Fieldmaps 1Psy 8960, Fall ‘06
Fieldmaps and distortion
• Using fieldmaps to correct distortion
• Using fieldmaps to predict through-slice dephasing
Fieldmaps 2Psy 8960, Fall ‘06
Scaled phase data: phase evolution during dTE
12
12
1212 )(
TETE
TETE
t
voxel
voxel
2
),(1),(
),(1
),(
12
12
yx
TETEyx
yxTETE
yx
TE = 5.19msTE = 7.65ms
Transverse plane
Fieldmaps 3Psy 8960, Fall ‘06
Functional imaging experiment
EPI data
Anatomical reference
Field map
Fieldmaps 4Psy 8960, Fall ‘06
Alignment b/w EPI and T1 anatomy
• Option 1: Assume they’re in the right place• Option 2: Use intensity-based or mutual information
algorithm to find rigid-body transformation of EPI to anat.
Fieldmaps 5Psy 8960, Fall ‘06
Perfectly aligned EPI still doesn’t match
Fieldmaps 6Psy 8960, Fall ‘06
Distortion prediction
EPI slice Field map
Fieldmaps 7Psy 8960, Fall ‘06
Distortion prediction
EPI slice Field map
Fieldmaps 8Psy 8960, Fall ‘06
Distortion prediction
EPI slice Voxel displacement map(Hz map divided by pixel BW)
ROTpixelBW
1
Fieldmaps 9Psy 8960, Fall ‘06
Distortion prediction
EPI slice Voxel displacement map(Hz map divided by pixel BW)
ROTpixelBW
1
Fieldmaps 10Psy 8960, Fall ‘06
Distortion correction
+ =
Fieldmaps 11Psy 8960, Fall ‘06
Unwarping
Ground truth Warped overlay Unwarped overlay
Fieldmaps 12Psy 8960, Fall ‘06
Before After
Fieldmaps 13Psy 8960, Fall ‘06
Before After
Fieldmaps 14Psy 8960, Fall ‘06
Predicting through-slice dephasing
Fieldmaps 15Psy 8960, Fall ‘06
Field map and gradients, inferior slice
Fieldmaps 16Psy 8960, Fall ‘06
Field map and gradients, middle slice
Fieldmaps 17Psy 8960, Fall ‘06
Field map and gradients: artifacts
Fieldmaps 18Psy 8960, Fall ‘06
Field map and gradients: artifacts
Fieldmaps 19Psy 8960, Fall ‘06
Predicting drop-out
• Need to know:– (Linear) field gradient in through-slice direction– Echo time
Fieldmaps 20Psy 8960, Fall ‘06