1. Psy 8960, Spring ‘07 Gradient and Spin Echoes1 Echoes, gradients and dephasing Gradient echoSpin echo

2. Psy 8960, Spring ‘07 Gradient and Spin Echoes2 Imaging gradients: gradient echo If the sample is a long pencil … A gradient (on the long axis) produces a distribution of frequencies (macroscopic)

3. Psy 8960, Spring ‘07 Gradient and Spin Echoes3 Imaging gradients: gradient echo If the sample is a long pencil … The opposite gradient produces the opposite distribution of frequencies

4. Psy 8960, Spring ‘07 Gradient and Spin Echoes4 Gradient echo pulse sequence Excitation pulseEcho S MTMT

5. Psy 8960, Spring ‘07 Gradient and Spin Echoes5 T 2 *: dephasing of spin isochromats Voxel (entire slide) = collection of isochromats Magnet field perturbation: distribution of frequencies (microscopic) time = 0

6. Psy 8960, Spring ‘07 Gradient and Spin Echoes6 T 2 *: dephasing of spin isochromats Range of resonant frequencies Decreased signal at echo (acquisition) time time = 30 ms

7. Psy 8960, Spring ‘07 Gradient and Spin Echoes7 T 2 *: intra-voxel dephasing due to field perturbation time Orientation of arrow indicates relative phase in rotating (128 MHz) reference frame.

8. Psy 8960, Spring ‘07 Gradient and Spin Echoes8 Spin Echo refocuses T 2 * dephasing Refocusing pulseExcitation pulseEcho S MTMT T2*T2* T2T2 Each isochromat gets shorter with time: T 2 decay

9. Psy 8960, Spring ‘07 Gradient and Spin Echoes9 Spin Echo: erasing magnetic field imperfections Imaging signal comes from protons on water molecules. Sensitive to macro- and microscopic variations in B 0. Hz Frequency map, zoomed in on lateral temporal cortex On resonance 100 Hz off resonance 250 Hz off resonance Air in auditory canals creates susceptibility-induced magnetic field gradients

10. Psy 8960, Spring ‘07 Gradient and Spin Echoes10 Spin Echo: erasing magnetic field imperfections Summing all spins through an axial slice creates rapid signal decay

11. Psy 8960, Spring ‘07 Gradient and Spin Echoes11 Spin Echo: erasing magnetic field imperfections Applying a 180 pulse at TE/2 refocuses the inhomogeneity-induced dephasing at TE t = 0 mst = TEt = TE/2

12. Psy 8960, Spring ‘07 Gradient and Spin Echoes12 SE EPI: reduction of through-slice dephasing Gradient echoSpin echo

13. Psy 8960, Spring ‘07 Gradient and Spin Echoes13 Parkes et al. 2005

14. Psy 8960, Spring ‘07 Gradient and Spin Echoes14 Traveling waves: V1

15. Psy 8960, Spring ‘07 Gradient and Spin Echoes15 Traveling waves

16. Psy 8960, Spring ‘07 Gradient and Spin Echoes16 Blurring & MTF