An obese 35-year-old female comes to the ED with worsening 7/10 headache over the past few months. The headache has progressed during this time. It is bilateral, pulsating, associated with nausea, worse when lying down or coughing and improves when she is upright. She reports 5-second blackouts of her vision, which occur occasionally when she changes position. A neurologic exam is unremarkable. Non-contrast head CT scan is normal. The most appropriate next step in the diagnostic workup is: