Recommended Imaging Modality for Adrenal Imaging
Non-contrast CT is the recommended first-line imaging modality for adrenal masses, with contrast-enhanced CT washout studies or chemical shift MRI as appropriate second-line options for indeterminate lesions. 1
Initial Imaging Approach
Non-contrast CT
- First-line imaging modality for adrenal masses 2, 1
- Highly specific for differentiating adenomas from malignant lesions 2
- Lesions with Hounsfield units (HU) ≤10 are highly likely to be benign adenomas 2, 1
- Risk of adrenocortical carcinoma is 0% when HU <10 1
- Advantages: relatively inexpensive, widely available, excellent sensitivity for small adrenal tumors 2, 3, 4
Second-line Imaging for Indeterminate Lesions
When non-contrast CT shows indeterminate results (HU >10):
Contrast-enhanced CT with Washout Protocol
- Measures contrast washout after 15 minutes 2, 1
- Adenomas typically show rapid washout (>60% absolute washout or >40% relative washout) 2, 5
- Sensitivity >95%, specificity >97% for adenoma detection 2
- Limitations:
- ~1/3 of pheochromocytomas may washout like adenomas
- ~1/3 of adenomas do not show typical washout pattern
- Some malignant masses can demonstrate adenoma-like washout 2
Chemical Shift MRI
- Excellent alternative for indeterminate adrenal masses 2, 1
- Exploits different frequency of protons in water and fat to detect microscopic fat 2
- Homogeneous signal intensity drop on opposed-phase images is diagnostic of lipid-rich adenoma 2
- Particularly useful when:
- Patient has contraindication to CT contrast
- Lesion is discovered incidentally on MRI
- Suspected pheochromocytoma (preferred over contrast CT due to risk of hypertensive crisis) 2
- Limitations: heterogeneous signal intensity drop can be seen in pheochromocytoma, adrenocortical carcinoma, and some metastases 2
Advanced Imaging Options
FDG-PET
- Useful for distinguishing potentially malignant lesions from benign tumors in radiologically indeterminate cases 2, 1
- Superior to MIBG and somatostatin-based methods for suspected malignant pheochromocytoma 2
- Particularly valuable when CT and MRI are inconclusive 2
- Shows high tracer uptake in malignant neoplasms due to increased glucose utilization 5
Algorithm for Adrenal Imaging
Start with non-contrast CT
- If HU ≤10: Likely benign adenoma, no further imaging needed
- If HU >10: Proceed to second-line imaging
Second-line imaging options
- Contrast-enhanced CT washout study (preferred if renal function normal)
- Chemical shift MRI (preferred if contrast contraindicated or suspected pheochromocytoma)
If still indeterminate
- Consider FDG-PET for further characterization
- Consider follow-up imaging in 3-6 months to assess for growth
Special Considerations
- Size matters: Lesions >4 cm with inhomogeneous appearance or HU >20 have higher risk of malignancy and should be considered for surgical removal 1
- Avoid biopsy: Adrenal mass biopsy is rarely indicated and should not be performed for suspected adrenocortical carcinoma due to risk of tumor seeding 2
- Functional status: All adrenal masses should undergo hormonal evaluation regardless of imaging appearance 1
- Follow-up: For benign-appearing lesions (<10 HU, <3 cm), repeat imaging in 6-12 months is recommended; no further follow-up needed if stable 2, 1
Pitfalls to Avoid
- Relying solely on imaging without hormonal evaluation
- Misinterpreting washout patterns without considering limitations
- Performing biopsy without excluding pheochromocytoma
- Neglecting follow-up imaging for indeterminate lesions
- Using ultrasound as primary imaging modality (limited sensitivity for small lesions) 2
By following this evidence-based approach to adrenal imaging, clinicians can accurately characterize adrenal masses while minimizing unnecessary procedures and optimizing patient outcomes.