[Mristudio-users] DTI method: VBA, MRIstudio vs. TBSS

[susumu mori]

Good question Gao.

Here is my thought (my personal opinion, of course);

1) you are comparing VBA and TBSS. These are methods to "define
corresponding pixels (or areas) across subjects", so that you can compare
pixel numbers such as FA and MD among different brains. This is called
"registration".
2) These methods can be classified from different point of views;
2-1) granularity: One extreme is to define the entire brain as one ROI. You
can get the whole brain volume, whole brain FA, or whole brain histogram.
While there is not much use of this approach, it is precise (do 10 times and
you get the same results) and accurate (nobody makes mistakes about where is
the whole brain except for some ambiguity about where you cut the ROI in the
brainstem). The other extreme is the pixel, which is the smallest unit. This
mean, you identify the corresponding pixels across subjects. Once you map
the entire 1 million pixels in one brain to the other, it is the same as
transforming one brain to the other (two brains now have the same shape).
This approach is called "normalization" and, of course, not accurate because
it is not possible to completely solve the system and accurately map all 1
million pixels. VBA and TBSS are based on this normalization procedure.
There are methods to ameliorate this accuracy issue. Usually VBA uses
filters to blur the information. In my understanding, TBSS "re-register"
nearby pixel information to the core of the white matter, which could be
considered as a sort of filtering, reducing the granularity and hopefully
increasing the accuracy.
2-2) Anatomy: When we do normalization, computer algorithm do not care about
anatomy. It just does whatever it thinks best to register pixels. This is
the pixel-based analysis. On the other hand, manual ROI is usually based on
anatomical information we can perceive. This is anatomy-based analysis.
Tractography-based analysis can also be considered as a kind of registration
method. We do, for example, tractography of the cortico-spinal tract in 10
subjects. Then we can define a group of pixels that belong to the CST and
compare the pixel values. In this way, we define a specific area across
subjects based on anatomy.
3) In MriStudio, DiffeoMap does pixel-based registration just as VBA. You
can do VBA analysis. In addition, you can superimpose our parcellation map
and divide the brain into about 150 areas. In terms of granularity, it is
much less than VBA (more than 1 million pixels vs 150 areas). This is also a
conversion to pixel-based to anatomy-based analysis.
4) Now going back to your question, VBA and TBSS are looking at the same
data with different point of view. First of all, the granularity is
different; TBSS reduces the information to the white matter core. Also, VBA
is completely pixel-based but TBSS, which is not completely anatomy-based
but has some anatomy-based factors by reducing the information to the core
of the white matter.
5) In my opinion, all methods described above have advantages and
disadvantages. I don't think any one of them is better than the other.
Quantification based on location information is definitely one of the most
difficult problems we are all facing.
6) On the other hand, your reviewer is correct, in a sense that it is always
important to compare different results to enrich your data and
interpretation. This is especially true if there is a tool widely used like
TBSS. However, it is not like, one method should be treated as the gold
standard and other approaches should give a similar result. If you compare
VBA and TBSS, you likely to get different results because as explained above
they are operating at the different granularity, precision, and accuracy.
MRI image anlaysis is very often a screening and hypothesis generating tool
rather than a tool to draw a conclusion. We are simply looking at 6MB
information based on water signal.

So, in conclusion,

> No, VBA is not out of date
> Yes, it is a good idea to compare results from widely used tools, but any
normalization-based method should not be considered as a gold standard in my
opinion. We just have to understand how they operate and what are their
advantages and disadvantages.

On Thu, May 13, 2010 at 8:07 AM, lion gao <gaolion at gmail.com> wrote:

> Dear Experts,
>
>
>
> I have one part of my thesis on DTI data analysis. The method I used is the
> voxel-based analysis (VBA) and I tried MRIstudio as well. One of the
> examiners pointed out that track-based spatial statistics (TBSS), as a state
> of art way, should have been considered.
>
>
> I am familiar with TBSS, only know that it may reduce systemic
> mis-registration inVBA and increasing papers published with the method. I am
> not sure whether:
> 1. VBA has become “out of date”,
> 2. and TBSS has become a “golden standard” for DTI data analysis.
>
> Can someone help to justify the situation or defense a little bit? Thank
> you very much in advance!
>
>
> Best wishes,
>
> Gao
>
> _______________________________________________
> Mristudio-users mailing list
> Mristudio-users at mristudio.org
> http://lists.mristudio.org/mailman/listinfo/
> Unsubscribe, send a blank email to:
> Mristudio-users-unsubscribe at mristudio.org
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.mristudio.org/pipermail/mristudio-users/attachments/20100513/5f5a046b/attachment-0001.html