Differential diagnosis between aneurysm and infundibular dilatation in the IC-PC region with 3D-CTA

In cases of asymptomatic internal carotid-posterior communicating artery (IC-PC) protrusions, it is sometimes difficult to differentiate infundibular dilatation (ID) from aneurysm by digital subtraction angiography. We applied three-dimensional CT angiography (SD-CTA) in 32 cases of these IC-PC protrusions. SOMATOM PLUS 4 was used under such conditions as to provide images with high spatial resolution. The shaded surface display (SSD) method was adopted to reconstruct the 3D images because of its advantage in separating overlapped vasculature. We also made reference to source images and maximum intensity projection (MIP) to make sure of our diagnoses. In all cases including 4 aneurysms and 28 IDs, we were able to distinguish between ID and aneurysm. The accuracy of 3D-CTA was confirmed by 9 surgical cases. Our technique was as follows: 1) To inject a high dose of diluted contrast medium rapidly to smaller arteries for opacification of contrast medium. 2) To exclude neighboring useless structures except for the very close structures such as posterior clinoid process from the target image focusing on the IC-PC region. 3) To observe the reconstructed image of MIP and SSD from various angles. The contralateral and craniocaudal view were valuable. 4) To change the threshold level gradually and observe the configurational changes of the apex of protrusion. Poorly developed PcomA was mostly delineated at the optimum threshold level. Otherwise, the apex of protrusion remained spherical in an aneurysm and became pyramidal in shape in an ID when the threshold level was gradually decreased. In conclusion, 3D-CTA was a useful modality for IC-PC protrusions to distinguish between ID and aneurysm.