Adrenal CT washout calculator
Enter the attenuation values from your triphasic adrenal CT to compute absolute and relative washout. The adenoma thresholds, the ≤10 HU unenhanced rule, and the caveats where washout misleads (pheochromocytoma, hypervascular metastases) are reproduced below so you can check the reasoning.
The thresholds
| Measurement | Formula | Adenoma threshold |
|---|---|---|
| Unenhanced attenuation | — | ≤ 10 HU = lipid-rich adenoma (no washout needed) |
| Absolute washout (APW) | (Enhanced − Delayed) / (Enhanced − Unenhanced) × 100 | ≥ 60% |
| Relative washout (RPW) | (Enhanced − Delayed) / Enhanced × 100 | ≥ 40% |
When washout misleads
| Pitfall | Why it matters |
|---|---|
| Pheochromocytoma | Can be lipid-poor AND show washout above adenoma thresholds — biochemical screening trumps washout when clinically suspected. |
| Hypervascular metastases | RCC and HCC metastases may mimic adenoma washout; in a patient with a known hypervascular primary, washout is unreliable. |
| Size and heterogeneity | Washout characterisation is intended for homogeneous lesions 1-4 cm. Lesions >4 cm, heterogeneous, necrotic or haemorrhagic lesions need dedicated workup regardless of washout. |
| Protocol | Enhanced HU at 60-75 s portal venous phase; delayed HU at 15 min (10-15 min acceptable). Different delays shift the thresholds the literature validated. |
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Source. Caoili EM, et al. Radiology 2002;222(3):629-633; Mayo-Smith WW, et al. J Am Coll Radiol 2017;14(8):1038-1044. View on PubMed
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