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Abdominal Imaging
Demonstration data

CT 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.

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📐 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.