[Mristudio-users] fiber tracking statistics

[ZhuangJohnny]

Dear Susumu
 
Thank you very much for the informative reply. It's much clearer to me now.
 
Many thanks and regards,

 


Date: Fri, 15 Oct 2010 07:13:42 -0400
From: susumu at mri.jhu.edu
To: mristudio-users at mristudio.org
Subject: Re: [Mristudio-users] fiber tracking statistics

In DTI-based tractography, there is no "true fiber length", because what we reconstructed in no way represents actual axonal fiber, which has a diameter of few micron, while our pixel size is 2mm.  


what we reconstruct may reflect the macroscopic configuration of "axonal bundles". Axonal bundles are large enough to visually appreciate it and 2mm pixels are small enough to visualize the large bundles. However, the large bundles we can see and we have named (like the "longitudinal fasciculus") is abstractive and somewhat artifactual concept. They don't have clear boundaries and definitions because at any point, axon (which is the real entity) can merge and exist. So, "the length of the axonal bundles" is often not undefinable. It may contain an axon that goes only 10 mm and also another one with 50 mm length.


If our image resolution is 2 mm and tractography gave 6mm fiber length, that simply means there are 3 pixels in row that has similar fiber orientation, pointing each other, with sufficiently high FA. Nothing more than that. It was terminated because of sharp turns or low FA, which could be due to real anatomy or by noise. There is nothing wrong with that; the results are based on our measurements and consistent image analysis algorithm.


If there is anything "wrong", it is our interpretation of the results. We often try to connect the measurement and analysis results to real biological entity. It is important but very difficult process. What we are observing is merely water molecule properties, and there is a large disconnection from biological entity. We can still quantify it and compare among different subjects non-invasively and three-dimensionally. We can't do it using histology. The downside is, our observation is disconnected from real biological entity and interpretation, only from MRI observation, is often not possible.


Going back to your question, the minimum fiber length is simply set by an arbitrary threshold. Any reconstruction results shorter than that is removed from subsequent analysis.


2010/10/15 ZhuangJohnny <mbiozl at hotmail.com>


Dear DTI studio experts
 
Currently, I'm using DTIstudio to perform fiber tracking in our sample.  The fiber statistics gave me a minimum fiber length of around 2mm in the selected fibre, i'm just wondering if this small value could reflect the true fiber length, or it may result from noise in the DTI data?  
 
Many thanks and regards,
 

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