Which Diffusion MR Measure is Best for Stroke Imaging

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There exist a large number of variations of images that can be derived from the DWI or DTI examination. These are loosely described as: ''isotropic'' (the combined or averaged diffusion-weighted images acquired along the x-, y-, and z-gradient axes; the ADC maps acquired from the low and high b-value images; the ''trace'' images or ADC maps similar to the averaged x-, y-, and z-axis diffusion-weighted images; exponential ADC ("eADC'' or ADC ''Expo'') maps in which the large ADC values (primarily seen in cerebrospinal fluid) are suppressed; and the DTI-derived anisotropic maps of relative anisotropy ''RA'', fractional anisotropy ''FA''. Recent studies, seeking to sort the relative merits of these measures to visualize the presence and extent of acute clinical stroke, found that the average absolute percentage changes for the isotropic strategies were all above 38% (Harris et al., 2004).

The ADC maps had the most significant difference (—42.4%). The DTI-derived anisotropic images had no significant differences in acute stroke. The authors concluded that anisotropic maps do not consistently show changes during the first 6 h of ischemic stroke, and that averaged or isotropic diffusion-weighted or ADC images using DWI were more appropriate for detecting hyperacute stroke. It was noted, however, that anisotropic images may be useful to differentiate hyperacute stroke from acute and subacute stroke.

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