[Mristudio-users] Mean FA

[Linda Lanyon]

Thanks for the comprehensive reply.
Yes, your assumption of what I meant - the mean streamline - is correct.
The weighted FA calculation would be useful, thanks Hangyi.

many thanks
Linda

________________________________
From: mristudio-users-bounces at mristudio.org [mristudio-users-bounces at mristudio.org] On Behalf Of susumu mori [susumu at mri.jhu.edu]
Sent: July 15, 2011 9:09 PM
To: DTI Studio, ROI Editor, DiffeoMap Questions/Support; Hangyi Jiang
Subject: Re: [Mristudio-users] Mean FA

Hi Linda,

I'm not sure what you exactly mean by "mean tract position". To calculate a mean, you need a population. The "population" could be across 10 different individuals, 10 repeated scans, 10 voxels within the same individual, etc.

I assume that you meant; "after tractography, it is common to obtain 10s of streamlines that belong to the same pathway and can we calculate a "mean streamline"  of the 10s of streamlines?"

If so, DtiStudio doesn't have such a function explicitly, but has the following functions;

1) After tractography (and thus obtained 10s of streamlines), this information is converted to 1/0 binary masking, in which voxels that contain the streamline are assigned "1". Then it reports mean FA of the masked voxels.
2) In the above method, a voxel that contains 1 streamline and 10 streamlines are both equally weighted (they are both assigned "1"). I believe that there is a function to perform "weighted FA calculation". Please ask Hangyi about the detail of this function. This may provide a similar result to calculating a mean tract and measuring FA.

To divide the tractography results, DtiStudio can report FA of a tract at each slice level. For example, if you have 50 axial slices, you can get a report of FA at 50 axial levels.  Then the problem is, you have to identify corresponding axial levels when you compare the reported slice-specific numbers across individuals. For example, the FA of the CST at axial slice #24 may correspond to slice #26 of another subject. When you get FA of the entire CST from the pons to the motor cortex, you don't have this problem. As soon as dividing the CST into "regions", you need spatial normalization across individuals to make sure you are comparing the same "regions" across individuals.

There are several ways to do this normalization. Probably the easiest way is to normalize the orientation and the size of the brains across the population using a linear transformation such that each axial level is more or less consistent and thus the slice-by-slice reporting can be population averaged. Or you can use our full brain segmentation using the DiffeoMap/RoiEditor pipeline such that the CST pathway can be divided into anatomical segments such as "pons", "midbrain", "internal capusule", "corona radiata".

Hope this helps.

On Fri, Jul 15, 2011 at 6:14 PM, Linda Lanyon <llanyon at eyecarecentre.org<mailto:llanyon at eyecarecentre.org>> wrote:
Is there a way in DTI Studio (or MRI Studio) to find a mean tract position for a bundle of fibers along the same pathway, and then to calculate mean FA along that tract (preferably divided into segments along the tract)?

The only method I can find in DTI Studio is to reduce the tracts down to a selected few and then perform statistics, which gives mean FA. - just wondering if there is a way to get the mean tract as I've seen this published with other software.

Thanks!
Linda

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Linda Lanyon PhD BSc(Hons) CEng CITP MBCS
Michael Smith Foundation for Health Research Post Doctoral Research Fellow
University of British Columbia, Human Vision & Eye Movement Laboratory
VGH Eye Care Centre, 3rd Floor Research Labs
2550 Willow St, Vancouver
Phone: +1-604-675-8865<tel:%2B1-604-675-8865>
Cell: +1-778-378-5545<tel:%2B1-778-378-5545>
Fax: +1 604-875-4663<tel:%2B1%20604-875-4663>
email: llanyon at eyecarecentre.org<mailto:llanyon at eyecarecentre.org>
web: http://www.neuroophthalmology.ca/UBCNeuroOp/JBarton/labstaff/Linda.htm

Director, Society for Canadian Women in Science and Technology: www.scwist.ca<http://www.scwist.ca>
SCWIST is a non-profit association that promotes, encourages and empowers women
and girls in science, engineering and technology.
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"If we knew what it was we were doing, it would not be called research, would it?"
Albert Einstein

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