Introduction: High-resolution research positron emission tomography (HRPET) is capable of 2 mm resolution and can potentially detect small corticotroph adenomas by 18F-flourodeoxyglucose (18F-FDG) uptake. To determine the sensitivity of this imaging modality , we compared the sensitivity of HRPET and MR-imaging for detection of pituitary adenomas in Cushing disease (CD).

Methods: Consecutive CD patients that underwent preoperative 18F-FDG-HRPET and MR-imaging (SE and high resolution SPGR sequences) were prospectively analyzed. Standardized uptake values (SUVs) were calculated from HRPET and compared to MR-imaging. Imaging findings were correlated to biochemical, operative and histological findings.

Results: Ten patients (7 female, 3 male) were included (mean age, 30.8±19.3). MR-imaging revealed a pituitary adenoma in 4 patients (40% of patients) on SE and in 7 patients (70%) on SPGR sequences. 18F-FDG-HRPET revealed increased 18F-FDG-uptake consistent with an adenoma in 4 patients (40 %; range, adenoma size 3 to 14 mm). Maximum SUV (SUVmax) was significantly higher for 18F-FDG-HRPET positive tumors (difference=5.1, 95% CI 2.1 to 8.1; P=0.004). HRPET-positivity was not associated with tumor volume (P=0.2) or dural invasion (P=0.5). 18F-FDG-HRPET positivity was correlated with midnight (P=0.01) and morning ACTH (P=0.04) levels. Both midnight and morning ACTH values correlated with quantitative SUVmax (R=0.9; P=0.001) and average SUV (R=0.8; P=0.01). HRPET positive adenomas had smaller response to preoperative CRH stimulation test (p=0.03). Three adenomas detected on MR-SPGR sequences were not detected by 18F-FDG-HRPET imaging. No SPGR negative adenomas were detected on 18F-FDG-HRPET. Two SE negative adenomas were detected on HRPET.

Conclusions: 18F-FDG-HRPET imaging can detect small functioning corticotroph adenomas and is more sensitive than SE MR-imaging. SPGR MR-imaging is more sensitive than 18F-FDG-HRPET and SE MR-imaging in the detection of CD-associated pituitary adenomas. The findings of this study also establish association between FDG uptake and the biochemical phenotype in CD adenomas.

Patient Care: 1. Reinforce the role of high resolution 3D gradient (SPGR) MRI imaging for diagnosis of Cushing's disease. 2. Newly discovered correlation between ACTH secretion phenotype and FDG uptake may lead to improved imaging or treatment options for Cushing's tumors.

Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of modern high resolution imaging for diagnosis of pituitary microadenomas in Cushing's disease. , 2) Discuss, in small groups, the relative sensitivities of imaging modalities including standard MRI (SE), high resolution MRI (SPGR) and high resolution PET imaging in Cushing's disease., 3) Identify effective imaging modalities for diagnosis of Cushing's Disease.

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