Fieldmaps and distortion Using fieldmaps to correct distortion

Fieldmaps and distortion • Using fieldmaps to correct distortion • Using fieldmaps to predict through-slice dephasing Psy 8960, Fall ‘ 06 Fieldmaps 1

Scaled phase data: phase evolution during d. TE TE = 7. 65 ms TE = 5. 19 ms Transverse plane Psy 8960, Fall ‘ 06 Fieldmaps 2

Functional imaging experiment Anatomical reference EPI data Psy 8960, Fall ‘ 06 Field map Fieldmaps 3

Alignment b/w EPI and T 1 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. Psy 8960, Fall ‘ 06 Fieldmaps 4

Perfectly aligned EPI still doesn’t match Psy 8960, Fall ‘ 06 Fieldmaps 5

Distortion prediction Field map EPI slice Psy 8960, Fall ‘ 06 Fieldmaps 6

Distortion prediction Field map EPI slice Psy 8960, Fall ‘ 06 Fieldmaps 7

Distortion prediction EPI slice Psy 8960, Fall ‘ 06 Voxel displacement map (Hz map divided by pixel BW) Fieldmaps 8

Distortion prediction EPI slice Psy 8960, Fall ‘ 06 Voxel displacement map (Hz map divided by pixel BW) Fieldmaps 9

Distortion correction + Psy 8960, Fall ‘ 06 = Fieldmaps 10

Unwarping Ground truth Psy 8960, Fall ‘ 06 Warped overlay Fieldmaps Unwarped overlay 11

Before Psy 8960, Fall ‘ 06 After Fieldmaps 12

Before Psy 8960, Fall ‘ 06 After Fieldmaps 13

Predicting through-slice dephasing Psy 8960, Fall ‘ 06 Fieldmaps 14

Field map and gradients, inferior slice Psy 8960, Fall ‘ 06 Fieldmaps 15

Field map and gradients, middle slice Psy 8960, Fall ‘ 06 Fieldmaps 16

Field map and gradients: artifacts Psy 8960, Fall ‘ 06 Fieldmaps 17

Field map and gradients: artifacts Psy 8960, Fall ‘ 06 Fieldmaps 18

Predicting drop-out • Need to know: – (Linear) field gradient in through-slice direction – Echo time Psy 8960, Fall ‘ 06 Fieldmaps 19

Psy 8960, Fall ‘ 06 Fieldmaps 20