What is an adrenal adenoma, and what are its CT and MRI features?
Adrenal adenoma is the most common benign neoplasm of the adrenal gland and is often detected incidentally on cross-sectional imaging. Some may be hyperfunctional (i.e., hormonally active) and symptomatic, although they are more commonly nonhyperfunctional.
On CT and MRI, a well-circumscribed adrenal gland nodule or mass is visualized, often homogeneous and ≤ 3 to 4 cm in size. However, calcifications are rarely visualized. Stability of nodule size compared to imaging studies obtained >6 to 12 months prior to the current study is a very useful feature to indicate a benign etiology, which emphasizes the importance of comparison to prior studies whenever available. The majority of adrenal adenomas, ≈70% of cases, contain intracellular microscopic lipid.
Specific diagnostic features of a lipid-rich adrenal adenoma include (1) attenuation of <10 HU on unenhanced CT and (2) uniform loss of signal intensity on out-of-phase T1-weighted images relative to in-phase T1-weighted MR images, both secondary to presence of microscopic lipid.