Advanced fronto-parietal dominant brain atrophy and chronic small vessel ischemic changes. Note lateral ventricle asymmetry. Thin, crescentic bilateral frontal subdural collections isodense to liquor. The cortical vein sign proves to be invaluable in outlining the boundary between the dilated subarachnoid space and the low density collections, and enables confident diganosis of bilateral subdural hygroma without iv. contrast in a patient with poor renal function.