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Adrenal Washout Calculator

Q: What is the difference between absolute and relative washout?

Absolute washout calculates contrast clearance as a percentage of the total enhancement above baseline density. It accounts for the lesion's starting density before contrast injection. Relative washout measures clearance purely from peak enhancement, ignoring baseline HU. Both are useful, but absolute washout is more specific for adenoma diagnosis. In practice, they track closely together, but absolute is preferred when available because it anchors the measurement to a physiologic baseline.

Q: Can I diagnose an adrenal adenoma with just one CT scan?

No. A single non-contrast scan cannot assess washout kinetics. You require at least two timed scans—one immediately after contrast and another at 15 minutes—plus a pre-contrast baseline for accurate absolute washout. Some institutions use alternative protocols like 10-minute delay or adrenal-to-spleen ratios, but the three-phase protocol described here remains the gold standard for adenoma characterization.

Q: What should I do if my adrenal lesion has borderline washout (50–59%)?

Borderline washout warrants consideration of other imaging features: lesion size, margins, homogeneity, and attenuation on non-contrast CT. Lesions smaller than 1 cm are almost always benign regardless of washout. Larger lesions or those with atypical features may benefit from follow-up imaging in 3–6 months or consensus review with an experienced radiologist. Clinical context—age, cancer history, symptoms—also guides management.

Q: Why do adenomas wash out contrast faster than metastases?

Adenomas are lipid-rich tumours with typical adrenocortical cells. These cells take up contrast quickly during the arterial phase but are not highly vascular, so contrast washes out rapidly as blood flow clears the lesion. Metastases, by contrast, have abnormal vascularity and cellular density that promotes sustained enhancement. The difference reflects underlying tissue biology rather than size or location, making washout a robust diagnostic criterion.

Q: Is the adrenal washout calculator safe for patients with contrast allergies?

The calculator itself is a mathematical tool—it poses no direct risk. However, the imaging protocol requires intravenous iodinated contrast, which carries a small risk of allergic reaction (roughly 0.1% for modern non-ionic agents). Patients with previous anaphylaxis should inform their radiologist before contrast administration. Premedication with corticosteroids and antihistamines can reduce risk. If contrast allergy is severe, alternative imaging (ultrasound, MRI without gadolinium) may be considered.

Q: Can this calculator work with MRI instead of CT?

The adrenal washout protocol as described is specific to CT and uses Hounsfield units, which are CT density values. MRI uses different signal intensities that cannot be directly converted to HU. However, similar washout principles apply to dynamic contrast-enhanced MRI with gadolinium. Different threshold criteria (not the same percentages) are used for MRI-based adenoma characterization. Always use CT-validated thresholds for CT imaging and MRI-validated criteria for MRI.

Adrenal washout measures how quickly contrast agent leaves an adrenal lesion during CT imaging. Radiologists use this metric to distinguish benign adenomas from suspicious masses like metastases. The technique relies on three timed scans: non-contrast baseline, immediate post-contrast (60–75 seconds), and delayed phase (15 minutes). Higher washout percentages suggest adenoma, reducing unnecessary follow-up imaging and patient anxiety.

Last updated: April 20, 2026

Creators Łucja Zaborowska, MD, PhD

Reviewers Małgorzata Koperska and Adena Benn

7,279 people find this calculator helpful

Understanding Adrenal Washout in CT Imaging

Adrenal lesions discovered incidentally on imaging present a diagnostic challenge. Benign adenomas contain lipid-rich cells that absorb contrast rapidly, then release it quickly as blood flow clears the area. Non-adenomatous lesions—such as metastases or pheochromocytomas—tend to enhance intensely but retain contrast longer because their vascularity and cellular composition differ fundamentally.

The washout protocol quantifies this contrast behaviour across three distinct timepoints. By measuring Hounsfield units (HU), a standardized density scale in CT imaging, radiologists can calculate how completely a lesion has "washed out" the injected contrast material. This creates an objective criterion that reduces observer bias and improves diagnostic confidence.

Two complementary metrics—absolute and relative washout—provide different perspectives. Absolute washout accounts for baseline density before contrast injection, while relative washout measures washout purely against the peak enhancement. Together, they form a robust framework for lesion characterization without requiring additional imaging or biopsy.

Adrenal Washout Formulas

Calculating washout requires Hounsfield unit measurements from all three CT phases. The two standard formulas are:

Absolute washout = 100 × ((Post-contrast HU − Delayed HU) ÷ (Post-contrast HU − Pre-contrast HU))

Relative washout = 100 × ((Post-contrast HU − Delayed HU) ÷ Post-contrast HU)

Pre-contrast HU — Hounsfield units measured on non-contrast baseline CT before any contrast injection
Post-contrast HU — Hounsfield units measured at 60–75 seconds after intravenous contrast administration (arterial-to-portal venous phase)
Delayed HU — Hounsfield units measured at 15 minutes post-contrast injection
Absolute washout — Percentage of contrast clearance relative to the initial baseline density and peak enhancement
Relative washout — Percentage of contrast clearance expressed as a proportion of maximum enhancement

Interpreting Washout Results

Diagnostic thresholds are well-established and reproducible across imaging protocols:

Absolute washout ≥60%: Highly suggestive of a benign adenoma. Very few non-adenomatous lesions achieve this level of washout.
Relative washout ≥40%: Also consistent with adenoma diagnosis. This threshold is slightly more sensitive but less specific than absolute washout.
Lower washout values: Warrant consideration of alternative diagnoses including metastatic disease, pheochromocytoma, or adrenocortical carcinoma. These lesions typically show persistent enhancement due to their vascular and cellular properties.

When washout values are borderline (absolute 50–59% or relative 35–39%), clinical correlation, imaging appearance, and follow-up imaging may be needed. Lesions smaller than 1 cm are often managed conservatively given the extremely low malignancy risk in that size range.

Adrenal Adenomas and Incidentalomas

Adrenal adenomas are benign tumours found in roughly 4–7% of abdominal CTs in middle-aged and older populations. Most are non-functioning and cause no hormonal symptoms. When discovered by chance during imaging for an unrelated condition, they are termed incidentalomas.

A minority of adenomas produce excess cortisol (Cushing syndrome), aldosterone (primary hyperaldosteronism), or catecholamines (pheochromocytoma). These functioning tumours typically present with clinical signs that prompt investigation. Regardless of function, the imaging appearance—particularly washout kinetics—determines whether further monitoring or intervention is needed.

The prevalence of adrenal incidentalomas increases with age and imaging frequency. This creates a practical need for objective diagnostic criteria like washout analysis. By confidently identifying benign lesions, the washout protocol reduces unnecessary repeat imaging, specialist referrals, and patient concern, while flagging genuinely suspicious masses for appropriate management.

Key Considerations for Washout Measurement

Accurate washout calculation depends on precise technique and careful result interpretation.

Scan timing consistency — The 60–75 second post-contrast timing is critical—this window captures portal venous phase enhancement when adenomas are maximally enhanced. Scans acquired too early (arterial phase) or too late (equilibrium) will skew washout percentages. Always verify acquisition times in your imaging report.
Hounsfield unit accuracy — Measurement location within the lesion matters. Select a region of interest that avoids fat planes, necrosis, or calcification. Small positioning differences can shift HU values by 10–20 units, especially in heterogeneous lesions. Use consistent technique across all three phases.
Borderline results require context — Washout values between 50–59% (absolute) or 35–39% (relative) are not diagnostically conclusive. Assess lesion size, morphology, and imaging characteristics. Lesions <1 cm with borderline washout are almost always benign. Larger lesions or those with atypical features may warrant follow-up imaging.
Non-contrast baseline cannot be omitted — Some protocols skip the non-contrast phase to reduce radiation. This prevents calculation of absolute washout, leaving only relative washout, which is less specific. Absolute washout is superior for adenoma diagnosis and justifies the modest additional dose.

Frequently Asked Questions

What is the difference between absolute and relative washout?

Absolute washout calculates contrast clearance as a percentage of the total enhancement above baseline density. It accounts for the lesion's starting density before contrast injection. Relative washout measures clearance purely from peak enhancement, ignoring baseline HU. Both are useful, but absolute washout is more specific for adenoma diagnosis. In practice, they track closely together, but absolute is preferred when available because it anchors the measurement to a physiologic baseline.

Can I diagnose an adrenal adenoma with just one CT scan?

No. A single non-contrast scan cannot assess washout kinetics. You require at least two timed scans—one immediately after contrast and another at 15 minutes—plus a pre-contrast baseline for accurate absolute washout. Some institutions use alternative protocols like 10-minute delay or adrenal-to-spleen ratios, but the three-phase protocol described here remains the gold standard for adenoma characterization.

What should I do if my adrenal lesion has borderline washout (50–59%)?

Borderline washout warrants consideration of other imaging features: lesion size, margins, homogeneity, and attenuation on non-contrast CT. Lesions smaller than 1 cm are almost always benign regardless of washout. Larger lesions or those with atypical features may benefit from follow-up imaging in 3–6 months or consensus review with an experienced radiologist. Clinical context—age, cancer history, symptoms—also guides management.

Why do adenomas wash out contrast faster than metastases?

Adenomas are lipid-rich tumours with typical adrenocortical cells. These cells take up contrast quickly during the arterial phase but are not highly vascular, so contrast washes out rapidly as blood flow clears the lesion. Metastases, by contrast, have abnormal vascularity and cellular density that promotes sustained enhancement. The difference reflects underlying tissue biology rather than size or location, making washout a robust diagnostic criterion.

Is the adrenal washout calculator safe for patients with contrast allergies?

The calculator itself is a mathematical tool—it poses no direct risk. However, the imaging protocol requires intravenous iodinated contrast, which carries a small risk of allergic reaction (roughly 0.1% for modern non-ionic agents). Patients with previous anaphylaxis should inform their radiologist before contrast administration. Premedication with corticosteroids and antihistamines can reduce risk. If contrast allergy is severe, alternative imaging (ultrasound, MRI without gadolinium) may be considered.

Can this calculator work with MRI instead of CT?