As said in the last section, tensor estimation is the first step before the computation of several scalar
maps you can visualize by clicking on one of the following buttons :
Fractional Anisotropy (FA) is a commonly used parameter to caraterize anisotropy in the
brain. Bright pixels represent areas where the anisotropy is high, and thus the areas where
brain white matter is concentrated. FA is calculated from the eigen values of the tensors
(
) as followed :
This button displays the FA map with a color code (FAc). FA pixel is colorized in relation
with the principal direction of diffusion (i.e. principal eigenvector of the tensor).
Consequently, the color represents an indication of the direction of the fiber bundles. A
red pixel indicates that in this location white matter is mainly orientated
Left-Right. Anterior-Posterior orientation is indicated by a green pixel and
Inferior-Superior orientation by a blue one.
There are other scalars that characterize anisotropy you can display. You can use the
Geodesic Anisotropy which is a home made version of the FA, it is also called log-FA
(lFA). You can also display the Apparent Diffusion Coefficient (ADC), it represents the
trace of tensors and can be written as followed :
NB: Even if you didn't click on the tensor estimation button, you can directly ``ask'' for the display of the scalar maps, MedINRIA will automatically see that tensors haven't been estimated yet and will perform the estimation. When saving the DTI-study, scalar maps are not saved but their computation is not time consuming.