A 35-year-old man is brought to the emergency department following a motor vehicle accident. He lost consciousness for 2 minutes, then regained consciousness and appeared well. After 4 hours, he develops severe headache, vomiting, and altered mental status. Non-contrast CT head shows a lens-shaped (biconvex) hyperdensity in the epidural space with mass effect. Which investigation should be performed next to assess for ongoing bleeding and guide urgent intervention?