USMLE Pocket medicine, Cushing Algorithm Question

[pzh200707]

In this picture, after the negative result of chest/abd MRI, why do we go directly to BIPSS, not pituitary MRI??

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“Simultaneous bilateral inferior petrosal sinus sampling (BIPSS) plays a crucial role in the diagnostic work-up of Cushing’s syndrome. It is the most accurate procedure in the differential diagnosis of hypercortisolism of pituitary or ectopic origin, as compared with clinical, biochemical and imaging analyses, with a sensitivity and specificity of 88–100% and 67–100%, respectively. In the setting of hypercortisolemia, ACTH levels obtained from venous drainage of the pituitary are expected to be higher than the levels of peripheral blood, thus suggesting pituitary ACTH excess as the cause of hypercortisolism.”

Bilateral inferior petrosal sinus sampling