Echoes gradients and dephasing Gradient echo Psy 8960

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

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

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

Gradient echo pulse sequence Excitation pulse Echo MT S Psy 8960, Spring ‘ 07 Gradient and Spin Echoes 4

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

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

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

Spin Echo refocuses T 2* dephasing Excitation pulse Refocusing pulse Echo Each isochromat gets shorter with time: T 2 decay MT T 2 * S Psy 8960, Spring ‘ 07 Gradient and Spin Echoes 8

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 B 0. On resonance 100 Hz off resonance 250 Hz off resonance Psy 8960, Spring ‘ 07 Gradient and Spin Echoes 9

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

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

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

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

Traveling waves: V 1 Psy 8960, Spring ‘ 07 Gradient and Spin Echoes 14

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

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