Patients with Cushing disease have hormone producing tumors in their pituitary gland. Often these tumors are so small they cannot be detected by magnetic resonance imaging (MRI). The inferior petrosal sinuses are small veins that drain the blood from the pituitary gland. By taking a small sample of blood from these sinuses, doctors can differentiate a small tumor in the pituitary gland from other tumors in the body producing the hormone. Patients with Cushing disease have high levels of the hormone ACTH in the petrosal sinuses. Patients with other causes of Cushing syndrome do not have increased levels of ACTH in the petrosal sinuses.