[Mristudio-users] CC in elderly patients

Thu Mar 11 15:01:50 EST 2010

Hi all,

We have noticed the same phenomena and also got several inquiries from
outside. Here is my understanding;

1) For young subjects, the shape of the CC has a clean U shape in coronal
sections. If you do fiber tracking, you usually can reconstruct the U-shape
trajectory, which makes some anatomical sense.
2) For the population with enlarged ventricles, such as elderly, AD, and
stroke patients, the trajectory of the CC becomes more like a "W" shape with
a sharp turn where the CC merges to the corona radiata. I believe that this
is the primary reason that most streamlines end up in going downward toward
the internal capsule, which doesn't make much anatomical sense.
3) Think about the 90 degree rotated "T" shape and the vertical line is the
corona radiata and the horizontal line is the CC. If the horizontal line
merges at the right angle, the probability of the tracking to go upward and
downward is 50/50. If the horizontal line is slightly tiled upward, the CC
tracking merges to the vertical line (corona radiata) and continues to
travel upward. It's as simple as this. Once we get a vector (or tensor)
field and ask a computer to reconstruct a path, the computer doesn't know
the anatomy. All it knows is just a bunch of vectors. One may think that
probabilistic tracking can deal with it but the connectivity strength could
be influenced by the merging angle too.
4) This exemplifies what tractography is. It is just a region growing (or
pixel-connection) tool based on pixel-by-pixel information, no matter which
algorithm we use.  It is just a feature extraction tool from MRI that
observes water molecules. Some results look right and some results do not.
5) Then many people ask, "how do we know the validity". Here we (both who
ask the question and who is asked the question) have to think very hard. To
show the validity, we need to compare our observation (in this case
tractography result) and gold standard (real anatomy). Then what do we
really mean by "real anatomy" or "real connectivity"? What is connectivity
anyway? First of all many of the macroscopic anatomical definition such as
"superior longitudinal fasciculs" and "corona radiata" are defined by human
eyes based on anatomical features, which doesn't have a clear definition and
boundary. It's like a cloud. Everybody can see it at the core but at the
boundary, we don't know how to define. Then how about the microscopic
anatomical definition? Yes, we have a very clear definition, which is axonal
connection. However, we know that one neuron communicate with multiple
neurons through a *branching* axon. We have 100 billion neurons. Even if one
neuron communicate with one neuron, this is a 100 billion x 100 billion
question. For MRI, even if you do 1,000-orientation Q-ball imaging, we are
talking about merely few GB of data. With a tensor fitting, what we have is
20 MB. On top of it, we are measuring water properties. So, we can
understand that we are on a very shaky ground when we talk about
"validation" because we can't even define what is the gold standard we are
shooting for and our data is so small compared to what we are interested in.
6) Then next question is, "why do we do tractography". Here, we should go
back and think why we do MRI first of all and what is the real power of it.
The real power is that we can examine the entire brain within 30 min
non-invasively and quantitatively. This is something no other modality,
certainly histology, can't do. By performing a feature extraction such as
tractography, we can quickly and quantitatively assess the anatomical
status.
7) As long as we apply the same data acquisition method and the same
analysis method, the results itself is always correct. There is nothing
wrong with the results whatever they are. In this example, in some
population, the CC goes downward while it goes upward in the normal
subjects. The tractography sensitively detected some differences between the
two population.
8) If we make mistake in this situation, that is when we interpret the data.
For example, if we interpret the data like, "in young brains, the CC goes up
and in aged brain the CC goes down because the connectivity changed", now
such a statement becomes the subject of "right or wrong".
9) So, it is very important to evaluate why you got different results
between two groups. The real reason could be something not the measurement
was intended. For the case of the CC trajectory, the real reason could be
"enlarged ventricles" and the DTI + tractography could be just an expensive
and indirect way to show it.
10) This is a very typical issue in MRI, not just DTI. When we observe
cortical thinning in T1, it may be just a change in contrast due to
myelination status or other things. What we really observe is the thinning
of T1-indicated-cortical-compartment and the direct translation into the
real cortical thickness is sometimes dangerous or even erroneous. As long as
we are dealing with 10 MB of water information, biological or cellular
interpretation always requires caution.
11) Fiber reconstruction is a high-level image analysis tool, which is
sensitive to so many anatomical factors including both micro and macroscopic
factors. The microscopic factors are myelination, axonal loss, cellular
loss, edema, etc that would affect the FA and anatomy. The microscopic
factors are multiple compartments (e.g. crossing fibers) within the 2 mm
pixel. So, in general, the interpretation would be difficult. It is a good
tool to survey the overall white matter anatomy but you need to exercise the
caution when you interpret it.

I guess what I mentioned above contains a lot of my personal opinions and
please take it as just one of the opinions, but hope it helps to understand
the issue of the downard CC projection.

Susumu

On Thu, Mar 11, 2010 at 11:52 AM, Jun Yi Wang <junyiwang2002 at yahoo.com>wrote:

> Hi Dafna,
>
> I only have a few subjects in 80s, but I have subjects from 16-65 years
> old.  I do see a substantial decrease of the amount of corpus callosum
> fibers projecting towards the cortex as we age.  Some fibers still project
> to the cortex although a lot of them project downwards instead.  I don't
> know whether this is caused by aging or DTI artifacts such as the
> termination of line propagation because of the lower FA or the crossing
> fiber issue.
>
> Jun Yi Wang
> Postdoctoral Research Scholar
> Fragile X Research and Treatment Center
> UC Davis M.I.N.D. Institute
> 530.747.3808 (Lab)
>
> ------------------------------
> *From:* Dafna Ben Bashat <dafnab at tasmc.health.gov.il>
> *To:* mristudio-users at mristudio.org
> *Sent:* Thu, March 11, 2010 3:19:20 AM
> *Subject:* [Mristudio-users] CC in elderly patients
>
>  Hi all,
>
> We are currently working on data from elderly stroke/TIA patients.
>
> In many of the TIA patients, who have good DTI data, for some reason the CC
> does not built well, we can't see the fan.
>
> This is not due to incorrect directions (the CST is built OK).
>
> ROI measurements give reasonable values. We even changed the flip-angel for
> the FACT, and got the same results.
>
> Any suggestions? Is it a known phenomenon in elderly / stroke / TIA
> patients?
>
>
>
> Thank you in advance.
>
>
>
> Dafna
>
>
>
>
>
> Dafna Ben Bashat Ph.D.
>
>
>
> Senior Physicist, In charge of MR systems
>
> The Wohl institute for Advanced Imaging
>
> Brain Imaging Center
>
> Tel Aviv Sourasky Medical Center
>
>
>
> Phone: 972-3-6973953 (o)          Fax: 972-3-6973080
>
>
>
>        972-52-4262515 (m)        Mail: dafnab at tasmc.health.gov.il
>
>
>
>
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