Imaging con tensore di diffusione: differenze tra le versioni

Versione delle 22:26, 25 ott 2010

Versione delle 22:27, 25 ott 2010

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Le principali applicazioni sono quelle nella visualizzazione della [[sostanza bianca]], per determinare la localizzazione, orientamento, e [[anisotropia]] dei tratti e fasci di fibre nervose. L'architettura dei gruppi di [[Assone|assoni]] in fasci paralleli, e le loro guaine di [[mielina]], facilitano la [[diffusione]] delle molecole d'acqua preferenzialmente lungo la loro principale direttrice. Questa diffusione orientata si denomina ''diffuzione anisotropica'' (dall'inglese: ''anisotropic diffusion'').

[[Image:DTI-sagittal-fibers.jpg|thumb|240px250px|TractographicRcostruzione reconstructionTrattografica ofdelle neuralconnessioni connectionsneurali via DTI]]

TheLa visualizzazione (imaging) di questa of this property is an extension of diffusion MRI. If a series of diffusion gradients (i.e. [[magnetic field]] variations in the MRI magnet) are applied that can determine at least 3 directional vectors (use of 6 different gradients is the minimum and additional gradients improve the accuracy for "off-diagonal" information), it is possible to calculate, for each [[voxel]], a [[tensor]] (i.e. a symmetric positive definite 3×3 [[matrix (mathematics)|matrix]]) that describes the 3-dimensional shape of diffusion. The fiber direction is indicated by the tensor's main [[eigenvector]]. This vector can be color-coded, yielding a cartography of the tracts' position and direction (red for left-right, blue for superior-inferior, and green for anterior-posterior). The brightness is weighted by the fractional anisotropy which is a scalar measure of the degree of anisotropy in a given voxel. Mean diffusivity (MD) or trace is a scalar measure of the total diffusion within a voxel. These measures are commonly used clinically to localize white matter lesions that do not show up on other forms of clinical MRI.

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Diffusion tensor imaging data can be used to perform [[tractography]] within white matter. Fiber tracking algorithms can be used to track a fiber along its whole length (e.g. the [[corticospinal tract]], through which the motor information transit from the [[motor cortex]] to the [[spinal cord]] and the peripheral [[nerve]]s). Tractography is a useful tool for measuring deficits in white matter, such as in aging. Its estimation of fiber orientation and strength is increasingly accurate, and it has widespread potential implications in the fields of cognitive neuroscience and neurobiology.