Imaging description

The normal steroid-producing cells of the adrenal cortex are rich in intracellular lipid because steroids are based on the cholesterol molecule. Adrenal adenomas are benign tumors derived from the adrenal cortex and also contain substantial amounts of intracellular lipid. Two of the critical imaging findings used to characterize adenomas are based on this fact; namely, adenomas are typically of low density (less than 10–18 Hounsfield Units) at non-enhanced CT and lose signal on opposed phase gradient-echo MRI when compared to in phase gradient-echo MRI [1, 2]. While both these signs are highly specific, there are some exceptions:

• Fluid-filled cystic or necrotic masses can be of low density on non-enhanced CT (Figures 39.1 and 39.2) [3].

• Other adrenal tumors can occasionally be rich in intracellular lipid, and demonstrate signal loss on opposed phase gradient-echo MRI, including adrenocortical carcinoma, metastases from primary tumors that contain intracellular lipid such as clear cell renal cell carcinoma or hepatocellular carcinoma (Figure 39.3), and pheochromocytoma [4–8].

Importance

The non-invasive characterization of adrenal adenomas by non-enhanced CT or opposed phase gradient-echo MRI is one of the major advances of modern genitourinary radiology, but erroneous diagnosis of an adenoma due to one of the rare exceptions detailed above could theoretically result in a missed opportunity for treatment of clinically important pathology, such as adrenocortical carcinoma or pheochromocytoma, or result in inappropriate treatment of the primary malignancy in the setting of unrecognized metastatic spread of clear cell renal cell carcinoma or hepatocellular carcinoma.