About adrenal CT washout
Adrenal washout is a CT technique used to characterize an adrenal nodule (an adrenal incidentaloma) by measuring how quickly contrast material leaves the lesion. It compares the attenuation of the mass, in Hounsfield units (HU), across three phases: an unenhanced scan, a contrast-enhanced scan taken about 60 seconds after injection, and a delayed scan taken roughly 15 minutes later. Benign lipid-poor adenomas take up and release contrast quickly, so they wash out faster than most non-adenomas.
The two washout formulas
- Absolute percentage washout (APW): (Enhanced HU − Delayed HU) ÷ (Enhanced HU − Unenhanced HU) × 100. It needs all three phases and is the preferred measure when an unenhanced scan is available.
- Relative percentage washout (RPW): (Enhanced HU − Delayed HU) ÷ Enhanced HU × 100. It uses only the enhanced and delayed phases, so it is applied when no unenhanced scan was obtained.
How to interpret the numbers
An APW of 60% or greater, or an RPW of 40% or greater, is the commonly used threshold consistent with a benign adrenal adenoma. Values below those thresholds are considered indeterminate and may warrant further imaging, follow-up, or additional workup. A separate rule also helps: an unenhanced attenuation of 10 HU or less already indicates a lipid-rich adenoma, so washout is most useful for lipid-poor masses whose unenhanced density is above 10 HU.
Measurement notes and limits
Place the region of interest over the same part of the lesion on each phase, avoiding the edge, calcification, or necrosis, so the HU values are comparable. Washout is a density-based estimate, not a diagnosis: some pheochromocytomas and, uncommonly, malignant lesions can also show high washout. Interpret every result with a radiologist alongside the clinical picture, lesion size, any prior imaging, and hormonal evaluation where relevant.
Frequently Asked Questions
What is adrenal washout on CT?
What is the difference between absolute and relative washout?
What washout values suggest a benign adenoma?
Can adrenal washout be wrong?
Common Tip Percentages
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| Restaurant (sit-down) | 15–20% | 20%+ for exceptional service |
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General Household Budget Ratios (50/30/20 Rule)
| Category | % of Net Income | Examples |
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| Needs | 50% | Rent, groceries, utilities, insurance |
| Wants | 30% | Dining out, entertainment, hobbies |
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Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |