الفهرس 
ANATOMY OF MAJOR BRAIN TRACTS.Only 14 pages are availabe for public view
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Abstract
Diffusion tensor imaging (DTI) is a magnetic resonance imaging technique that can be used to characterize the orientation properties of the diffusion process of water molecules. Usually, the information is contracted to two types of parameters: diffusion anisotropy, which represents the amount of directionality and orientation of the axis along which water molecules move preferentially.
Recent developments in post-processing algorithms for DTI allow study of the three-dimensional (3D) configuration of major white matter tracts ”fiber tractography” for which good agreement with post-mortem anatomic studies has been reported.
Various types of DTI visualization schemes have been postulated. Orientation-based color coding is a visualization approach in which the image brightness represents diffusion anisotropy, while a red-green-blue color scheme indicates tract orientation, assuming that the preferential diffusion axis coincides with the fiber orientation.
Because at present there are no other imaging modalities that can provide equivalent information, DTI is expected to become an important tool for the study of brain anatomy and the diagnosis of various white matter abnormalities.
The involvement of the white matter tracts can often be clearly identified in brain tumor patients by using anisotropic maps and so-called ‘‘diffusion tractography,’’. White matter involvement by a tumor can be categorized as being displaced, invaded, infiltrated, disrupted and edematous.
DTI can distinguish the edematous areas with intact fibers mostly found in metastases from the disrupted fibers mostly found in high-grade gliomas as the DROP in FA values of the area infiltrated by cell tumors is lower than in the peri-tumoral edema.
DTI tractography is gaining support as a pre-operative method of evaluating tumors closely related to eloquent area. The combination of DTI and fMRI might allow to map an entire functional circuit precisely. Even though fMRI locates eloquent cortical areas, determination of the course and integrity of the fiber tracts remains essential to the surgical planning.
DTI may provide an improved way to monitor intra-operative surgical procedures as well as their complications. Furthermore, evaluation of the response to treatment with chemotherapy and radiation therapy might also be possible.
FA of the infracted area is significantly lower than the contralateral side, and that in the white matter decreased more significantly. The involved white matter tract in infarction may be either compressed, shifted or interrupted. The involved severity of the tracts is positively correlated with the severity of muscle strength loss. This can be used to evaluate the prognosis of rehabilitative therapy.
DTI was found more sensitive than T2-weighted imaging to WD (Wallerian Degeneration). Diffusion anisotropy is reduced both in the primary lesion and in the areas of WD
DTI can show an altered cortico-cerebellar circuit in patients with chronic stroke involving more than one-third of the unilateral hemisphere with crossed cerebellar diaschisis (CCD), which is hardly demonstrated by conventional MR images.
Previous research demonstrated the potential utility of DTI in qualifying and quantifying neuropathology in TBI, in which diffuse axonal injury is common. In chronic moderate to severe TBI, reduced FA has been reported, even in the absence of observable lesions in standard structural MRI. Although the specifics are still not well understood, FA is believed to reflect the underlying changes in the white matter including the degree of myelination, axonal density and/or integrity.
DTI has been widely used in studies conducted with the purpose of better understanding the pathogenesis of MS, its natural course, and the nature and location of WM abnormalities, as well as the correlation between MS lesions and the clinical outcome measures.
In general, MS patients present an increased amount of water diffusion and a decreased anisotropy of diffusion direction in the region of the lesions, in the surrounding lesion tissue, and in the remote normal-appearing white matter (NAWM) and normal-appearing grey matter (NAGM). These changes have been found to be particularly high in patients presenting a more severe course of the disease, such as secondary progressive MS (SPMS), than in less severe courses, such as early-onset MS and relapsing remitting MS (RRMS) patients.
Recent studies applying DTI to patients with chronic epilepsy and hippocampal sclerosis have demonstrated increased diffusivity and reduced anisotropy in sclerotic hippocampi suggesting loss of structural organization and expansion of the extra-cellular space. DTI also revealed higher diffusivity and lower anisotropy in the zones of MCD (malformations of cortical development) and in surrounding brain that appeared normal on conventional MRI and may prove valuable for identifying epileptogenic foci as well as more optimally defining the extent of the lesion for surgical resection.
Anisotropy is lower in the genu and splenium in patients with presumed AD than in sex- and age-matched controls, probably due to axonal loss or demyelination in these areas. There is also presumed axonal degeneration in the cognitive tracts when compared with tracts associated with motor function.
DTI helps in viewing areas of statistically significant decreased anisotropy in patients with schizophrenia widely distributed in frontal and centrum semi-ovale white matter, the corpus callosum, and the anterior internal capsule.
Patients with obsessive compulsive disorders demonstrated significantly lower FA bilaterally in the anterior cingulate gyrus white matter, in the parietal region (supra-marginal gyri), right posterior cingulate gyrus, and left occipital lobe.
DTI has been found to be superior to conventional MRI in differentiating dys-myelinating disorders such as Pelizaeus-Merzbacher disease from de-myelinating disorders, such as Krabbe’s disease and Alexander’s disease. Diffusional anisotropy is present in dys-myelinated lesions but is lost in demyelinated lesions.