Echoes gradients and dephasing Gradient echo Psy 8960
- Slides: 16
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
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