Psy 8960, Fall ‘06 Fieldmaps1 Fieldmaps and distortion What is a fieldmap? How can we predict distortion? How can we correct distortion?

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Presentation transcript:

Psy 8960, Fall ‘06 Fieldmaps1 Fieldmaps and distortion What is a fieldmap? How can we predict distortion? How can we correct distortion?

Psy 8960, Fall ‘06 Fieldmaps2 Field map comes from 2 GE FLASH images

Psy 8960, Fall ‘06 Fieldmaps3 Ratio of phase images gives 

Psy 8960, Fall ‘06 Fieldmaps4 Data from Siemens scanner is scaled weird

Psy 8960, Fall ‘06 Fieldmaps5 Scaled phase data: phase evolution during dTE TE = 5.19ms TE = 7.65ms Transverse plane

Psy 8960, Fall ‘06 Fieldmaps6 Distortion and signal drop-out vs. bandwidth: low resolution

Psy 8960, Fall ‘06 Fieldmaps7 Distortion and signal drop-out vs. bandwidth: low resolution

Psy 8960, Fall ‘06 Fieldmaps8 Distortion and signal drop-out vs. bandwidth: high resolution

Psy 8960, Fall ‘06 Fieldmaps9 Distortion and signal drop-out vs. bandwidth: high resolution

Psy 8960, Fall ‘06 Fieldmaps10 Mystery … solved Theory: power calibration applies saturation pulse before image acquisition. If operator delays before starting scan, magnetization returns to equilibrium and 1 st image has T 2 * contrast. If operator does not delay, magnetization is not at equilibrium, and contrast is T 2 * with T 1.

Psy 8960, Fall ‘06 Fieldmaps11 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.

Psy 8960, Fall ‘06 Fieldmaps12 Motor mapping prelim. study