What is the recommended CT protocol for imaging the adrenal glands?

CT Protocol for Adrenal Imaging

The recommended CT protocol for adrenal gland imaging begins with unenhanced (non-contrast) CT as the first-line modality, with lesions measuring ≤10 Hounsfield Units (HU) considered benign adenomas requiring no further imaging. 1, 2

Initial Imaging: Unenhanced CT

• Unenhanced CT is the mandatory first step for all adrenal lesions, as it provides the critical attenuation measurement needed to characterize the mass 1, 2
• Lesions with attenuation ≤10 HU are diagnostic of lipid-rich adenomas and require no additional imaging 2, 3
• The American College of Radiology assigns an appropriateness rating of 8/9 (highly appropriate) for non-contrast CT in adrenal incidentaloma evaluation 2
• Unenhanced CT can also identify characteristic features of myelolipomas (fat density) and cysts, which are readily diagnosed by their imaging appearance alone 3, 4

Second-Line Imaging for Indeterminate Lesions (>10 HU)

When the unenhanced attenuation exceeds 10 HU, proceed with one of two equally appropriate second-line protocols:

Option 1: Delayed Enhancement (Washout) CT Protocol

• Perform contrast-enhanced CT followed by delayed imaging at 15 minutes to calculate washout characteristics 2, 3
• Adenomas demonstrate >60% absolute washout or >40% relative washout at 15 minutes post-contrast 2, 3
• The American College of Radiology assigns an appropriateness rating of 8/9 for delayed enhancement CT 2
• Calculate absolute washout using: [(enhanced HU - delayed HU) / (enhanced HU - unenhanced HU)] × 100 3

Option 2: Chemical Shift MRI

• Chemical shift MRI detects microscopic fat by exploiting different proton frequencies in water and fat 2, 3
• Homogeneous signal intensity drop on out-of-phase images is diagnostic of lipid-rich adenoma 2, 3
• The American College of Radiology assigns an appropriateness rating of 8/9 for chemical shift MRI 2
• MRI is preferred in pregnant patients, young adults, and children when radiation exposure is a concern 5

Critical Technical Considerations

ROI Placement in Heterogeneous Nodules

• Adrenal-protocol CT has poor diagnostic performance for heterogeneous nodules regardless of ROI placement method 6
• For heterogeneous lesions, the standard single large ROI in the nodule's center showed sensitivity of only 57.3% and specificity of 84.1% for adenoma diagnosis 6
• Heterogeneous nodules often require alternative approaches such as biopsy or surgical resection rather than relying on washout characteristics 6

Size Thresholds

• Lesions >4 cm have increased malignancy risk and warrant more aggressive evaluation even if imaging suggests benign features 1, 3
• The 4 cm cutoff is based on association with extra-adrenal disease and genetic syndromes 1

Important Pitfalls and Limitations

False Negatives and Positives

• Approximately 1/3 of pheochromocytomas may demonstrate washout in the adenoma range, potentially leading to false-negative results 2, 3
• Malignant masses (adrenocortical carcinoma, hypervascular metastases) can sometimes show washout patterns mimicking adenomas 2, 3
• About 1/3 of true adenomas do not demonstrate characteristic washout patterns 3

Mandatory Pre-Procedure Precautions

• All patients must undergo biochemical testing (plasma or 24-hour urinary metanephrines) before any invasive procedure, including biopsy, to exclude pheochromocytoma and prevent hypertensive crisis 2, 3
• Hormonal evaluation is required for all adrenal incidentalomas regardless of imaging characteristics, as imaging cannot reliably predict functionality 2

Specific Protocol Timing

• Delayed imaging should be obtained at 15 minutes post-contrast for washout calculations 2, 3
• Some older protocols used 30-minute delayed imaging, which showed complete separation of adenomas (<37 HU) from nonadenomas (>41 HU), but 15-minute protocols are now standard 7
• Portal venous phase imaging at 60-90 seconds shows significant overlap between adenomas and nonadenomas and is not useful for differentiation 7

When Biopsy Is Considered

• Biopsy should not be part of initial workup and has limited clinical value 1
• Consider biopsy only for hormonally inactive masses with non-benign imaging where pathology would directly change management (e.g., suspected metastasis in cancer patients) 1
• Always exclude pheochromocytoma biochemically before biopsy 2, 3