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Understanding Gradient and Spin Echoes in Imaging

Explore the principles behind gradient and spin echoes in imaging, including dephasing of spin isochromats and techniques for erasing magnetic field imperfections. Learn about the distribution of frequencies and how they affect imaging signals.

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Understanding Gradient and Spin Echoes in Imaging

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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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