[Mristudio-users] tensor calculation - automatic outlier rejection

[susumu]

Hi Venkateswaran,

 

The outlier rejection is still work-in-progress. We are about to post
Version 3, in which there will be only one threshold to simplify the
function. Meanwhile please use very large numbers for all thresholds to
avoid the software to reject any images. 

 

However, there are two very handy functions in "verify the results" section
of the window, which are two checkboxes with "create images of ,,,,".

 

After the calculation, please click the "Original-ADC-Mean-STD" button. In
this window, you can see your raw data and "theory image", which tells how
the image should look like calculated from all data, and "error image",
which is the difference between the raw and theory data. These images can be
very informative to detect corrupted images, typically observed around the
forth ventricle (brainstem, cerebellum, and often up to the splenium of the
corpus callosum). For the corrupted images, you can simply click the bad
image and large red X mark appears. These images won't be included for
tensor calculation. 

 

In our experience, maybe well more than 50% of DTI data do not have
significantly corrupted images and you don't need to worry. There are,
however, sometimes subjects who have several corrupted images. Once
affected, it is common to see multiple severely corrupted ones. My
experience says pediatric brains are more prone to the artifacts. There are
sometimes signal leak from outside MRI that causes a spike in the k-space
and produce "zebra" patterns. This should also be eliminated. 

 

The "error images" also sensitively detect subject motions and eddy current
issues. These problems appear as a ring around the brain. If the subject
moved in the middle of the scan, half the error images show bright edge in
one side of the brain and dark edge in the other side. The rest of the
images have the same edges but with the opposite bright/dark edge locations.

 

You can try different threshold levels to see if the software can
automatically detect the one you thought bad. The version 3 has better
automatic detection, but these functions are still beta.

 

Hope this helps.

 

Susumu

 

  _____  

From: mristudio-users-bounces at mristudio.org
[mailto:mristudio-users-bounces at mristudio.org] On Behalf Of Rajagopalan,
Venkateswaran
Sent: Friday, September 05, 2008 12:50 PM
To: mristudio-users at mristudio.org
Subject: [Mristudio-users] tensor calculation - automatic outlier rejection

 

Dear All,

 

Recently i downloaded latest version of DTI studio. When i tried to do
Tensor calculation the usual DTI- map options appear first and when fill in
the details and say ok another window pops up asking for outlier rejection
and when i say yes another window pops up where there are many options to
fill in.  It will be of great help to me if  anyone could please tell me
what this new automatic outlier rejection option is for and where i can get
details regarding how to choose these different options and under what
circumstances should i go for this as i couldn't  find their details in the
manual.

 

Thanks

 

venkateswaran

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