Pre-contrast
HU
60-75 sec post-contrast
HU
15 min delayed
HU
🤓 HU (Hounsfield Unit) is a relative quantitative measurement of radio density used in the interpretation of CT images.
Results
Absolute washout
%
An absolute washout higher than 60% is strongly suggestive of adrenal adenoma.
Relative washout
%
Relative washout higher than 40% is strongly suggestive of adrenal adenoma.

Adrenal Washout Calculator

By Joanna Michałowska, PhD candidate

The adrenal washout calculator estimates adrenal washout using a non-contrast scan, a contrast-enhanced scan with a delay of 60-75 seconds, and a delayed scan at 15 minutes. We use it primarily to diagnose adrenal adenoma.

If you find this calculator helpful, then check out our other nephrology tools. With the GFR calculator you can find the glomerular filtration rate to diagnose kidney malfunction, and the FENa calculator can help you establish the cause of acute kidney failure.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment.

The adrenal washout calculator

Adenomas (both lipid-rich and lipid-poor) rapidly wash out contrast, whereas non-adenomas (e.g., metastases) generally enhance quickly but take longer to washout. Therefore, you can use the adrenal washout formula to distinguish adrenal adenomas from non-adenomas.

Adrenal washout protocol consists of three elements:

  1. A non-contrast scan;
  2. A contrast-enhanced scan with a delay of 60-75 seconds; and
  3. A delayed scan at 15 minutes.

Interpretation and characteristic:

  • Absolute washout higher than 60% is highly suggestive of an adrenal adenoma;

  • Relative washout higher than 40% is highly suggestive of an adrenal adenoma;

  • Any adrenal lesion higher than 120 HU (Hounsfield Unit) with washout should not be diagnosed as an adenoma;

  • Pheochromocytomas may also wash out, however the presence of intracytoplasmic lipid in pheochromocytomas (< 10 HU on non-contrast CT) is extremely rare;

  • In patients with renal cell carcinoma, hepatocellular carcinoma (and other tumors with hypervascular metastases) washout measurements may have significantly reduced specificity;

  • A non-calcified, non-hemorrhagic lesion with pre-contrast HU higher than 43 is suspicious for malignancy, regardless of the washout value; and

  • Washout characteristics should not be used to diagnose inhomogeneous lesions with large areas of necrosis or hemorrhage.

Adrenal washout formula

There are two Adrenal washout formulas:

Absolute = 100 * [(post HU - delayed HU)/(post HU - pre HU)] and

Relative = 100 * [(post HU - delayed HU)/post HU]

where,

Absolute - absolute washout;

Relative - relative washout;

post HU - post-contrast Hounsfield units;

delayed HU - delayed phase Hounsfield units; and

pre HU - pre-contrast Hounsfield units.

In this adrenal washout calculator we present both values using percentages. You can find the interpretation of the results below the calculator.

Adrenal adenoma

Adrenal adenoma, or, in other words, adrenocortical adenoma, is a benign tumor of the adrenal glands. Adrenal adenomas can either be functioning (hormonally active) or non-functioning (hormonally inactive). As they are usually asymptomatic, most reported cases are diagnosed fortuitously through autopsy, or during medical imaging (CT scan, magnetic resonance, etc). Therefore, sometimes they are called "adrenal incidentalomas". Hormonally active adenomas can present symptoms, including Cushing syndrome and hypertension.

Based on CT scan findings and autopsy series, the prevalence of adrenal incidentalomas ranges from 0.35% to 2.3%. It is slightly more often diagnosed in women than men, and the mean age of diagnosis is 57 years. The prognosis for patients with adrenal adenoma is good, and non-functioning adrenal adenomas rarely need treatment. Complications may occur for hormonally active lesions, which include the Cushing syndrome, virilization, or hyperaldosteronism.

Adrenal calculator - a practical example

Let's calculate an example adrenal washout with the adrenal washout calculator. We will use the following values:

  • Pre-contrast: 32 HU,

  • Post-contrast: 192 HU, and

  • Delayed phase: 105HU

  1. Calculating absolute adrenal washout:

Absolute = 100 * [(post HU - delayed HU)/(post HU - pre HU)]

Absolute = 100 * [(192 - 105)/(192 - 32)]

Absolute = 100 * (87/160)

Absolute = 100 * (87/160)

Absolute = 100 * 0.54

Absolute = 54%

  1. Calculating relative adrenal washout:

Relative = 100 * [(post HU - delayed HU)/post HU]

Relative = 100 * [(192 - 105)/192]

Relative = 100 * (87/192)

Relative = 100 * 0.45

Relative = 45%

Because relative washout is higher than 40%, the result is strongly suggestive of adrenal adenoma.

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Adrenal Washout Calculator