Fiber Tracking & DTI Flashcards
Structural connectivity in the brain is simply
physical white matter fiber connections between remote brain areas.
If two brain areas need to exchange information, we assume that structural connectivity is a prerequisite.
functional connectivity is simply
statistical dependencies in neural activation time series of remote brain areas.
What is measured with diffusion tensor imaging(DTI)?
How water travels along the white matter tracts in the brain.
Why can water diffusion be measured with DTI?
Water molecules diffuse around the brain all the time, and in some parts they diffuse randomly, in other parts they diffuse with a specific directionality. This is because myelin (which is insulating the axons) is fat, and fat is hydrophobic (avoids water). Therefore, a water molecule will never go towards a myelin molecule, which causes the water to diffuse parallel to the axons.
completely unrestricted diffusion = isotropic
restricted diffusion = anisotropic
Isotropic vs anisotropic diffusion
What is it? where in the brain do we see most of each?
Isotropic diffusion is completely random and seen for example in the ventricles where there are no axons.
Anisotropic diffusion is directed and seen very prominently at the corpus callosum where a lot of axons pass.
What is a tensor?
What is it good for?
A matrix of numbers. In DTI, the tensor is fitted to the activity in a voxel in order to represent the direction of diffusion.
From the fitted tensor, three values of direction can be calculated.
What does the bottom image represent?
For each voxel, a tensor has been fitted which represents the diffusion of water molecules. The brain is then color coded in terms of directionality and intensity of the tensor.
Fiber Assignment by Continuous Tracking (FACT)
What is it?
One limitation?
Algorithm used to find fibers in the brain based on e.g. DTI. This is done by following tensor direction from one voxel to the next by aligning the “principle diffusion gradients”(directions) of the tensors.
A downside of this simple approach is that if there are two fiber bundles crossing eachother in the xovel, the tensor will be isotropic.
What does the picture show?
How a connectome is made from fiber tracts (B) and a structural scan (A) .
In this example, the brain is divided into specific brain regions using a brain atlas(C), and the fiber tracts and structural scan is combined (D). Thereby, we can count the connection between different regions of interest (E). We can also create a matrix showing how each cortical area is interconnected (F).
How many percent of possible connections in the brain are found in all participants of the connectome project?
7%!
20% of possible connections are never found, and the rest is distributed as the curve shows.
Different ways to weight the connections between regions of interest.
Number of streamlines
Streamline volume density
Fiber distance
Fractional anisotropy
Number of streamlines: we weight the connection by counting how many streamlines there are between two ROIs
Streamline volume density: Same as number of streamlines but adjusted to the size of the ROIs, as larger brain areas usually have more connections.
Fiber distance: When we have the fiber tracts, we can measure the distance between connected ROIs, which can also be used for weighting
Fractional anisotropy: By averagine together direction values from the tensors that make up the fiber tracts, it is possible to weight the connection by how anisotropic the connection is.
0=isotropic
1=anisotropic
What is this?
The rich club!
A brain hub which has been found using DTI and structural connectivity. Found in almost all species and is one of the first brain networks that form in infants, and the last one we loose when we grow old.
From the paper: “Growing evidence supports the critical roles of a set of brain regions within the brain network, known as the brain’s cores or hubs. These regions require high energy cost but possess highly efficient neural information transfer in the brain’s network and are termed the rich-club. The rich-club of the brain network is essential as it directly regulates functional integration across multiple segregated regions and helps to optimize cognitive processes.”
What is the main function of the rich club?
Communication between brain regions and executive control.
When connections in the rich club are taken away, the communicability in the brain decreases much more than for other connections.
When correlating structural and functional connectivity on a voxel-wise basis, which brain network stands out the most?
The default mode network.
Can brain regions have functional connectivity without structural connectivity?
Yes! two brain regions do not need direct fiber tracts to be functionally correlated, this can happen through adjacent brain regions and brain networks.
