Echoes,%20gradients%20and%20dephasing

1
Echoes, gradients and dephasing
Gradient echo
Spin echo
2
Imaging gradients gradient echo
If the sample is a long pencil
A gradient (on the long axis) produces a
distribution of frequencies (macroscopic)
3
Imaging gradients gradient echo
If the sample is a long pencil
The opposite gradient produces the opposite
distribution of frequencies
4
Gradient echo pulse sequence
Excitation pulse
Echo
MT
S
5
T2 dephasing of spin isochromats
Voxel (entire slide) collection of isochromats
Magnet field perturbation distribution of
frequencies (microscopic)
time 0
6
T2 dephasing of spin isochromats
Range of resonant frequencies
Decreased signal at echo (acquisition) time
time 30 ms
7
T2 intra-voxel dephasing due to field
perturbation
Orientation of arrow indicates relative phase in
rotating (128 MHz) reference frame.
time
8
Spin Echo refocuses T2 dephasing
Refocusing pulse
Excitation pulse
Echo
Each isochromat gets shorter with time T2 decay
MT
T2
T2
S
9
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
10
Spin Echo erasing magnetic field imperfections
Summing all spins through an axial slice creates
rapid signal decay
11
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
12
SE EPI reduction of through-slice dephasing
Gradient echo
Spin echo
13
Parkes et al. 2005
14
Traveling waves V1
15
Traveling waves
16
Blurring MTF