FIG 3. Example of a DSA false-positive finding for basilar occlusion in a patient with a relatively isolated posterior circulation due to bilateral vertebral artery occlusion, a hypoplastic right posterior communicating artery, and a significant left proximal P1 stenosis.

A, Frontal late arterial phase DSA image shows selective injection of a dominant right vertebral artery. The basilar artery is not opacified. The right vertebral artery appears to terminate in the right posterior inferior cerebellar artery (arrow). Note that the patent right posterior inferior cerebellar artery supplies some blood flow to the right anterior inferior cerebellar artery and a small tonsillar loop branch to the contralateral left posterior inferior cerebellar artery.

B, Frontal middle phase DSA image shows selective injection of the left vertebral artery, which terminates in an extracranial muscular branch near the skull base (arrow). No flow is seen in the intracranial segment of the left vertebral artery indicating occlusion of this vessel segment.

C, Lateral projection, right ICA injection, middle arterial phase DSA image shows minimal retrograde filling of the distal basilar artery (arrow) through the posterior communicating artery, but there is absence of flow in the remainder of the basilar artery, suggesting segmental occlusion of this vessel.

D, CTA was performed 1 day after DSA. This volume-rendered 3D CTA image in posteroanterior projection (anatomic orientation) shows bilateral distal vertebral artery occlusions (solid black arrows) and significant focal origin stenoses of both the left proximal P1 and the left superior cerebellar artery (white arrow). CTA image also demonstrates minimal flow in what appears to be a small segment of a hypoplastic distal right intracranial vertebral artery, distal to the origin of the right posterior inferior cerebellar artery. The CTA image shows that the basilar artery is entirely patent and stenosis-free (open black arrow). This was verified on the gray-scale 2D source image.

E, Volume-rendered 3D CTA image, craniocaudal projection with anatomic orientation, demonstrates a prominent left posterior communicating artery (arrow) and absent right posterior communicating artery. The left P1 is small in caliber.

F, Targeted volume-rendered 3D CTA image in the anteroposterior projection demonstrates significant proximal focal left P1 and superior cerebellar artery stenoses (arrow).

G, TOF MRA image with frontal MIP shows absent flow signal (arrow) in the expected region of the basilar artery, suggesting occlusion.