1. What is Washout Percentage?

The washout percentage is a key parameter in adrenal gland CT imaging used to differentiate adrenal adenomas from other adrenal masses. It measures the rate of contrast material washout from the adrenal lesion.

2. How Does the Calculator Work?

The calculator uses the washout percentage formula:

Where:

• \( \text{Peak} \) — Peak enhancement value in Hounsfield Units (HU)
• \( \text{Delayed} \) — Delayed phase enhancement value in HU
• \( \text{Pre} \) — Pre-contrast enhancement value in HU

Explanation: This formula calculates the percentage of contrast washout between the peak enhancement phase and the delayed phase, normalized by the overall enhancement from pre-contrast levels.

3. Importance of Washout Calculation

Details: Washout percentage is crucial for characterizing adrenal masses. Adenomas typically show rapid washout (>60% absolute washout, >40% relative washout), while malignant lesions show slower washout.

4. Using the Calculator

Tips: Enter all three HU values (Peak, Delayed, Pre) measured from adrenal CT scans. Ensure measurements are taken from the same region of interest in the adrenal mass.

5. Frequently Asked Questions (FAQ)

Q1: What is considered diagnostic for adrenal adenoma?
A: Absolute washout >60% or relative washout >40% is highly suggestive of adrenal adenoma.

Q2: What are the typical CT phases for washout calculation?
A: Pre-contrast, portal venous phase (60-70 seconds for peak), and delayed phase (15 minutes).

Q3: Can washout percentage differentiate all adrenal masses?
A: While highly accurate for adenoma vs metastasis, some pheochromocytomas and rare tumors may show atypical washout patterns.

Q4: What are limitations of washout calculation?
A: Less reliable for small lesions (<1 cm), heterogeneous masses, or with technical variations in CT acquisition.

Q5: How does this relate to chemical shift imaging?
A: Washout calculation and chemical shift MRI are complementary techniques for adrenal mass characterization.