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