Fogging phenomenon (cerebral infarct)

Last revised by Frank Gaillard on 25 Nov 2023

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 a number of processes occurring simultaneously including the migration of lipid-laden macrophages and leukocytes into the infarcted tissue, proliferation of capillaries, extravasation of red blood cells out of damaged capillaries and a decrease in edema 1-3,7

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 the onset of infarction 5


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