The greater the anisotropy the more directional and linear the diffusion of water molecules. Water molecules will diffuse differently through space depending on the tissue type, components, structure, architecture, and integrity these principles allow clinically significant imaging to occur, particularly the DTI. Anisotropy is when there is a directionality in the diffusion of water present, and the movement of water is no longer random (such as water movement along straws placed in a glass). Isotropy is defined as uniformity in all directions and when applied to water molecules, isotropy occurs when the diffusion of water is entirely uninhibited (such as water movement in a glass of water). Random thermal motion, also known as Brownian motion, is water molecular diffusion in three-dimensional space. DTI does not require contrast and is available on almost all modern MR scanners with relatively quick scan times for this sequence. The first application of DWI to the human brain was performed in 1986 and since has become the gold standard for detecting acute stroke. DTI is a variant of diffusion-weighted imaging (DWI) which utilizes a tissue water diffusion rate for image production. Specifically, diffusion tensor imaging (DTI) has become increasingly studied and utilized in recent years and has become incorporated by many radiologists into routine clinical practice with most research performed on traumatic brain injury. Advanced magnetic resonance (MR) neuroimaging modalities are becoming more available and useful as their value in the diagnosis and prognosis of central nervous system diseases is more fully understood and studied.
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