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 with large areas of necrosis or hemorrhage cannot be characterized by their washout pattern 6.
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