Nerve Pathways with MR Tractography8

A. CHeRUBiNi, G. LucciOHeNti, F. Fasano, G. E. HAGBeRG, P. PeRAN, F. Di SALLe, F. Esposito, T. SCARABiNO, U. SABAtiNi

The neuroradiological interpretation of magnetic resonance (MR) images relies on a complex semeiotics that is based on the morphological and signal characteristics of normal and pathological brain and on the detailed knowledge of the ultrastructural and functional organization of the central nervous system (CNS). The study of the brain's cortical organization is facilitated by the presence on its surface of fissures that divide it into lobes and sulci that circumscribe in each lobe a number of convolutions or gyri. Identification of the encephalic nuclei, grey matter formations lying deep in the hemispheres, is also facilitated by their characteristic morphology, their symmetric position with respect to the midline, and the presence of specialized white structures such as the internal, external and extreme capsule that mark their borders.

Detailed white matter evaluation is more challenging than the study of cortical organization, because it does not exhibit anatomical landmarks excepting the contiguous cortical gyri, the ventricular systems and the base nuclei; however, it does contain fibres with distinct anatomical courses and functional significance that include projection and association systems. Identification of nerve fibre bundles is essential in neuro-physiology and to study CNS diseases, and knowledge of their spatial relationships with lesions requiring surgical treatment is crucial to preserve functional pathways and the activities they subserve.

Standard MR techniques provide an accurate representation of the brain's macroscopic anatomy, but not detailed information on white matter anatomy. By contrast, study of the anisotropic diffusion of water molecules (diffusion tensor imaging; DTI) [1] and tractography (fibre tracking) provide data on its microscopic organization and make it possible to reconstruct axonal tracts using diffusion-weighted MR images [2-6]. Since tractography is currently the sole method affording non-invasive study of the 3D architecture of axons in vivo, it has potential applications to several fields of neurophysiology, neurology and neurophysiology to visualize and quantify physiological mechanisms and pathological processes.

Herein we illustrate the main tractographic techniques and discuss their enormous potential and current limitations.

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