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Arachnoid Cyst (AC)

Last Updated: October 1, 2018

Figure 1: This left middle crania fossa extra-axial lesion follows CSF on all sequences and is in a very common location for arachnoid cyst. Note how the temporal lobe is displaced but is without edema (top left and right). No diffusion restriction is present that would suggest this lesion to be epidermoid cyst (bottom). Given the degree of mass effect, for most other lesions one would expect evidence of cerebral herniation, a finding not typically seen with arachnoid cysts.


Figure 2: This arachnoid cyst posterior to the right cerebellar hemisphere demonstrates T2 hyperintensity (top row left) and completely suppresses on FLAIR (top row right), matching the CSF in the 4thventricle on both of these sequences. It exhibits no DWI hyperintensity (bottom row), distinguishing this arachn...