On T1-weighted images epidermoids are generally slightly hyperintense or isointense relative to gray matter. The lesions are usually isointense relative to CSF on T2-weighted images, but they may be slightly hyperintense. Now, diffusion-weighted imaging can be used to differentiate these entities, because epidermoids have markedly restricted diffusion and, therefore, high signal intensity on the diffusion-weighted trace images. The free water in arachnoid cysts has low signal intensity.
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