fieldmaps and distortion
DESCRIPTION
Fieldmaps and distortion. What is a fieldmap? How can we predict distortion? How can we correct distortion?. Field map comes from 2 GE FLASH images. Ratio of phase images gives . Data from Siemens scanner is scaled weird. TE = 7.65ms. Scaled phase data: phase evolution during dTE. - PowerPoint PPT PresentationTRANSCRIPT
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Fieldmaps 1Psy 8960, Fall ‘06
Fieldmaps and distortion
• What is a fieldmap?• How can we predict
distortion?• How can we correct
distortion?
![Page 2: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/2.jpg)
Fieldmaps 2Psy 8960, Fall ‘06
Field map comes from 2 GE FLASH images
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Fieldmaps 3Psy 8960, Fall ‘06
Ratio of phase images gives
![Page 4: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/4.jpg)
Fieldmaps 4Psy 8960, Fall ‘06
Data from Siemens scanner is scaled weird
![Page 5: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/5.jpg)
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
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Fieldmaps 6Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: low resolution
![Page 7: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/7.jpg)
Fieldmaps 7Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: low resolution
![Page 8: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/8.jpg)
Fieldmaps 8Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: high resolution
![Page 9: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/9.jpg)
Fieldmaps 9Psy 8960, Fall ‘06
Distortion and signal drop-out vs. bandwidth: high resolution
![Page 10: Fieldmaps and distortion](https://reader036.vdocuments.us/reader036/viewer/2022070400/56813590550346895d9cfdc4/html5/thumbnails/10.jpg)
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.
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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.
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Fieldmaps 12Psy 8960, Fall ‘06
Motor mapping prelim. study