Imaging of hyperfunctioning adrenal masses

Adrenal imaging not only aims to detect hyperfunctioning adrenal lesions responsible for distinct clinical syndromes (pheochromocytoma, Cushing's syndrome or primary aldosteronism) but also to differentiate malignant from benign disease. Computed tomography (CT), MRI and high-resolution ultrasound imaging methods are available for detection of adrenal masses with high sensitivities. Although the risk of malignancy increases with the size of the adrenal mass, this criterion does not discriminate malignant from benign disease with sufficient specificity. Attenuation values on unenhanced CT, contrast washout characteristics on CT and chemical-shift imaging on MRI help differentiate adrenal adenomas and nonadenomas. Adrenal lesions with equivocal results on CT and MRI can be characterized further by functional nuclear medicine imaging. Radiopharmaceutical agents specifically detect masses derived from the adrenal cortex (e.g.,¹³¹I-6-iodomethyl-norcholesterol) and from the adrenal medulla (e.g., ¹²³I- and ¹³¹I-meta-iodobenzylguanidine). Unspecific tracers, such as ¹⁸F-fluorodeoxyglucose, are very useful in discriminating malignant and benign adrenal tumors.