Gradient and Spin Echoes 1Psy 8960, Spring ‘07

Echoes, gradients and dephasing

Gradient echo Spin echo

Gradient and Spin Echoes 2Psy 8960, Spring ‘07

Imaging gradients: gradient echo

If the sample is a long pencil …

isochromatvoxel MS

A gradient (on the long axis) produces a distribution of frequencies (macroscopic)

Gradient and Spin Echoes 3Psy 8960, Spring ‘07

Imaging gradients: gradient echo

If the sample is a long pencil …

isochromatvoxel MS

The opposite gradient produces the opposite distribution of frequencies

Gradient and Spin Echoes 4Psy 8960, Spring ‘07

Gradient echo pulse sequenceExcitation pulse Echo

S

MT

Gradient and Spin Echoes 5Psy 8960, Spring ‘07

T2*: dephasing of spin isochromats

Voxel (entire slide) = collection of isochromats

isochromatvoxel MS

Magnet field perturbation: distribution of frequencies (microscopic)

time = 0

Gradient and Spin Echoes 6Psy 8960, Spring ‘07

T2*: dephasing of spin isochromats

Range of resonant frequencies

isochromatvoxel MS

Decreased signal at echo (acquisition) time

time = 30 ms

Gradient and Spin Echoes 7Psy 8960, Spring ‘07

T2*: intra-voxel dephasing due to field perturbation

time

*2Tt

eS

Orientation of arrow indicates relative phase in rotating (128 MHz) reference frame.

Gradient and Spin Echoes 8Psy 8960, Spring ‘07

Spin Echo refocuses T2* dephasingRefocusing pulseExcitation pulse Echo

S

MT

T2*

T2

Each isochromat gets shorter with time: T2 decay

Gradient and Spin Echoes 9Psy 8960, Spring ‘07

Spin Echo: erasing magnetic field imperfections

Imaging signal comes from protons on water molecules.

Sensitive to macro- and microscopic variations in B0.

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

Gradient and Spin Echoes 10Psy 8960, Spring ‘07

Spin Echo: erasing magnetic field imperfections

Summing all spins through an axial slice creates rapid signal decay

Gradient and Spin Echoes 11Psy 8960, Spring ‘07

Spin Echo: erasing magnetic field imperfections

Applying a 180 pulse at TE/2 refocuses the inhomogeneity-induced dephasing at TE

t = 0 ms t = TEt = TE/2

Gradient and Spin Echoes 12Psy 8960, Spring ‘07

SE EPI: reduction of through-slice dephasing

Gradient echo Spin echo

Gradient and Spin Echoes 13Psy 8960, Spring ‘07

Parkes et al. 2005

*2T

t

e

Gradient and Spin Echoes 14Psy 8960, Spring ‘07

Traveling waves: V1

Gradient and Spin Echoes 15Psy 8960, Spring ‘07

Traveling waves

Gradient and Spin Echoes 16Psy 8960, Spring ‘07

Blurring & MTF