The main differential consideration for new neurological deficits following subarachnoid hemorrhage is vasospasm-associated ischemia (Fig. 39.2). Although infarction secondary to vasospasm and posterior reversible encephalopathy can both appear as bilateral hypoattenuating foci on CT, infarcts typically demonstrate perfusion deficits, while posterior reversible encephalopathy tends to show elevated perfusion. Diffusion-weighted imaging can also be useful for characterizing these lesions, whereby acute infarcts display restricted diffusion, while posterior reversible encephalopathy more commonly demonstrates elevated diffusivity.