A young pt admitted to casualty with history of trauma. MR study shows two signal voids in the Sylvian fissures on either side on T2 as well as T1w images.
Diagnosis ? Bilateral MCA Aneurysms ?
Answer : No, as pt's non contrast 3 D TOF MR Angiography of brain normal, the thin axial CT sections in corresponding region nicely demonstrates that the low signal intensity in sylvian fissures on MR is signal voids due to intra cranial air and not flow void of aneurysms. Note the susceptibility Artifact due to air on MR Diffusion.
FLOW VOID AND SIGNAL VOID
Low signal intensity in Sylvain fissures like those on T2WI images could be due to signal void of air or flow void aneurysms. The bright rim evident on the diffusion scan indicates susceptibility Artifact that would not be expected in association with an aneurysm.
Reference: Practical MR Physics and case file of MR artifacts and pitfalls, Alexander C. Mamourian, MD