Free water correction

[Rodolphe]

Hello, 

 by looking at your image results on http://mig.cs.ucl.ac.uk/index.php?n=Tutorial.NODDImatlab, I noticed that the NDI and ODI in Figure 3 seems to have some free water correction in ventricles, but I cannot find any explanation for it and I don't think it is implemented in the Toolbox. 

Could anyone help me with that? 

Thank a lot.

[Hui Zhang]

Hi,

Actually, that figure came from running the current version of NODDI toolbox on the example dataset we provide.  For voxels in the ventricles, they contain only free water (FWF = 1), no tissue.  Remember that NDI and ODI reflect properties of the tissue space.  So when there exists no tissue at all, values for NDI and ODI can no longer be uniquely determined.  In other words, the values of NDI and ODI in the ventricles are arbitrary.  The same is true whenever FWF is close to 1.

On a related note, we have recently developed tissue-weighted mean to take into account the importance of FWF to the interpretation of NDI and ODI. Here is a link to a short blog explaining the idea:

https://csparker.github.io/research/2021/11/16/Tissue-weighted-mean.html

Hope this helps.

Gary

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