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Diffusion Tensor Imaging

Practice Essentials

Diffusion-weighted imaging (DWI) is a well-established magnetic resonance imaging (MRI) method for diagnosing cerebral ischemia. DWI is a routine protocol in most institutions that perform neuroimaging; normal states and abnormal conditions are easily interpreted through the use of DWI in conjunction with the use of apparent diffusion coefficient (ADC) imaging by correlating findings of hyperintensity on DWI images with findings of hypointensity on ADC images (see the images below).
 Imaging and interpretation of water diffusion have improved with the development of diffusion tensor imaging (DTI). DTI allows direct in vivo examination of aspects of the tissue microstructure. DTI takes advantage of diffusion anisotropy to provide excellent details of the brain; for example, it enables mapping of the orientation of the white-matter tracts.

Normal brain appearance in axial DWI (left) and AD

Normal brain appearance in axial DWI (left) and ADC (right) images in a 35-year-old man.

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Axial DWI image demonstrates a typical wedge-shape

Axial DWI image demonstrates a typical wedge-shaped, cortical-based, hyperintense lesion in the left temporoparietal lobes consistent with acute infarct.

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DWI is also useful in the investigation of other brain disorders such as epilepsy, multiple sclerosis, brain abscesses, brain tumors, mild traumatic brain injury, and hypertensive encephalopathy (see the images below).

Axial fluid-attenuated inversion recovery (FLAIR)

Axial fluid-attenuated inversion recovery (FLAIR) image (top left), diffusion-weighted image (DWI) (top right), and axial and sagittal T1-weighted (T1W) images (bottom) in a 40-year-old man with a history of intravenous drug abuse and fever demonstrate multiple enhancing focal brain lesions in the gray-white matter junction (arrow) compatible with septic emboli. The lesions are hyperintense on both FLAIR and DWI images.

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Axial diffusion-weighted image (DWI) (top left), T

Axial diffusion-weighted image (DWI) (top left), T2-weighted (T2W) image (top right), fluid-attenuated inversion recovery (FLAIR) image (bottom left), and contrast-enhanced T1W (bottom right) image demonstrate a right convexity meningioma, which appears hypointense on DWI image. The perilesional brain edema (arrow) is hyperintense on T2W and DWI sequences.

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Diffusion, or brownian movement, denotes the random motion of molecules. All molecules exhibit such motion at temperatures greater than absolute zero. Diffusion is termed isotropic if the motion is equal in all directions. However, water diffuses asymmetrically in the white matter—that is, diffusion is restricted perpendicular to the long axis of the axons. By contrast, water diffuses faster along the Z axis (see the image below). This property is known as anisotropy; it may be used to define the direction of the axons in a particular voxel.

Diffusion ellipsoid. Three eigenvectors are demons

Diffusion ellipsoid. Three eigenvectors are demonstrated, with the principal eigenvector along the Z direction. Courtesy of Dr Andrei I. Holodny, MD.

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Diffusion in structured tissue, such as white matter, is anisotropic. DTI can be used to measure anisotropy per voxel and provides the directional information relevant for magnetic resonance tractography or fiber tracking in vivo. DTI allows for direct examination of the brain microstructure and has become a useful tool for investigation of brain disorders such as stroke, epilepsy, MS, brain tumors, and demyelinating and dysmyelinating disorders. However, further improvements in the technique and in postprocessing analysis are needed to increase the widespread utility of DTI in both research and clinical applications.

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