When the ADC values are calculated, they are done so in one direction at a time. This is not necessarily related to anisotropy. Higher ADC values mean it can go very far without interruption, and it takes the signal with it. More diffusion yields a darker pixel because you lose signal as the water molecule can go anywhere. On the right is a single direction of the diffusion - note that the brightness varies dramatically in the white matter depending on the alignment of the fibers. On the left is the image for the mean B0 image which is not sensitive to diffusion direction. This is not related to anisotropy.īelow are some raw images from our DTI scan collected with our 3-Tesla Siemens Trio. There are multiple volumes, but each at a different diffusion direction. You can see that each gradient direction is acquired for each slice. You can view the raw DICOM files by dragging and dropping them onto MRIcron (or MRIcron's icon in the dock, if you use OSX). The MD map shows that the ventricles have high diffisuity (looking like Figure A above) while the gray and white matter have lower diffisuity (looking like B above). the tensors look like Figure D above), while the gray matter is more isotropic (i.e. Note that the FA is extreme in regions of white matter (e.g. The figure below shows the fractional anistropy and mean diffisuity of a group of healthy individuals. The other is to use the data for tractography to illustrate the shape and direction of fiber tracts (comparing C versus D). One is to describe it as having normal or abnormal ADC values per pixel (e.g. Therefore, are two ways to describe and analyze diffusion data. For example, in figure C below the water is moving isotropically, while in D the water is moving anisotropically. the brain's white matter) the diffusion properties are anisotropic - water will move in the direction of the fibers much more rapidly than orthogonal to the fibers. On the other hand, in some areas such as along the fiber bundles of neurons (e.g. For example, in the figure below, water is diffusing faster in A than B. inside cells) water is constrained, and the mean diffusion (apparent diffusion coefficient, or ADC) is slow, while in regions such as the ventricles the motion of the cerebral spinal fluid is unconstrained and relatively fast. Consider the drawing A below, where the red line shows the typical diffusion of a water molecule, randomly moving around - the black ball shows this molecules diffusion speed and direction. However, in practice different sequences can give dramatically different results (especially in terms of regional variability) so you probably want to use existing databases primarily as a starting point for developing a set of templates based on your own population, scanner and protocol.įurthermore, analysis methods for DTI remain primitive. This means that one can compare results to different atlases from other centers, for example the Hokkaido University Fractional Anisotropy Database. Furthermore, in theory DTI values should be relatively calibrated - healthy adults should show similar values regardless of which scanner is used. DTI appears to be a tremendously important tool, as it is sensitive to acute brain injury (you can see strokes that can not be detected by conventional MRI) and it correlates with white matter integrity. Diffusion imaging measures the random motion of water (and other liquids with hydrogen). This page describes Diffusion Tensor Imaging was written by Leigh Morrow, Paul Morgan and Chris Rorden. Download and extract the sample dataset that includes the DTI data (in NIfTI format: DTI30s010: 30 direction, 2mm) - it is ready to analyze.FSL installed on your computer (included with Lin4Neuro).To complete this tutorial you will want the following: Diffusion Tensor Imaging Analysis using FSL
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