Abdominal Imaging
Live dataCT Adrenal Washout
Characterises an indeterminate adrenal nodule as a lipid-poor adenoma using absolute and relative contrast washout on multiphase CT.
Adenomas characteristically de-enhance (wash out) contrast faster than non-adenomas. Absolute percentage washout (APW) requires an unenhanced series; relative percentage washout (RPW) can be computed from the enhanced and delayed phases alone. If the unenhanced attenuation is ≤10 HU the lesion is already a lipid-rich adenoma and washout analysis is unnecessary.
📐 Formula
APW = (enhanced − delayed) / (enhanced − unenhanced) × 100; RPW = (enhanced − delayed) / enhanced × 100
🎯 When to Use
- Indeterminate adrenal nodule that is not diagnostically lipid-rich (unenhanced >10 HU).
- Dedicated adrenal CT protocol with unenhanced, portal-venous and 15-minute delayed phases.
- Incidental adrenal mass requiring benign/malignant characterisation without biopsy.
⚙️ How It Works
- Measure the mean attenuation of the nodule on each available phase using an ROI over most of the lesion.
- An unenhanced value ≤10 HU indicates a lipid-rich adenoma; no washout calculation is needed.
- APW = (enhanced − delayed) / (enhanced − unenhanced) × 100 when the unenhanced phase is available.
- RPW = (enhanced − delayed) / enhanced × 100 when only enhanced and delayed phases are available.
📊 Interpreting the Result
- APW ≥ 60% is diagnostic of a lipid-poor adenoma (high specificity).
- RPW ≥ 40% is diagnostic of a lipid-poor adenoma when no unenhanced series exists.
- Values below these thresholds are indeterminate and warrant further work-up (chemical-shift MRI, PET, follow-up or biopsy).
⚠️ Limitations & Pitfalls
- Washout thresholds are less reliable for large (>4 cm), heterogeneous or necrotic masses.
- Some hypervascular metastases, pheochromocytomas and adrenal cortical carcinomas can show high washout, mimicking adenoma.
- Requires standardised phase timing; mistimed acquisitions invalidate the calculation.
📚 References
- Caoili EM, Korobkin M, Francis IR, et al. Adrenal masses: characterization with combined unenhanced and delayed enhanced CT. Radiology. 2002;222(3):629-633.
- Boland GW, Blake MA, Hahn PF, Mayo-Smith WW. Incidental adrenal lesions: principles, techniques, and algorithms for imaging characterization. Radiology. 2008;249(3):756-775.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.