A pneumatized anterior clinoid mimicking an aneurysm on MR imaging. Report of two cases.

The advent of magnetic resonance (MR) imaging has marked a new era in neuroimaging--particularly in terms of diminishing the need for more invasive diagnostic procedures. A cautionary note should be sounded, however, about an important limitation of standard spin-echo MR studies. Two patients were referred for angiography because MR imaging indicated the presence of a "paraclinoid aneurysm." In retrospect, these findings were due instead to a pneumatized anterior clinoid. Angiography could have been avoided had this pitfall been recognized, and had a gradient-echo flow-imaging protocol been utilized. This latter approach (which does not replace spin-echo imaging) is more sensitive to flowing blood and thus allows differentiation of an air space from a nonthrombosed aneurysm.