Lipid Poor Adrenal Adenoma

Facts

• Most adenomas can be characterized as lipid rich on precontrast scan, which will show attenuation value of less than 10 HU
• Some adenomas are lipid poor, therefore demonstrate a higher attenuation value. This can be further characterized by performing an adrenal mass CT protocol
• Adrenal mass CT consists of 1) precontrast, 2) portovenous phase postcontrast (60-70 sec) and 3) delayed phase (10-15 min)
• Measurement of region of interests (ROI) over > half of the mass at different phases. Values are used to calculate absolute and relative percentage washout

Absolute Percentage Washout (APW) = 100 x (portovenous attenuation - delayed attenuation)/(delayed attenuation - precontrast attenuation)

Relative Percentage Washout (RPW) = 100 x (portovenous attenuation - delayed attenuation)/delayed attenuation

Adenomas typically show rapid washout

• APW > 60% OR
• RPW > 40%

Reference:

Johnson PT, Horton KM, Fishman EK. Adrenal imaging with multidetector CT: evidence-based protocol optimization and interpretative practice. RadioGraphics 2009;29:1319-1331.