Do you have any parallel imaging capability on your scanner?  If so, set
GRAPPA to 2 and this will reduce distortions with some SNR cost.
Additionally, if you are able, you can increase bandwidth to reduce
distortions at an SNR cost.  Also make sure your TE is at the minimum
possible for your given parameters.  

Peace,

Matt.

-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Mark Pinsk
Sent: Tuesday, October 13, 2009 11:00 AM
To: [log in to unmask]
Subject: Re: [FSL] signal voids in unwarped image

Thanks for the information Mark!   If our distortions seem so bad, I wonder
if you (or anyone) wouldn't mind looking at our scanning parameters?  Maybe
there are some parameters that aren't optimal, and I could tweak them to
reduce the stretching a bit?   

We will also try the reversal phase encoding undistortion down the line, but
I'd like to make sure our parameters are optimal first.

I'm on a Siemens 3T Allegra.  Here is my protocol for a b0 image
acquisition:

Routine Tab:
Slices: 66
Dist. factor: 0 %
Position: Isocenter
Orientation: Transversal
Phase enc dir: A >> P
Rotation: 0 deg
Phase oversampling: 0 %
FoV read: 256 mm
FoV phase: 100 %
Slice thickness: 2 mm
TR: 10,000 ms
TE: 95 ms
Averages: 1
Concatenations: 1
Filter: None
Coil Elements: HE

Contrast Tab:
MTC: 0
Magn. preparation: None
Reconstruction: Magnitude
Fat suppr.: Fat sat.
Measurements: 1
Delay in TR: 0 ms

Resolution Tab:
Base resolution: 128
Phase resolution 100 %
Phase partial Fourier: 6/8
Filter 1 Raw Filter: Off
Filter 2 Large FoV: Off
Filter 3 Normalize: Off
Filter 4 Elliptical filter: Off
Interpolation: 0

Geometry Tab:
Mult-slice: Interleaved
Series: Interleaved
Special sat: None

System Tab:
Save uncombined: 0
Scan at current TP: 0
Scan region position: H
Scan region position: 0 mm
MSMA: S-C-T
Sagittal: R>>L
Coronal: A>>P
Transversal: F>>H
Head 3T / HE: 1
Shim mode: Standard
Confirm freq. adjustment: 0
Assume Silicone: 0
Ref. amplitude [1H]: 154.807 V
Adjust volume:
 Position: Isocenter
 Orientation: Transversal
 Rotation: 0 deg
 R>>L: 256 mm
 A>>P: 256 mm
 F>>H: 132 mm

Physio Tab:
1st Signal/Mode: None

Diff Tab:
Diffusion mode: MDDW
Diff. weightings: 1
b-value: 0 s/mm2
Diff. weighted images: 1
Trace weighted images: 0
Average ADC maps: 0
Individual ADC maps: 0
Noise level: 40
Diff. directions: 1

Sequence Tab:
Introduction: 0
Averaging mode: Long term
Bandwidth: 1776 Hz/Px
Free echo spacing: 0
Echo spacing: 0.61 ms
EPI factor: 128
RF pulse type: Normal
Gradient mode: Fast