Diffusion Tensor Imaging and Tractography

When six or more gradients are used (gradient orientation is not only along the three main axes, but also in different, diagonal, directions: zx, zy, xy,...), DTI and TI are obtained.

Tensor imaging follows the multidirectional diffusion of water in tissues. To follow anisotropic diffusion, the DTI can be displayed as an ellipsoid whose three main axes constitute a system of orthogonal coordinates. The longest main axis of the diffusion ellipsoid is the value (eigenvalue) and direction (eigenvector) of peak diffusion (the longer the diffusion, the greater the anisotropy), whereas the shortest axis is the value and direction of minimum diffusion. If the three values are equal, the diffusion is called isotropic and the diffusion tensor will be sphere-shaped.

As discussed in detail below, DTI is used to depict structured water movements within tissues, especially in bundles of white matter fibres, whose orientation is of particular interest in stroke and tumours. This information can be processed to produce colour maps of white matter fibre bundles.

Tractography uses the main direction of diffusion measured by the tensor to calculate the pathways of white matter fibres. In TI, signal intensity represents the value of anisotropy (the greater the anisotropy, the

Fig. 7.3. ADC map acquired at 3.0 T. Low-grade astrocytoma: a SE T1 after contrast administration, b FLAIR image, c ADC map; compared with glioblastoma: d SET1 after contrast administration, e FSET2 image, f ADC map. The ADC is higher in the tumour compared with the surrounding parenchyma, in the oedema compared with the tumour, in the cystic areas compared to solid tumour components and in the low-grade compared to the high-grade solid tumour areas

Fig. 7.3. ADC map acquired at 3.0 T. Low-grade astrocytoma: a SE T1 after contrast administration, b FLAIR image, c ADC map; compared with glioblastoma: d SET1 after contrast administration, e FSET2 image, f ADC map. The ADC is higher in the tumour compared with the surrounding parenchyma, in the oedema compared with the tumour, in the cystic areas compared to solid tumour components and in the low-grade compared to the high-grade solid tumour areas whiter the image), while colour codifies the prevalent direction of the diffusion. Tracing is obtained first by defining the positions of interest in the white matter pathways and then, following the interpolated directions of peak diffusion, by drawing a continuous path through the initial position. These pathways, depicted by dedicated software in three dimensions, show the anatomy of white matter fibres.

The possibility of determining the anisotropy of each diffusion ellipsoid and then undertaking a more elaborate analysis of the relations between the diffusion ellipsoids enables non-invasive in vivo investigation of the connections between axonal fibres and the functions of different brain regions. In addition, fibre tracking with DTI could be used instead of, or in combination with, functional MRI (fMRI) [24]. For example, combined study with fMRI provides more consistent approaches for analysing brain function and anatomy [25].

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