Estimation of vertebral arterial asymmetry by computed tomography.

In 80 patients with no stenotic lesions in the vertebrobasilar arterial system, a study was made of the relationship between the deviation of the basilar artery (BA) from the midline on computed tomography (CT) and the right-to-left vertebral arterial caliber difference on angiograms. In 66 patients (83%), the BA was visible on plain CT films, and 55 of them showed deviation of the BA to either side. In 44 of these patients (80%), the vertebral artery (VA) contralateral to the side of BA deviation had a larger caliber compared with the ipsilateral one on angiograms. In 6 patients whose unilateral VA terminated in the posterior inferior cerebellar artery (PICA) and showed an extremely small caliber compared to the contralateral one, the BA was always deviated to the side of the smaller VA. Our data suggest that the deviation of the BA on plain CT films may represent a good indicator for estimating the right-to-left VA caliber difference. At the time of vertebral angiography, injection of contrast medium should preferably be made from the larger VA in order to avoid laminar flow in the BA and to shorten the procedure. Prior estimation of the right-to-left VA caliber difference by CT may be of great benefit to the angiographic procedure.