Fieldmaps 1Psy 8960, Fall ‘06
Fieldmaps and distortion
• What is a fieldmap?• How can we predict
distortion?• How can we correct
distortion?
Fieldmaps 2Psy 8960, Fall ‘06
Field map comes from 2 GE FLASH images
Fieldmaps 3Psy 8960, Fall ‘06
Ratio of phase images gives
Fieldmaps 4Psy 8960, Fall ‘06
Data from Siemens scanner is scaled weird
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 6Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: low resolution
Fieldmaps 7Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: low resolution
Fieldmaps 8Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: high resolution
Fieldmaps 9Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: high resolution
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.
Fieldmaps 12Psy 8960, Fall ‘06
Motor mapping prelim. study