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

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Presentation transcript:

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

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)

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

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

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

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

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.

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

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

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

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

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

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

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

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

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