Fat matters: practical imaging approach to microscopic and macroscopic fat in adrenal nodules

Fat-containing adrenal nodules are frequently encountered in clinical practice and most often represent benign entities, particularly adrenal adenomas or myelolipomas. Although most adrenal nodules are adenomas regardless of imaging appearance, accurate characterization of microscopic and macroscopic fat remains important because rare non-adenomatous and malignant lesions may also contain fat and can create diagnostic pitfalls in selected clinical contexts. Importantly, even when imaging features support a benign adrenal adenoma, hormonal evaluation may still be required because functional status cannot be determined by imaging alone. This review provides an imaging centered overview of fat-containing adrenal nodules on computed tomography (CT) and magnetic resonance imaging (MRI), with emphasis on distinguishing microscopic from macroscopic fat. Microscopic fat is most commonly associated with adrenal adenomas and can be detected using chemical-shift MRI or unenhanced CT attenuation measurements. Homogeneous signal loss on chemical-shift imaging strongly supports adenoma; however, emerging evidence indicates that certain heterogeneous signal loss patterns may also be compatible with benignity, particularly in patients without a history of malignancy. Macroscopic fat most commonly indicates adrenal myelolipoma, although small foci of macroscopic fat may rarely occur in other adrenal neoplasms due to myelolipomatous degeneration. Key imaging features, diagnostic pitfalls, and differential considerations are reviewed, including metastases from lipid-containing primary tumors, collision tumors, adrenocortical carcinoma and retroperitoneal mimics. A practical imaging-based framework integrating CT attenuation, chemical-shift MRI findings, fat proportion, lesion morphology, and clinical context is proposed to facilitate systematic evaluation of fat-containing adrenal nodules across incidental, oncologic and problem-solving scenarios. Integration of imaging findings with guideline-recommended biochemical evaluation and clinical context remains essential for accurate diagnosis and appropriate patient management.