![]() fractional anisotropy – FA, mean diffusivity – MD, Eigen values and Eigen vectors, etc.) for each subject and quality controlling along the way to remove scans with abnormalities and artifacts. Measurement of an ellipsoid tensor further permits a retrospective analysis, to gather information about the process of diffusion in each voxel of the tissue and finally it enables mapping of the orientation of the white-matter tracts.īy pre-processing, we are referring to converting your images from the raw DICOM to a standarized format (NIfTI) and then compute diffusion metric images (i.e. The ellipsoid formalism functions also as a mathematical method of organizing tensor data. Diffusion is fast along the length of (parallel to) an axon, and slower perpendicularly across it. This property is known as anisotropy it may be used to define the direction of the axons in a particular voxel. By contrast, water diffuses faster along the Z axis (imagine an ellipsoid). ![]() However, water diffuses asymmetrically in the white matter-that is, diffusion is restricted perpendicular to the long axis of the axons. This scalar defines the amount of diffusion weighting in the experimen, the higher the b-value the greater the diffusion weighting.ĭiffusion is termed isotropic if the motion is equal in all directions (like in a sphere). The amplitude and spacing, called the intensity and duration of the diffusion gradients are characterized or summarized by a b-value. So, two pulses are used, first a 180º pulse occurs, followed by a second matching magnetic pulse (called the diffusion gradient) used to rephrase the nuclear spin, preserving the signal of the echo. To sensitize MRI images to diffusion, instead of a homogeneous magnetic field, the homogeneity is varied linearly by a pulsed field gradient. This causes many of the protons in water molecules to precess simultaneously, producing signals in MRI. In a typical MR image, water molecules in a sample are excited with the imposition of a strong magnetic field. These barriers are cell membranes, myelin sheaths, white matter fiber tracts, and protein molecules. In the human brain, water diffusion is a three-dimensional process that is not truly random because the diffusional motion of water is impeded by natural barriers. All molecules exhibit this motion at temperatures greater than absolute zero. The physics of the DWI are based on measuring the Brownian movement of water molecules within a point of tissue, which is the “ramdom walk” or motion that each molecule experiences. large axonal connections between brain regions, and can therefore reveal microscopic details about tissue architecture, either normal or in a diseased state. Because water flow is constrained by the axons (white matter) in the brain, it can be used to , it can be used to visualize and quantify the neural pathways, i.e. It involves using a special scanning and reconstruction sequence to image the flow (or, more specifically, constraints in the flow) of water through the brain. Diffusion Weighting Imaging, or DWI, also referred to as diffusion tensor imaging or DTI, is a MRI method that maps water molecule diffusion patterns and can be done on most research MRI scanners.
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