What is the recommended protocol for a CT (Computed Tomography) scan of the adrenals?

CT Protocol for Adrenal Imaging

Begin with non-contrast CT of the abdomen as the primary imaging modality, measuring attenuation in Hounsfield Units (HU) to characterize the lesion, with masses <10 HU considered benign adenomas requiring no further imaging. 1, 2

Initial Imaging Protocol

Non-Contrast CT (First-Line)

• Non-contrast CT is the gold standard initial test for characterizing adrenal masses, as lipid-rich adenomas demonstrate low attenuation values typically <10 HU 1, 2
• Measure attenuation using region of interest placement on the adrenal mass 1
• Lesions measuring <10 HU are definitively benign adenomas and require no additional imaging 1, 2, 3
• Histographic analysis can improve sensitivity: if ≥5% of pixels measure <0 HU, the lesion is very likely an adenoma, even after contrast administration 1

When Non-Contrast CT is Indeterminate (>10 HU)

For masses measuring >10 HU on non-contrast CT, proceed to second-line imaging with either delayed contrast-enhanced washout CT or chemical shift MRI. 1, 2

Second-Line Imaging Options

Option 1: Delayed Contrast-Enhanced Washout CT

• Administer intravenous contrast followed by delayed imaging at 15 minutes 1
• Calculate absolute percentage washout: adenomas demonstrate >60% washout at 15 minutes (sensitivity >95%, specificity >97%) 1
• Alternative protocol uses 30-minute delay (sensitivity 97%, specificity 100%) 1

Critical limitations to recognize:

• Approximately 1/3 of pheochromocytomas may washout like adenomas 1
• Approximately 1/3 of adenomas do NOT washout in the adenoma range 1
• Some malignant masses (adrenocortical carcinoma, hypervascular metastases) can washout like adenomas 1

Option 2: Chemical Shift MRI

• Exploits different proton frequencies in water versus fat to detect microscopic lipid 1, 2
• Homogeneous signal intensity drop between in-phase and out-of-phase imaging is diagnostic of lipid-rich adenoma 1, 2
• Particularly useful for lesions with CT density 10-30 HU, where 89% can be correctly characterized 1
• Heterogeneous signal drop is less reliable, as microscopic fat can occur in pheochromocytoma, adrenocortical carcinoma, and some metastases 1

Size-Based Considerations

Lesions <3 cm

• Non-contrast CT with follow-up imaging at 6-12 months if indeterminate 1
• If benign-appearing (<10 HU), no further follow-up required 1

Lesions 3-5 cm

• Non-contrast CT followed by delayed enhancement CT or chemical shift MRI if indeterminate 1
• Follow-up imaging at 3-6 months if initial characterization is benign 1
• Consider biopsy if CT and MRI remain inconclusive 1

Lesions >4-5 cm

• Larger lesions should be removed surgically due to increased malignancy risk 1
• Suspect adrenocortical carcinoma if >4 cm with irregular margins or internal heterogeneity 1
• Use adrenal protocol CT or MRI to assess size, heterogeneity, lipid content, contrast washout, and margin characteristics 1

Special Imaging Protocols

For Surveillance in Cancer Patients

• Diagnostic chest CT that includes the adrenals with contrast (preferred) or without contrast 1
• No added benefit of full abdomen/pelvis CT over chest CT through the adrenals for surveillance 1

Lesions with Characteristic Features

• Myelolipomas and cysts containing macroscopic fat are readily diagnosed by CT imaging features alone and require no further workup 2, 3

Follow-Up Imaging Recommendations

• Benign non-functional adenomas <4 cm with <10 HU: no further imaging or functional testing required 1
• Benign lesions ≥4 cm: repeat imaging in 6-12 months 1
• Indeterminate non-functional lesions: repeat imaging in 3-6 months versus surgical resection (shared decision-making) 1
• Lesions growing >5 mm/year: consider adrenalectomy after repeating functional workup 1
• Lesions growing <3 mm/year: no further imaging required 1

Critical Pitfalls to Avoid

• Never perform adrenal biopsy without first excluding pheochromocytoma biochemically, as biopsy can precipitate fatal hypertensive crisis 1, 2, 3
• Do not routinely biopsy suspected adrenocortical carcinoma due to tumor seeding risk 1
• Biopsy is only appropriate when diagnosis of metastatic disease would change management and non-invasive imaging is inconclusive 1
• FDG-PET is not recommended for routine surveillance and should only be considered if CT and MRI are inconclusive 1
• Some malignancies (including renal cell carcinoma) are not PET-avid 1