Chemistry Reference
In-Depth Information
Figure 15.
Reproduced from Bammer et al. [92]. Figure 15a is a high-resolution DTI image.
Figure 15b is a color-coded fractional anisotropy map. The anterior-posterior direction is represented
by red, left-right by green and cephalo-caudal by blue. (See insert for color representation of the figure).
Diffusion tensor imaging has been successfully applied to quantitatively evalu-
ate the integrity of highly ordered tissue
in vivo
in both normal and diseased states
[94]. One method by which the diffusion measurements obtained in DTI can be
summarized is through the calculation of diffusivity and anisotropy. The ADC rep-
resents overall diffusivity and can be derived from the trace of the diffusion tensor.
Anisotropy is represented by FA, which is a measure of the degree of diffusion di-
rectionality in the tissue microstructure (Fig. 15). Water ADC and FA in the white
matter of the brain are found to change significantly from childhood to adulthood
[95, 96]. Fiber tractography is another application of DTI. It involves the assign-
ment of associations between neighboring voxels based on eigenvalue and eigen-
vector information. In simple terms, it is assumed that the eigenvector associated
with the largest eigenvalue is aligned with the direction of the WM fiber bundle [97].
3. Functional Imaging
Contrast based on blood oxygenation level, volume and flow changes have been
used to indirectly detect regional brain activation. Functional MRI (fMRI) provides
maps of changes in cerebral blood flow that are interpreted in terms of regions being
activated by sensory, motor, or cognitive tasks [98]. The acquired signals contain
hemodynamic modulations that mean their spatial and temporal characteristics
are biased by complex geometry and varying delivery rates of the vasculature. The
resulting temporal delays create spatial dispersions of the detected brain activity
using MRI [99]
