02.08.2020

things that restricted diffusion

This largely stems from the initial popularisation of DWI in stroke, which presented infarcted tissue as high signal on isotropic maps and described it merely as "restricted diffusion", implying that the rest of the brain did not demonstrate restricted diffusion, which is clearly not true. Hence they are not rephased and they lose some of their signal. After the 180-degree pulse, however, they are exposed to the exact same gradient (because they have not changed location) which undoes all the effects of the first (since they have flipped 180-degrees). For example, water within The way in which diffusion information is extracted from the tissue is to first obtain a T2* weighted image with no diffusion attenuation. The restricted diffusion in abscesses is thought to be due to the presence of viscous fluid containing cellular debris, bacteria, inflammatory cells, and mucoid proteins. "To make matters worse, many are not aware of the concept of A much safer and more accurate way of referring to diffusion restriction is to remember that we are referring to actual As opposed to essentially free diffusion of water kept inside a container, diffusion of water inside brain tissue, for example, is hindered primarily by cell membrane boundaries. A great deal of confusion exists in the way the clinicians and radiologists refer to diffusion restriction, with both groups often appearing to not actually understand what they are referring to.The first problem is that the term "diffusion-weighted imaging" is used to denote a number of different things: Additionally, confusion also exists in how to refer to abnormal restricted diffusion. The principal mechanisms are thought to be:The vast majority of neoplasms do not restrict diffusion or change it only mildly. The more easily water can diffuse (i.e. The differential diagnosis includes acute ischemic stroke, abscess, empyema, epidermoid cyst, lymphoma, intracerebral hematoma, encephalomyelitis, diffuse axonal injury, meningioma, and multiple sclerosis. And from the Stokes-Einstein equations diffusivity is inversely proportional to viscosity. An incomplete listing by category of disease is shown below, with the most common examples highlighted in red:The mechanisms responsible for restricted diffusion are incompletely understood and depend on the particular disease being considered. For example, in acute cerebral infarction it is believed that the decrease in ADC values is the result of a combination of water moving into the intracellular compartment (where its diffusion is more impeded by organelles than it is in the extracellular space) and the resulting cellular swelling narrowing the extracellular space The further an individual water molecule diffuses during the sequence the more it will be exposed to varying gradient strength and the more it will be dephased reducing the amount of signal returned. As a result, the diffusion of intracellular water molecules is more restricted than that of extracellular water molecules. The restricted diffusion in abscesses is thought to be due to the presence of viscous fluid containing cellular debris, bacteria, inflammatory cells, and mucoid proteins. Animal models have shown an inverse correlation between cellular density and ADC. The factors affecting rate of diffusion are: concentration, temperature, mass of the particle and properties of the solvent in which diffusion occurs. This occurs at a much smaller scale than a single voxel. Intramyelinic edema with separation of myelin sheath layersReversible DWI+ lesion of the splenium due to intramyelinic edema post seizureA wide range of toxic, metabolic, and demyelinating processes may result in restricted diffusion within the white matter. Many different entities can cause increased signal on diffusion weighted imaging. Diverse disorders such as hyperglycemia, osmotic demyelination syndrome, methotrexate encephalopathy, heroin toxicity, and phenylketonuria all share a final common pathway of brain injury, mediated by the release of excitotoxic amino acid neurotransmitters, especially glutamate. The ADC map, in contrast, is related to the natural logarithm (ln) of the isotropic DWI divided by the initial T2* signal (b=0). A few interesting tumors, including lymphomas and some highly malignant gliomas, may show significant restriction of diffusion and thus appear remarkably bright on DW images. The further they are able to move the less successfully they will be rephased and the less signal will remain. The aforementioned process generates four sets of images: a T2* b=0 image and three diffusion-weighted images (one for each X, Y and Z direction) with the T2* signal attenuated according to how easily water can diffuse in that direction.

The high signal seen on DW images in these conditions may be fully reversible and does not necessarily imply cell death (as it does in infarction). Animal models have shown an inverse correlation between cellular density and ADC. isotropic map, b=0, ADC) Faster movement equals faster diffusion.

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