Acute: Subtle low signal (hypointense) on T1, often difficult to see at this stage, and high signal (hyperintense) on spin density and/or T2-weighted and proton density-weighted images starting 8 h after onset; should follow vascular distribution. Mass effect maximal at 24 h, sometimes starting 2 h after onset, even in the absence of parenchymal signal changes. No parenchymal enhancement with paramagnetic contrast agent. Territorial intravascular paramagnetic contrast enhancement of "slow-flow" arteries in hyperacute infarcts; at 48 h, parenchymal and meningeal enhancement can be expected.Subacute (1 wk or older): Low signal on T1, high signal on T2-weighted images. Follows vascular distribution. Revascularization and blood-brain barrier breakdown may cause parenchymal enhancement with contrast agents.
Old (several weeks to years): Low signal on T1, high signal on T2. Mass effect disappears after 1 mo. Loss of tissue with large infarcts. Parenchymal enhancement fades after several months.
Source: Strokecenter
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