Adrenal masses most often are detected by CT scan. In patients with clinical or biochemical evidence of a hyperfunctioning adrenal endocrine disorder, the CT scan typically is tailored to evaluate only the adrenal glands, often with thin sections of 3 to 5 mm. More commonly, adrenal masses are detected on CT scans performed for a variety of indications on patients without an adrenal endocrine disorder, usually with slice thickness of 7 to 10 mm. In this latter group of patients, especially if there is no evidence of an underlying extra-adrenal primary malignancy, a CT scan–detected adrenal mass often is referred to as an incidentaloma.

Although most adrenal masses first are detected on CT scan, characterization of these masses sometimes is augmented by MR imaging or by radionuclide scintigraphy. In addition, percutaneous biopsy of an adrenal mass sometimes is obtained if a diagnosis of adrenal metastasis will determine whether surgical resection of a primary extra-adrenal primary neoplasm is performed. This article reviews the imaging findings of a variety of adrenal masses and the relative roles and indications of CT scan, MR imaging, radionuclide scintigraphy, and percutaneous adrenal biopsy in their detection and characterization.

A minority of adrenal masses have specific CT scan features that permit a precise histologic diagnosis of myelolipoma, hemorrhage, or adrenal cyst. Adrenal myelolipomas are benign neoplasms that are composed of mature fat and myeloid tissue, as in normal bone marrow. Most are detected as asymptomatic incidental findings on CT scan, but large lesions can cause pain or result in intratumoral hemorrhage. The diagnosis can be made confidently when CT scan density measurements confirm discrete regions of fat attenuation (−30 to −100 H) within an adrenal mass (Fig. 1)

Acute adrenal hemorrhage is the other entity that often shows specific CT scan features, especially on unenhanced scans. Bilateral adrenal hemorrhage can be associated with sepsis, hypotension, recent surgery, severe trauma, or a bleeding disorder (especially as a complication of anticoagulation therapy). Unilateral adrenal hemorrhage most often is caused by blunt trauma, usually on the right side, but sometimes is caused by adrenal vein ligation at the time of orthotopic liver transplantation. Acute hemorrhage is characterized by its high CT scan attenuation value (50 to 90 H), and is appreciated easily on unenhanced scans (Fig. 2). On enhanced CT scan, however, many nonhemorrhagic adrenal masses have an attenuation value indistinguishable from acute hemorrhage. In the appropriate clinical setting a diagnosis of adrenal hemorrhage on enhanced CT scan must be confirmed by an unenhanced CT scan on a subsequent day or by interval decrease in size of the adrenal mass(es) on follow-up imaging in the weeks following the presumed hemorrhagic insult.

Although adrenal cysts often are listed as one type of adrenal mass with highly specific CT scan features, in practice it is uncommon to make this diagnosis. As is described later in this article, adrenal adenomas often have an attenuation value on unenhanced CT scan similar to adrenal cysts. In addition, many adrenal cysts are really pseudocysts caused by prior hemorrhage into a normal or abnormal gland, and these cystic masses often show a thickened wall, nodularity, septations, or soft-tissue components making it difficult to exclude a neoplasm (Fig. 3)<sup>38 Johnson C.D., Baker M.E., Dunnick N.R. CT demonstration of an adrenal pseudocyst J Comput Assist Tomogr 1985 ;  9 : 817

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Only a small minority of adrenal masses have highly specific CT scan features, which suggest a histologic diagnosis. The morphologic features of most adrenal masses are nonspecific, and the final diagnosis often is based on a combination of clinical features, correlative imaging studies, follow-up CT scans, or in some cases percutaneous adrenal biopsy. It is useful to divide patients with adrenal masses into those with and without hyperfunctioning adrenal endocrine syndromes. In patients with a hypersecretory adrenal syndrome, a unilateral adrenal mass often is removed surgically on the basis of CT scan findings alone, although functional confirmation of the nature of the mass by radionuclide scintigraphy sometimes is obtained.