الفهرس 
Structural and Functional Anatomy of the CerebrumOnly 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.
One of the most important applications of DTI and fiber tarctography is demonstration of the relationship between white matter structure and function. For example, how corticospinal tractography would become one of clinical tools to indicate the PMA location using appropriate seed areas and FA values.
It is also used to study the course and the evaluation of the somatotopic organization of corticospinal tracts (CST) in the posterior limb of the internal capsule (PLIC) and cerebral peduncle (CP) by using regions of interest (ROI) at expected areas of the pons and expected areas of the lateral half of the PLIC.
The involvement of the white matter tracts especially the corticospinal tract which is the most important motor tract can often be clearly identified in brain tumor patients by using anisotropic maps and so-called ‘‘diffusion tractography’’. Corticospinal tract involvement by a tumor can be categorized as being displaced, invaded, infiltrated, disrupted and edematous.
DTI and fiber tractography 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 and corticospinal 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 to avoid intraoperative injury.
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.
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-FT demonstrates abnormal CNS development through change in FA; for example: decreased anisotropy of white matter adjacent to the malformed cortex and an aberrant course of major fiber pathways in cortical dysplasia. Increased anisotropy of dysplastic gray matter in heterotopia.
Through tractography, it is possible to quantify the degree of damage to the corticospinal tract and to visualize the corticospinal tract in 3 dimensions. Therefore, it is possible to predict motor impairment and recovery in patients with (intracerebral haemorrhage) ICH using this technique.
FA values and fibre bundles were significantly reduced in the CST but not the non-motor tracts in patients with amyotrophic lateral sclerosis (ALS). DTI can visualise axonal degeneration and may provide useful and quantitative supplementary information on upper motor neuron (UMN) disease in the study of ALS patients.
Conclusion
Development of DTI and fiber tractography allows direct examination in vivo of some aspects of brain microstructure. It has been shown to be of value in studies of brain anatomy and fiber connectivity. It has become interesting for investigations of different brain pathology such as brain tumors, cerebral ischemia, trauma, developmental CNS anomalies, intracerebral haemorrhage and amyotrophic lateral sclerosis. However further improvement in the technique is needed to increase the utility of DTI in both research and clinical application.
Finally corticospinal tractography is a rapid and non invasive method of brain mapping using routine MRI scanners making this technique likely to become one of the major methods for identifying the PMA, especially in patients who have several cortical dysfunctions.