Fieldmaps and distortion

1
Fieldmaps and distortion

• What is a fieldmap?
• How can we predict distortion?
• How can we correct distortion?

2
Field map comes from 2 GE FLASH images
3
Ratio of phase images gives ??
4
Data from Siemens scanner is scaled weird
5
Scaled phase data phase evolution during dTE
TE 5.19ms
Transverse plane
6
Distortion and signal drop-out vs. bandwidth
low resolution
7
Distortion and signal drop-out vs. bandwidth
low resolution
8
Distortion and signal drop-out vs. bandwidth
high resolution
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Distortion and signal drop-out vs. bandwidth
high resolution
10
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.
11
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.

12
Motor mapping prelim. study