Imaging Studies for Cushing Syndrome

Initial Imaging for ACTH-Dependent Cushing Syndrome

The American College of Radiology recommends MRI sella without and with IV contrast as the first-line imaging study for suspected Cushing disease, with high-resolution pituitary protocols 4, 5
3T MRI is preferred over 1.5T MRI where available due to improved detection of microadenomas 1, 2, 3
Technical refinements that improve detection include spoiled gradient-recalled (SPGR) acquisition echo with 1 mm slice intervals, fluid attenuation inversion recovery (FLAIR) sequences, and T1-weighted turbo spin echo (TSE) sequences 1, 2

Bilateral Inferior Petrosal Sinus Sampling (BIPSS) should be performed when pituitary lesions <6 mm are detected on MRI, MRI is negative or equivocal, or there is discordance between biochemical testing and imaging findings 6, 7, 8
BIPSS is not necessary when pituitary tumors ≥10 mm are detected on MRI and dynamic testing results are consistent with Cushing disease 6, 7
For tumors 6-9 mm, expert opinions differ, but most recommend BIPSS 6, 7
When ectopic ACTH syndrome is suspected, neck-to-pelvis thin-slice CT scan is recommended, and 68Ga-DOTATATE PET/CT is useful for localizing ectopic ACTH-secreting neuroendocrine tumors, with approximately 65% detection rate 6, 7

PET has been explored as an alternative to or in combination with MRI for localizing corticotroph adenomas, with 18F-FDG PET/CT comparable to standard fast spin echo MRI in detecting pituitary lesions 1, 2, 9
PET coregistration with volumetric MRI (PET/MRCR) combines functional and anatomical imaging, and 11C-methionine PET may permit more accurate localization of radiotracer uptake sites 1, 2, 9

7T MRI is not widely available and there is currently no justification for re-imaging on 7T MRI if no tumor is detected on 1.5T/3T MRI 1, 2, 3
No single imaging test can absolutely differentiate between pituitary and ectopic ACTH-secreting tumors; clinical context and biochemical test results must guide management 6, 7
Functional imaging will likely prove a better approach than MRI alone in the future, but more data are needed to define the use of different ligands in various clinical settings 1, 2, 3