Note.—Susceptibility-weighted imaging may help differentiate hemorrhagic transformation from cortical necrosis, and diffusion-weighted imaging findings may be falsely negative in patients with hyperacute or acute posterior circulation or lacunar stroke. In the hyperacute stages, a large area of enhancement in the deep gray matter or basal ganglia may indicate impending hemorrhagic transformation.
* Increased parenchymal cortical enhancement should end around 48 hours after stroke.
† Arterial and meningeal enhancement should end around 1 week after stroke.
http://radiographics.rsna.org/content/32/5/1285.full
Note.—Susceptibility-weighted imaging may help differentiate hemorrhagic transformation from cortical necrosis, and diffusion-weighted imaging findings may be falsely negative in patients with hyperacute or acute posterior circulation or lacunar stroke.
* Cortical necrosis usually resolves by 3 months after stroke and rarely persists for more than a year.
† If parenchymal enhancement persists for more than 8 weeks, other causes should be considered.
Radiology Reference 