[Mristudio-users] Optic radiations

[susumu]

Hi Christian

 

The optic radiation is a difficult tract to reconstruct. Maybe you can get a
good result in one subject but you may find a poor result in the other.
That's why we didn't include it in our protocol paper in Neuroimage 30, 630,
2007. The reasons are following;

 

1)       It has quite a sharp turn where it exits the LGN and merges into
the sagittal stratum (a huge sheet-like bundle running sagittally, which
includes the inferior fronto-occipital fasciculu, inferior longitudinal
fasciculus, projections from the splenium of the corpus callosum, and
posterior thalamic radiation). With 2-2.5 mm resolution, the turn is quite
sharp.

2)       The way it exits the thalamus is oblique with respect to the
coronal, sagittal, and axial slices and it is difficult to get the
cross-section within one plane in a very selective manner.

3)       As also pointed in #1, it merges to the sagittal stratum and
therefore it is difficult to extract it selectively.

 

To solve the #2 issue, you may need to define the LGN or the posterior
thalamus three-dimensionally as one of the ROIs.

 

Sorry if this is not really a solution. It is one of the limitations of the
DTI-based tractography. You are interested in connection between A - B and
between A and B there is a non-specific large bundle (A - large bundle - B).
In this kind of situation, depending on how the A and B branches come out
from the large bundle, you may get only a portion of the fiber you want. The
size of the reconstruction may also be influenced by status of unrelated
fibers in the large bundle. In general, there are several approached you may
take; 

 

1)       If there is an existing knowledge that the A-B connection is the
majority of the large bundle, you may just have to measure the size of the
large bundle using the ROI delineation or the tractography of the bundle
itself (not using ROIs in A and B, because there is no guarantee that it
actually increases the specificity in this situation). In your case, this
means measurement of the sagittal stratum or the posterior thalamic
radiation.

2)       Identify regions where the tract is more pure before merging to the
large bundle (e.g. A - large bundle or large bundle - B regions). For
example, if you are interested in the corticospinal tract, the lower pons
region is anatomically known to be more pure CST. For the optic radiation,
it is where the OR comes out from the LGN or exits to the CF, although I'm
not sure if you can see a nicely defined tract in such regions.

 

Susumu

 

  _____  

From: mristudio-users-bounces at mristudio.org
[mailto:mristudio-users-bounces at mristudio.org] On Behalf Of Christian Bieck
Sent: Tuesday, October 28, 2008 1:49 PM
To: mristudio-users at mristudio.org
Subject: [Mristudio-users] Optic radiations

 

Hello,

Since my last inquiry seemed to have escaped notice, I write you again 
in the hopes of finding a solution to my problem. 
As I wrote in my last email, I'm analyzing the FA values of the optic 
radiations and have, so far, extracted them by using 
anatomical landmarks and the DTIStudio functions AND, OR and NOT. The 
focus of my research concerns the posterior parts of the optic 
radiations ending in the primary visual cortex, but I find myself 
unable to extract them properly. Using the ROI-Operations in the Fiber 
Tab during Fiber Tracking oftentimes results in the removal of entire 
fiber tracts instead of just the anterior parts and any attempts to 
work with ROI Editor have not been successful so far. 
I have seen at least one paper in which the anterior/posterior parts 
and their FA values were examined via DTIStudio, so I know it's 
possible, but I'm completely puzzled as how to actually do it. 
I would appreciate it *very* much if you could point me in the right 
direction. 
Thank you for your time. 

Cheers,
Christian Bieck

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