Hepatic Steatosis (CT)
Estimates hepatic fat using absolute liver attenuation and the liver–spleen difference on non-contrast CT.
Fat lowers the CT attenuation of the liver. On non-contrast CT a normal liver measures 50–65 HU and is a few HU denser than the spleen. A low absolute liver value, or a liver that is substantially less dense than the spleen, indicates hepatic steatosis.
📐 Formula
L − S = liver attenuation − spleen attenuation (HU)
🎯 When to Use
- Incidental assessment of hepatic fat on any non-contrast abdominal CT.
- Screening potential living liver donors and follow-up of fatty liver disease.
- Distinguishing diffuse steatosis from focal fat sparing/deposition when spleen serves as an internal reference.
⚙️ How It Works
- Place ROIs in uniform hepatic parenchyma, avoiding vessels, and in the spleen on the same non-contrast series.
- Record the mean liver and spleen attenuation in HU.
- Compute the liver–spleen difference (L − S).
- Interpret using both the absolute liver value and L − S.
📊 Interpreting the Result
- Liver–spleen difference < −10 HU (liver more than 10 HU less dense than spleen) indicates moderate-to-severe steatosis.
- Absolute liver attenuation < 40 HU also indicates moderate-to-severe steatosis.
- Absolute liver attenuation < 48 HU corresponds to roughly >30% macrovesicular fat.
⚠️ Limitations & Pitfalls
- Only non-contrast CT thresholds are valid; contrast enhancement invalidates absolute cut-offs.
- Iron overload, amiodarone, copper or glycogen raise liver density and can mask coexisting steatosis.
- CT is insensitive to mild steatosis; MRI proton-density fat fraction is more accurate for low fat levels.
📚 References
- Kodama Y, Ng CS, Wu TT, et al. Comparison of CT methods for determining the fat content of the liver. AJR Am J Roentgenol. 2007;188(5):1307-1312.
- Boyce CJ, Pickhardt PJ, Kim DH, et al. Hepatic steatosis (fatty liver disease) in asymptomatic adults identified by unenhanced low-dose CT. AJR Am J Roentgenol. 2010;194(3):623-628.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.