53The Brownian motion of water molecules within the brain has, for many years, beenimaged with a specialized MR technique termed diffusion-weighted imaging (DWI)(see Case 29). This technique often incorporates information about diffusion in theslice, read, and phase direction, which limits the amount of magnitude and direction-al information that can be extracted. Diffusion tensor imaging (DTI) builds on the DWItechnique to provide greater details of the directional component of the diffusion-based motion resulting in a diffusion analysis from which information on the micro-scopic structure of white-matter tracts can be inferred.

Once again, 3 T MR imaging systems prove superior to 1.5 T systems by providinghigher SNR and improving the clinician’s ability to quantify diffusion motion and direc-tion. This SNR improvement is demonstrated in a normal volunteer in Figs. 53–1Aand 53–1B where data from a single diffusion slice with a b-value of 1000 s·mm2 isillustrated at 1.5 T and 3 T, respectively.

Calculating the tensor of all 12 diffusion directions in the above experiment makesit possible to process a variety of images related to the magnitude and direction ofmolecular diffusion. Figures 53–2A and 53–2B demonstrate the magnitude data of thediffusion tensor from the above experiment due to anisotropy. This image, termed afractional anisotropy (FA) image, displays the magnitude of diffusion on a voxel byvoxel basis. Although similar, the 3 T (B) image provides a better delineation of structur-al borders, for example the external capsule (arrow, B), due to increased SNR.

Figure 53–1

Brain: Diffusion Tensor ImagingStuart H. Schmeets

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Within white matter tracts, diffusion is impeded perpendicular to the long axis ofthe fibers. Because DTI provides directional information within each voxel, the struc-ture of white-matter tracts can also be calculated and reconstructed from the diffusiondata. The images in Figs. 53–3A and 53–3B are reconstructed from the data sets illus-trated in Fig. 53–2. However, in these images the white-matter tracts have been cal-culated starting at the level of the medulla and reconstructing in the caudal to cranialdirection. Note once again that the improvement in SNR plays a role in providing addi-tional information at 3 T, regarding existence of crossing tracts (arrow, B), that canonly be visualized in a limited way at 1.5 T (A).

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Figure 53–2

Figure 53–3

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