What is the recommended diagnostic workup for adrenal nodules, including the role of MRI (Magnetic Resonance Imaging)?

MRI for Adrenal Nodules: When to Order

MRI (specifically chemical shift MRI) should be ordered as second-line imaging for adrenal nodules that are indeterminate on non-contrast CT (Hounsfield units >10), serving as an alternative to washout CT to characterize the lesion. 1

Initial Imaging Strategy

• Non-contrast CT is the first-line imaging modality for all adrenal nodules to measure Hounsfield units (HU) and provide initial characterization 2
• Lesions with HU ≤10 are definitively benign (lipid-rich adenomas) and do not require MRI or any further imaging unless they are ≥4 cm 1, 2
• Lesions with HU >10 are indeterminate and require second-line imaging 1

When MRI is Indicated

Chemical shift MRI should be ordered when:

• The adrenal mass shows HU >10 on non-contrast CT, making it indeterminate for adenoma versus other pathology 1, 2
• You need an alternative to washout CT (either option is acceptable per guidelines) 1
• The patient has suspected pheochromocytoma, where MRI is preferred over contrast-enhanced CT due to risk of hypertensive crisis from IV contrast 1
• Radiation safety is a concern, particularly in young adults, children, or pregnant patients where low-dose CT or chemical shift MRI should be preferred 1, 3

How Chemical Shift MRI Works

• Chemical shift MRI detects microscopic fat by exploiting different proton frequencies in water versus fat 1
• Homogeneous signal intensity drop on opposed-phase images is diagnostic of lipid-rich adenoma 1
• Heterogeneous signal intensity drop is more controversial, as minute amounts of microscopic fat can occur in pheochromocytoma, adrenocortical carcinoma, and some metastases 1
• However, heterogeneous microscopic fat still indicates high likelihood of benignancy, particularly in patients without prior cancer (0% malignancy rate in one multicenter study) 4

Critical Limitations of Alternative Imaging

Washout CT has significant pitfalls that make MRI attractive:

• Approximately 1/3 of pheochromocytomas may washout in the characteristic range of an adenoma (false negative) 1
• Approximately 1/3 of adrenal adenomas do not washout in the adenoma range (false positive) 1
• Malignant masses (adrenocortical carcinoma, hypervascular metastases) can also washout in the adenoma range 1

When MRI is NOT Needed

• Benign non-functional adenomas <4 cm with HU <10 require no further imaging 1, 2
• Myelolipomas and masses containing macroscopic fat identified on initial CT require no further imaging 1
• Lesions that are clearly malignant on CT (large, heterogeneous, irregular margins) should proceed directly to surgical planning rather than additional characterization 1

Common Pitfalls to Avoid

• Never order MRI before excluding pheochromocytoma hormonally if planning contrast-enhanced studies, as IV contrast can precipitate hypertensive crisis 1
• Do not skip hormonal evaluation even when ordering MRI for radiologic characterization—all adrenal incidentalomas require functional testing regardless of imaging appearance 1, 2
• Do not order MRI for routine follow-up of already-characterized benign lesions <4 cm, as this provides no additional benefit 1, 3
• Be aware that heterogeneous signal loss on chemical shift MRI does not definitively exclude malignancy in patients with prior cancer, particularly hepatocellular carcinoma or renal cell carcinoma which may contain fat 4