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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