DTI stands for Diffusion Tensor Imaging. This is an imaging technique based on the 3D shape of water diffusion. Free diffusion occurs equally in all directions. This is termed "isotropic"
diffusion. If the water diffuses in a mileu having barriers, the diffusion will be uneven. In such a case, the relative-mobility of the molecules from the origin has a shape different from a sphere, most frequently an ellipsoid. Barriers can be many things --cell membranes, axons, myelin, etc; but in white matter the principal barrier is the myelin sheath of axons. Bundles of axons provide a barrier to perpendicular diffusion and a path for parallel diffusion along the orientation of the fibers. This is termed "anisotropic"
Anisotropic diffusion is expected to be increased in areas of high mature axonal order. Conditions where the myelin or the structure of the axon are disrupted, such as trauma, tumors, and inflamation reduce anisotropy, as the barriers are affected by destruction or disorganization.
Anisotropy is mesured in several ways. One way is by a ratio called "fractional anisotropy" (FA). An anisotropy of "0" corresponds to a perfect sphere, whereas 1 is an ideal linear diffusion. Well defined tracts have FA larger than 0.20. Few regions have FA larger than 0.90. The number gives us information of how assymmetric the diffusion is but says nothing of the direction.
Each anisotropy is linked to a orientation of the predominant axis (predominant direction of the diffusion). Post-processing programs are able to extract this directional information.
This additional information is difficult to represent on 2D grey-scaled images. To overcome this problem a color code is introduced . Basic colors can tell the observer how the fibers are oriented in a 3D-coordinate system: This is termed an "anisotropic map." Our software encodes the colors in this way:
- Red indicates directions in the X axis: right to left or left to right.
- Green indicates directions in the Y axis: posterior to anterior or from anterior to posterior.
- Blue indicates directions in the Z axis: foot-to-head direction or viceversa
Notice that the technique is unable to discriminate the "positive" or "negative" direction in the same axis.
"Fractional anisotropy image." White pixels (dots) correspond to high values of fractional anisotropy (FA). Dark pixels correspond to low values of FA. White matter consists of axons, which serve as barriers for free water diffusion, and explains the high anisotropy which is bright compared to cortex where there is less structure and more free diffuse resulting in lower sign.
Color coded "fractional anisotropy map." Pixels are now displayed in colors to reveal the predominant direction of the diffusion tensor. Green shows fibers oriented anterior-to-posterior or posterior-to-anterior. Red shows fibers oriented right-to-left or left-to-right (e.g.: corpus callosum). Blue shows fibers oriented head-to-foot or viceversa (e.g.: the pyramidal tracts in the corona radiata, lateral to the corpus callosusm).