencies (macroscopic) Gradient and Spin Echoes

  3. Imaging gradients: gradient echo If the sample is a long pencil … The opposite gradient produces the opposite distribution of frequencies Gradient and Spin Echoes

  4. Gradient echo pulse sequence Excitation pulse Echo MT S Gradient and Spin Echoes

  5. T2*: dephasing of spin isochromats Voxel (entire slide) = collection of isochromats Magnet field perturbation: distribution of frequencies (microscopic) time = 0 Gradient and Spin Echoes

  6. T2*: dephasing of spin isochromats Range of resonant frequencies Decreased signal at echo (acquisition) time time = 30 ms Gradient and Spin Echoes

  7. T2*: intra-voxel dephasing due to field perturbation Orientation of arrow indicates relative phase in rotating (128 MHz) reference frame. time Gradient and Spin Echoes

  8. Spin Echo refocuses T2* dephasing Excitation pulse Refocusing pulse Echo Each isochromat gets shorter with time: T2 decay MT T2 T2* S Gradient and Spin Echoes

  9. Spin Echo: erasing magnetic field imperfections Imaging signal comes from protons on water molecules. Air in auditory canals creates susceptibility-induced magnetic field gradients Frequency map, zoomed in on lateral temporal cortex Hz Sensitive to macro- and microscopic variations in B0. On resonance 100 Hz off resonance 250 Hz off resonance Gradient and Spin Echoes

  10. Spin Echo: erasing magnetic field imperfections Summing all spins through an axial slice creates rapid signal decay Gradient and Spin Echoes

  11. 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 Gradient and Spin Echoes

  12. SE EPI: reduction of through-slice dephasing Gradient echo Spin echo Gradient and Spin Echoes

  13. Parkes et al. 2005 Gradient and Spin Echoes

  14. Traveling waves: V1 Gradient and Spin Echoes

  15. Traveling waves Gradient and Spin Echoes

  16. Blurring & MTF Gradient and Spin Echoes

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