[Mristudio-users] checkered artifact

[Dresner, Alex]

Hi Mark,
Your first advisor was correct, these are Fourier truncation artifacts.  They show up most prominently around high signal areas--so it's the bright fluid, not specifically flow, that is the problem.  They show up in your FA maps because those are generated relative to the non-DWI or B0 image.  There are two physics solutions and one image processing approach:
1) increase your native (not just reconstructed) resolution to limit the ringing.  (It's also called Gibbs ringing, but that doesn't help you address it except in searching the literature.)  Since the ringing is in both directions, it will cost you some time (and SNR) to increase the resolution.  Are you using parallel imaging to accelerate the acquisition?  If so, you might be able to reduce the artifact just by cutting down on the acceleration factor.
2) suppress the bright fluid signal in your non-DWI image.  A few people have suggested using an isotropically-weighted b=20 instead of a b=0 image so suppress large free fluid signals; this is still minimal compared to the b=800 or b=1000 so this shouldn't produce a quantitative difference.
3) More filtering of your data.  There are more smoothing filters available than you could possibly test.  Simple interpolation will reduce the choppy (pixelated, checkered) appearance but will not address the underlying ringing.  The scanner should have some interpolation/filtering/ringing tools available; if these are already on maybe they're not performing the way you think they should.

Should you be concerned? I'll leave that to the processing and results experts.  But now you know more about it...

Regards,
Alex

____________________
M. Alex Dresner, Ph.D.
MR Clinical Scientist
Philips Healthcare
____________________




-----Original Message-----
From: mristudio-users-bounces at mristudio.org [mailto:mristudio-users-bounces at mristudio.org] On Behalf Of Mark A. Pinsk
Sent: Thursday, August 04, 2011 3:16 PM
To: mristudio-users at mristudio.org
Subject: [Mristudio-users] checkered artifact

Hi everyone,

We're seeing an artifact in our DTI images (most visible in the non-DWI images, but they show up everywhere including the calculated FA maps).  I assume this will negatively impact any tracking analyses.   It's a checkered pattern that originates from high intensity regions (i.e. ventricles) and goes in both the frequency and the phase encode directions.  Links to snapshots below.   I was told by one person that they are Fourier truncation artifacts, but I was also told by another person that they are flow artifacts.

Can someone clarify for me (1) what they are, (2) should I be concerned, and (3) how do I avoid them ?

http://dl.dropbox.com/u/3889124/dti_snapshot1.tiff
http://dl.dropbox.com/u/3889124/dti_snapshot2.tiff

Thanks very much!
Mark


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