Echoes,%20gradients%20and%20dephasing - PowerPoint PPT Presentation

Title: Echoes,%20gradients%20and%20dephasing


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Echoes, gradients and dephasing
Gradient echo
Spin echo
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Imaging gradients gradient echo
If the sample is a long pencil
A gradient (on the long axis) produces a
distribution of frequencies (macroscopic)
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Imaging gradients gradient echo
If the sample is a long pencil
The opposite gradient produces the opposite
distribution of frequencies
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Gradient echo pulse sequence
Excitation pulse
Echo
MT
S
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T2 dephasing of spin isochromats
Voxel (entire slide) collection of isochromats
Magnet field perturbation distribution of
frequencies (microscopic)
time 0
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T2 dephasing of spin isochromats
Range of resonant frequencies
Decreased signal at echo (acquisition) time
time 30 ms
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T2 intra-voxel dephasing due to field
perturbation
Orientation of arrow indicates relative phase in
rotating (128 MHz) reference frame.
time
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Spin Echo refocuses T2 dephasing
Refocusing pulse
Excitation pulse
Echo
Each isochromat gets shorter with time T2 decay
MT
T2
T2
S
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Spin Echo erasing magnetic field imperfections
Imaging signal comes from protons on water
molecules.
Air in auditory canals creates susceptibility-indu
ced magnetic field gradients
Frequency map, zoomed in on lateral temporal
cortex
Hz
Sensitive to macro- and microscopic variations in
B0.
On resonance
100 Hz off resonance
250 Hz off resonance
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Spin Echo erasing magnetic field imperfections
Summing all spins through an axial slice creates
rapid signal decay
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Spin Echo erasing magnetic field imperfections
Applying a 180 pulse at TE/2 refocuses the
inhomogeneity-induced dephasing at TE
t 0 ms
t TE
t TE/2
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SE EPI reduction of through-slice dephasing
Gradient echo
Spin echo
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Parkes et al. 2005
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Traveling waves V1
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Traveling waves
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Blurring MTF