Images below require Macromedia's Flash Player to view

Neuroradiology Case of the Week

Case 161

Ajay Malhotra , MD and PL Westesson MD, DDS, PhD

Clinical Presentation: A 52-year-male with a history of MVC on 9/11 had loss of consciousness with seizure activity. He also had left hemiparesis. CT of the head was done on 9/14 and repeated on 9/23. An MRI of the head was performed on 9/17.

Radiological Findings: Initial Head CT of 9/14 (Figs. 1A-C) showed multiple small hypodense lesions in the right parieto-temporo-occipital lobes, left occipital and left frontal lobe representing acute infarcts.
     MRI on 9/17 confirmed areas of restricted diffusion (Figs. 3A-C) in the above mentioned areas consistent with embolic infarcts. Post-contrast images (Figs. 4A-C) revealed patchy enhancement, consistent with subacute infarcts.
      On repeat CT of 9/23 (Figs. 2A-C) the above mentioned lesions were no longer apparent.

Diagnosis: Fogging phenomenon on CT

Discussion: "Fogging" is the temporary loss of visibility of an infarct on CT which occurs in the subacute phase at about 2 weeks after stroke. It occurs in up to 40% of medium to large infarcts on CT. Cerebral infarcts therefore may be overlooked or grossly underestimated if the scan is performed during the second and third week after stroke.
     Increase of x-ray attenuation on day 10 is known as the fogging effect and appears to be a favorable prognostic factor. Fogging is generally considered to be due to macrophage invasion and proliferation of capillaries within the infarct area, but probably also represents partial restoration of some viable tissue
     Reduction in infarct extent on T2-weighted MR which may be attributed to "fogging" occurs in a significant proportion of patients with cortical infarcts. This may lead to an underestimation of true final infarct extent. This suggests that true infarct extent on T2-weighted MR can probably only be assessed on scans obtained beyond 7 weeks after stroke. A contrast enhanced MRI may reveal striking enhancement 1-2 weeks following ictus with no mass effect. Subacute infarcts typically reveal gyral parenchymal enhancement, beginning towards end of first week and persisting for 6-8 weeks.

References:

1. Skriver EB, Olsen TS. Transient disappearance of cerebral infarcts on CT scan, the so-called fogging effect.Neuroradiology 1981;22:61-65. [Medline]
2. Becker H, Desch H, Hacker H, Pencz A. CT fogging effect with ischemic cerebral infarcts. Neuroradiology 1979;18:185-192. [Medline]
3. Ackerman RH, Lev MH, Alpert NM, et al. Infarct fogging on CT: prevalence, features, and PET/MRI correlates. In: Proceedings of the 28th Annual Meeting of the American Society of Neuroradiology; 1990; Los Angeles, CA.