Adrenal Washout (CT) Calculator
Calculate absolute washout and relative washout of adrenal lesions from three-phase CT Hounsfield Unit (HU) values — with adenoma criteria interpretation, step-by-step formula breakdown, and example cases.
Enter HU values
Non-contrast phase (pre-contrast).
Portal venous phase (± 60–90 seconds after contrast).
Delayed phase, usually 15 minutes after contrast.
(adenoma criterion ≥ 60%)
(adenoma criterion ≥ 40%)
Example cases
Click to load values into the calculator above.
Unenhanced 10 HU, portal venous 80 HU, delayed 30 HU → absolute 71.43%, relative 62.50% → meets adenoma criteria.
Unenhanced 10 HU, portal venous 100 HU, delayed 70 HU → absolute 33.33%, relative 30.00% → below both thresholds, needs further evaluation.
Three CT phases in the adrenal protocol
| Phase | Timing | Role in the formula |
|---|---|---|
| Unenhanced | No contrast | Denominator of absolute washout; also assesses intracellular fat content (≤ 10 HU suggestive of lipid-rich adenoma). |
| Portal venous | ± 60–90 seconds | Peak enhancement value; numerator of both formulas and denominator of relative washout. |
| Delayed | 15 minutes | Measures how fast contrast washes out; numerator of both formulas. |
Adenomas wash out contrast quickly (high washout). Non-adenoma lesions (e.g., metastasis, adrenocortical carcinoma, pheochromocytoma) tend to retain contrast, giving low washout or even progressive enhancement.
Formulas
Absolute washout = (HU portal venous − HU delayed) ÷ (HU portal venous − HU unenhanced) × 100%
Relative washout = (HU portal venous − HU delayed) ÷ HU portal venous × 100%
Interpretation: absolute ≥ 60% or relative ≥ 40% → consistent with adenoma (benign). Below both thresholds → indeterminate, needs further evaluation by a radiologist.
Frequently asked questions (FAQ)
What is adrenal washout?
What is the adrenal absolute washout formula?
What is the adrenal relative washout formula?
What washout thresholds are consistent with adenoma?
When should relative washout be used instead of absolute washout?
Can this calculator diagnose an adrenal tumor?
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