A 52-year-old man with hypertension, central obesity, proximal muscle weakness, and easy bruising is suspected of having Cushing syndrome. His 24-hour urinary free cortisol is elevated at 180 μg/day (normal <50 μg/day), and late-night salivary cortisol is elevated. Which investigation is most appropriate as the NEXT step to determine the etiology (ACTH-dependent vs. ACTH-independent)?