CT · Adrenal

Adrenal Washout

Absolute and relative washout from region-of-interest HU values to characterise an adrenal nodule on a dedicated washout CT protocol. Decision-support only — always verify clinically.

Calculation type

Both APW & RPW

Use when a true unenhanced phase is available (preferred).

Absolute washout only (APW)

Use when unenhanced phase is available and you want APW only.

Relative washout only (RPW)

Use when no unenhanced phase is available.

Attenuation values (Hounsfield units)

Unenhanced (pre-contrast) HU

Enhanced HU (≈60–75 s portal venous)

Delayed HU (≈15 min)

Formulae, thresholds & caveats

APW = (Enhanced − Delayed) / (Enhanced − Pre-contrast) × 100

RPW = (Enhanced − Delayed) / Enhanced × 100

Thresholds: APW ≥ 60% and RPW ≥ 40% are suggestive of an adrenal adenoma. Performance figures from Caoili et al. report sensitivity/specificity around 86%/92% (absolute) and 82%/92% (relative) for lesions with pre-contrast attenuation ≥ 10 HU.

Caveats: washout is unreliable in heterogeneous nodules, and roughly 30–45% of phaeochromocytomas overlap with lipid-poor adenomas. Lesions > ~120 HU or with macroscopic fat should not be characterised as adenomas on washout alone.

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Enter attenuation values on the left to see the washout calculation.