Radiopaedia.org

The vertebral arteries are of similar size, with no dissection or focal stenosis from origin to basilar artery. The proximal half of the basilar artery is small calibre, but this is a congenital anomaly secondary to a large persistent trigeminal artery arising from the cavernous portion of the right internal carotid artery. There is fusiform dilatation of the terminal basilar artery, the maximum transverse dimension of 6 mm. There is no saccular, conventional aneurysm. Both carotid arteries are of normal calibre, with no fibromuscular disease or stenosis.

Conclusion

1. Large trigeminal artery from right ICA.
2. 6 mm fusiform aneurysm of terminal basilar artery-no saccular component, likely low risk, and not likely to benefit from endovascular intervention.