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