Is this sufficient to indicate brain death?
No. Although it is highly suspicious and clearly has a terrible prognosis, as the CTA captures circulation at one point in time it is conceivable that blood flow to the brain is merely very delayed.
What imaging tests are suitable to establish brain death?
Angiography or nuclear medicine perfusion studies.
At the same time as the initial CT brain a CT angiogram was performed demonstrating no opacification of the intracranial ICA and no opacification of the dural venous sinuses. Note prominent (normal) opacification of the external carotid artery branches in the scalp.