[Mristudio-users] Absence of CST in subcortical motor cortex

[susumu mori]

Hi Venkat,

If you apply the same method to both groups and only one of the groups show
the truncated CST, it tells there is some difference between the two groups
detected by DTI.

I think this is a fact. Our interpretation, on the other hand, may not be a
fact and there are many ways to interpret the same data. I believe this is
the source of your question.

When we do MRI and find something (e.g. FA drops, ADC increases, fibers
thinner, etc), we try to directly relate it to underlying neuroanatomy. For
example, FA tells something about myelination and tractography about
connectivity. However, MRI/DTI measures water properties at each pixel,
which is a decidedly indirect indicator of any cellular entity. The level of
biological question we are asking often require astronomical amount of data.
For example, if we want to know myelination status, we are asking questions
in micrometer scale of 100 billion axons. On the other hand, DTI data is
merely 20 MB of information. This is a huge degeneration problem. You can
predict what would happen in DTI/MRI if you know the anatomy, but it's much
tougher to define what really happens from DTI/MRI results.

By applying the same tractography protocol, you found consistent difference
between two groups. If you use Wakana's protocol, there are more subjects in
the patient group that return "null". If you quantify the results in some
way (e.g. the number of "null" subjects, the average number of pixels, etc),
you may be able to find a statistical difference. So far so good. However,
you have to interpret this data, where the difficulty arises. If you
literary take the results and claim, "there are more patients who lack the
CST", that may not be true. This is because there could be multiple reasons
that lead to the same tractography results; there is one area with low FA,
enlargement of the ventricle made the CST trajectory more tortuous, thinning
of one anatomical area made the tractography less reproducible (partial
volume effect), reduction of the CST axon leads to the higher contribution
of a small population of crossing fibers, and so on.

By having only 20 MB of the data, MRI/DTI/tractography can only be a
screening tool, I think. To obtain more information, you may need to do more
analysis to come closer to the true reason of your results. You can modify
the CST protocol so that you can get more fibers. If you lower the second
ROI away from the motor cortex, you get more fiber with an expense of less
specificity to the CST. You can monitor FA values along the fiber. You can
do voxel-based analysis to find a group of pixels that accounts for the
different tractography results. If you can identify the reason (e.g.
locations of pixels with different FA/vector values), you have more specific
information about the difference between the two groups.

Sorry that I'm not providing a solution to your answer. If your control
group shows good CSTs, then I think you are not making a mistake. It's
possible that the corona radiata has lower FA in the patients. I would try
to find out the reason of the truncated CST.

Susumu

On Tue, Apr 13, 2010 at 12:16 PM, Rajagopalan, Venkateswaran <
rajagov2 at ccf.org> wrote:

>
> Dear All,
>
> In our study we tried to evaluate changes in CST between our patient and
> control groups using DTI metrics. We reconstructed CST using tracography and
> measured changes in DTI metrics.
>
> We used same protocol for DTI imaging and the DTI images were processed the
> same way in DTI studio like setting same the FA threshold for tracking in
> all the subjects, tracts reconstructed using Wakana et al's two ROI approach
> i.e we placed our first ROI in cerebral peduncle and then the second ROI in
> subcortical Motor cortex, used cut operation in all the subjects, then
> similar kind of outlier rejection of bad DW images.By this i mean same
> processing steps.
>
>
> The question i have is in some of these patients CST doesn't go all the way
> up to motor cortex i.e.after reconstruction CST is available only between
> cerebral peduncle to top of lateral ventricle and after that ( i.e. higher
> up) CST is absent, this is seen only in some of our patients not in all the
> cases. Can i attribute this to the disease process or is there somewhere i
> am making mistake in my image processing steps.
>
> Thanks in advance.
>
> venkat
>
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