Subarachnoid Hemorrhage — Star Pattern

A 52-year-old woman with chronic smoking and hypertension presents to the emergency department with sudden-onset "thunderclap" headache described as "the worst headache of my life" while exercising, accompanied by brief loss of consciousness and vomiting. Neck stiffness is noted on examination. Non-contrast CT head is performed urgently. The structure marked **A** in the diagram shows hyperattenuating blood (60–80 HU) in a classic star or pentagon configuration. Which of the following is the most appropriate NEXT diagnostic step if the CT is negative or performed >6 hours after symptom onset?