Adrenal Washout
Calculate absolute and relative adrenal washout from CT Hounsfield Unit values to characterize adrenal masses. Enter pre-contrast, post-contrast, and delayed HU for results.
About This Calculator
The Adrenal Washout Calculator estimates absolute and relative washout percentages using CT Hounsfield Unit (HU) measurements to characterize adrenal masses and differentiate benign adrenal adenomas from non-adenomatous lesions such as metastases, pheochromocytomas, and adrenocortical carcinomas.
Adrenal adenomas — both lipid-rich and lipid-poor — rapidly wash out contrast material, whereas non-adenomas generally enhance quickly but take longer to wash out. The adrenal washout protocol consists of three CT acquisitions: a non-contrast scan, a contrast-enhanced scan with a 60-75 second delay (portal venous phase), and a delayed scan at 15 minutes.
Two formulas are used: Absolute Washout = 100 × (Post HU − Delayed HU) / (Post HU − Pre HU) and Relative Washout = 100 × (Post HU − Delayed HU) / Post HU. An absolute washout higher than 60% or a relative washout higher than 40% is strongly suggestive of an adrenal adenoma. A pre-contrast HU above 43 is suspicious for malignancy regardless of washout. Lesions with HU exceeding 120 on any phase should not be diagnosed as adenomas.
This tool is designed for radiologists, clinicians, and medical students who need quick washout calculations during CT interpretation. Always correlate results with clinical history, laboratory findings, and full imaging features. Washout characteristics have reduced specificity in patients with renal cell carcinoma or hepatocellular carcinoma with hypervascular metastases.
Frequently Asked Questions
What is adrenal washout in CT imaging?
Adrenal washout refers to the rate at which contrast material clears from an adrenal mass after injection. Adenomas typically wash out contrast rapidly, while malignant lesions and metastases tend to retain contrast longer. The washout percentage is calculated using Hounsfield Unit (HU) measurements from pre-contrast, post-contrast (60-75 seconds), and delayed (15 minute) CT scans.
What do the absolute and relative washout percentages mean?
Absolute washout uses all three HU measurements: 100 × (post HU - delayed HU) / (post HU - pre HU). Relative washout uses only post-contrast and delayed values: 100 × (post HU - delayed HU) / post HU. An absolute washout greater than 60% or a relative washout greater than 40% is strongly suggestive of a benign adrenal adenoma.
How accurate is this adrenal washout calculator?
This calculator uses the standard adrenal washout formulas published in radiology literature and validated in clinical studies (Radiopaedia, AJR). The absolute and relative washout calculations are mathematically exact. However, results should always be interpreted by a qualified radiologist or clinician in the context of the full clinical picture.
Can I rely on this for medical diagnosis of adrenal masses?
No. This calculator is for educational and reference purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment by a qualified healthcare provider. Adrenal mass characterization requires correlation with clinical history, laboratory findings, and expert radiologist interpretation.
What if my pre-contrast HU is above 43?
A non-calcified, non-hemorrhagic adrenal lesion with a pre-contrast HU greater than 43 is suspicious for malignancy regardless of the washout value. Washout characteristics should not be the sole criterion for diagnosis in such cases, and further evaluation including biopsy or advanced imaging may be necessary.
Are there any limitations to adrenal washout measurements?
Yes. Washout measurements have reduced specificity in patients with renal cell carcinoma or hepatocellular carcinoma with hypervascular metastases. Pheochromocytomas may also demonstrate washout mimicking adenomas. Inhomogeneous lesions with large areas of necrosis or hemorrhage should not be evaluated using washout criteria. Lesions with HU above 120 on any phase are not consistent with adenoma.
What Hounsfield Unit values are typically seen in adrenal adenomas?
Lipid-rich adenomas typically show pre-contrast attenuation of 10 HU or less. Lipid-poor adenomas have pre-contrast attenuation between 10 and 43 HU. Both types demonstrate rapid contrast washout with absolute washout exceeding 60% or relative washout exceeding 40%. The presence of intracytoplasmic lipid in pheochromocytomas with pre-contrast HU below 10 is extremely rare.