[Mristudio-users] fiber tracking statistics

[susumu mori]

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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