Fogging phenomenon (cerebral infarct)

The fogging phenomenon is seen on non-contrast CT of the brain and represents a transient phase of the evolution of cerebral infarct where the region of cortical infarction regains a near-normal appearance. 

During the first week following a cortical infarct, hypoattenuation and swelling become more marked, resulting in significant mass effect and clear demarcation of the infarct often with vivid gyral enhancement ⁶. 

As time goes on, the swelling starts to subside and the cortex begins to increase in attenuation. This is believed to occur as the result of migration of lipid-laden macrophages into the infarcted tissue with proliferation of capillaries and a decrease in edema ^1-3. At 2 to 3 weeks following an infarct, the cortex regains near-normal density and imaging at this time can lead to confusion or missed diagnosis ^4,5. Fogging has been demonstrated in around 50% of cases ⁴. 

If in doubt, the administration of IV contrast will demarcate the region of infarction ². 

A similar phenomenon is also seen on T2 MRI sequences of the brain, and is believed to be due to similar cellular processes, as the timing is similar ³. It has been found to occur in approximately 50% of patients between 6 and 36 days (median 10 days) after onset of infarction ⁵.