Adrenal CT washout calculator
Characterise an incidental adrenal nodule from its CT attenuation. Enter the unenhanced, portal-venous and 15-minute delayed Hounsfield units to get the absolute and relative percentage washout, the adenoma thresholds, and a report line you can paste straight into your CT report.
Use a region of interest over most of the nodule on each phase. Without an unenhanced phase the tool reports the relative washout only. An unenhanced value of 10 HU or less is already a lipid-rich adenoma.
Report line
How adrenal CT washout works
An adrenal protocol acquires an unenhanced phase, a portal-venous phase (about 60–75 seconds) and a delayed phase at 15 minutes. Adenomas enhance and then wash out contrast faster than most non-adenomas, so the percentage washout separates a benign adenoma from an indeterminate nodule.
Relative washout (RPW) = (enhanced − delayed) ÷ enhanced × 100
Thresholds
| Finding | Interpretation |
|---|---|
| Unenhanced ≤ 10 HU | Lipid-rich adenoma (benign); washout not required |
| Absolute washout ≥ 60% | Lipid-poor adenoma (benign) |
| Relative washout ≥ 40% | Lipid-poor adenoma (benign), when no unenhanced phase |
| Below the thresholds | Indeterminate — further work-up may be needed |
The washout criteria assume a homogeneous, non-calcified nodule without macroscopic fat. Some non-adenomas — including pheochromocytoma and occasional metastases — can also show washout, so read the numbers with the morphology and the clinical context.
Sources: 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.