Fogging Effect
- “Fogging effect” is a well known phenomenon consisting of transient disappearance of a cerebral infarction on CT images.
- Few case reports have described the same phenomenon on T2-weighted MRI and DWI, usually occurring in the subacute stages of a cerebral infarction (2–20 days) and mainly involving cortical or posterior cerebral fossa ischemia.
- In cortical infarction, “fogging effect” mainly consists of a reduction and shading of the ischemic regions, while in posterior cerebral fossa infarction, MRI findings may apparently disappear.
- The causes of this “imaging effect” are poorly understood. Different hypotheses have been suggested:
- Reparative processes occurring in subacute stages may “hide” the underlying infarction.
- Diapedesis of red blood cells containing paramagnetic hemoglobin (deoxy- and metahemoglobin) may account for T2* susceptibility effects.
- Mild hemorrhagic transformation may occur and change the signal.
- In conclusion, attention should be paid when evaluating a patient with infarction involving the brainstem in the 20–30 day period (subacute stage) from the ictus.
