A 57-year-old woman presents to the emergency department with new-onset seizures. She was witnessed by her husband to have a generalized seizure lasting approximately 1 minute. She has smoked 1 pack of cigarettes a day for over 40 years. In the past 3 months, she has lost 25 lb in weight. On examination, she appears thin and nervous but findings on her neurologic examination are otherwise essentially within normal limits. Plain radiographs of the chest obtained in the emergency department show a 4-cm nodule in the upper lobe of her right lung. To exclude cerebral metastasis as a cause of her seizure, what should the next test requested be?