psy 8960, fall ‘06 fieldmaps1 fieldmaps and distortion what is a fieldmap? how can we predict...
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Fieldmaps 1Psy 8960, Fall ‘06
Fieldmaps and distortion
• What is a fieldmap?• How can we predict
distortion?• How can we correct
distortion?
Fieldmaps 5Psy 8960, Fall ‘06
Scaled phase data: phase evolution during dTE
12
12
1212 )(
TETE
TETE
t
voxel
voxel
),(1
),(12
yxTETE
yx
TE = 5.19msTE = 7.65ms
Transverse plane
Fieldmaps 10Psy 8960, Fall ‘06
Mystery … solved
Theory: power calibration applies saturation pulse before image acquisition. If operator delays before starting scan, magnetization returns to equilibrium and 1st image has T2* contrast. If operator does not delay, magnetization is not at equilibrium, and contrast is T2* with T1.
Fieldmaps 11Psy 8960, Fall ‘06
When distortion matters
• Many data sets are best visualized in 3D
• 3D anatomy is usually a FLASH-family sequence– 64 or 128 higher pixel bandwidth– often acquired on a different day
(different day = different fieldmap)
• Go ahead and try to align a distorted EPI to a FLASH volume.