Comparative evaluation of conventional and dynamic magnetic resonance imaging of the pituitary gland for the diagnosis of Cushing's disease.

Objectives: The ability of MRI to detect pituitary ACTH-secreting adenomas in patients with Cushing's disease is limited. Owing to different dynamics of contrast enhancement between adenomas and normal pituitary tissue, it has been suggested that obtaining images within seconds after gadolinium (Gad) injection using dynamic procedures increases the sensitivity of MRI in the detection of pituitary microadenomas. The objective of this study was to compare the ability of conventional magnetic resonance imaging (CMRI) and dynamic MRI (DMRI) to detect ACTH-secreting pituitary adenomas.

Design: Twenty-six consecutive patients with ACTH-dependent Cushing's syndrome and 10 normal subjects were investigated. According to the results of inferior petrosal sinus sampling, 21 patients had Cushing's disease and five had ectopic ACTH syndrome. Patients with Cushing's disease were operated regardless of the results of imaging studies. All underwent identical MRI and DMRI procedures using a 1.0 T magnet. Image sampling time during DMRI was 19 sec. Scans were randomly mixed and analysed blind, retrospectively and independently by two experienced radiologists. The clarity of the images was assessed by the analysis of agreement among radiologists. MRI findings were compared to surgical and histopathological findings.

Results: Surgical exploration identified three macrodenomas and 14 microadenomas. One microadenoma was found at pathological examination after subtotal hypophysectomy and no tumour was found in three cases. According to the combined opinion of radiologists, the three macroadenomas were identified equally well with CMRI and DMRI. Eight ACTH-secreting microadenomas were detected with CMRI and 11 with DMRI. The three microadenomas detected with DMRI only were visualized within 60 sec following Gad injection. No false positives occurred with CMRI. Three false positives were obtained with DMRI: one in a patient with ectopic ACTH syndrome while a silent microprolactinoma and normal tissue were found at the site of the radiological abnormality in two patients with Cushing's disease. In our study, the sensitivity of DMRI is greater than that of CMRI (0.67 vs. 0.52) but is associated with a loss in specificity (0.80 vs. 1.00). False positives may result from the increased sensitivity of DMRI which detects incidental pituitary lesions, technical artefacts or lowest clarity of images, as suggested by a lower observer agreement of DMRI (Kappa statistic 0.66 vs. 0.83). Overall, the two MR procedures had equivalent diagnostic power (0.72).

Conclusions: In our hands, dynamic procedures did not improve the usefulness of MRI in Cushing's syndrome.