- Tractography
In
neuroscience , tractography is a procedure to demonstrate theneural tract s. It utilizes special techniques ofmagnetic resonance imaging (MRI), and computer-basedimage analysis . The results are presented in two- and three-dimensional images.In addition to the long tracts that connect the
brain to the rest of thebody , there is a complicated 3D formed by short connections among different cortical andsubcortical regions. The existence of these bundles has been revealed byhistochemistry and biological techniques onpost-mortem specimens.Brain tracts are not identifiable by direct exam, CT, orMRI scans. This difficulty explains the paucity of their description inneuroanatomy atlases and the poor understanding of their functions.The MRI sequences utilized look at the symmetry of brain water diffusion. Bundles of fiber tracts make the water diffuse asymmetrically in a
tensor , the major axis parallel to the direction of the fibers. The asymmetry here is calledanisotropy . There is a direct relationship between the number of fibers and the degree of anisotropy.MRI technique
Tractography is performed utilizing Diffusion Weighted Imaging, an MR technique which is sensitive to the
diffusion ofwater in the body, and can be used to reveal its 3D shape. Free diffusion occurs equally in all directions. This is termed "isotropic " diffusion. If the water diffuses in a medium with barriers, the diffusion will be uneven, which is termed "anisotropic " diffusion. In such a case, the relative mobility of themolecules from the origin has a shape different from asphere . This shape is often modeled as anellipsoid , and the technique is then calledDiffusion Tensor Imaging . Barriers can be many things--cell membranes, axons, myelin, etc; but in white matter the principal barrier is themyelin sheath ofaxons . Bundles of axons provide a barrier to perpendicular diffusion and a path for parallel diffusion along the orientation of the fibers.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 , andinflammation reduce anisotropy, as the barriers are affected by destruction or disorganization.Anisotropy is measured 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 information of how aspherical the diffusion is but says nothing of the direction.
Each anisotropy is linked to an 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". The software could encode 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 fromanterior to posterior.
*Blue indicates directions in the Z axis: foot-to-head direction or vice versaNotice that the technique is unable to discriminate the "positive" or "negative" direction in the same axis.
External links
* [http://www-sop.inria.fr/asclepios/software/MedINRIA/index.php MedINRIA - A free software (non-commercial use only) for tractography]
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