Adrenal washout can be calculated using the density value of an adrenal mass on non-enhanced, portal venous phase and 15 minutes delayed CT scans (density measured in Hounsfield units (HU)). It is primarily used to diagnose adrenal adenoma.
- absolute washout
- [(HUportal venous phase) - (HUdelayed)] / [(HUportal venous phase) - (HUnon-enhanced)] x 100
- >60% washout is highly suggestive of adrenal adenoma
- relative washout
- [(HUportal venous phase) - (HUdelayed)] / (HUportal venous phase) x 100
- >40% washout is highly suggestive of adrenal adenoma
Pheochromocytomas may also wash out, but should have a different clinical presentation. They also may have a higher absolute attenuation on the contrast phase (arterial or portal venous). Any adrenal lesion >120 HU with washout should not be diagnosed as an adenoma.
Lesions that are inhomogeneous or with large areas of necrosis or hemorrhage cannot be characterized by their washout pattern 6.
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