On contrast-enhanced CT, which imaging feature is most specific for diagnosing hepatic steatosis?

Explanation

## Hepatic Steatosis on Contrast-Enhanced CT **Key Point:** The most specific CT finding for hepatic steatosis is a decrease in liver attenuation relative to the spleen (liver-to-spleen ratio <1.0), with preserved normal enhancement of the portal vein and hepatic vessels. ### Quantitative and Qualitative CT Criteria for Steatosis | Finding | Specificity | Notes | | --- | --- | --- | | Liver HU < Spleen HU (liver-to-spleen ratio <1.0) | 95% | Most specific; requires unenhanced and arterial phase imaging | | Liver HU < 40 on unenhanced CT | 85% | Suggests moderate-to-severe steatosis | | Normal vascular enhancement | High | Rules out cirrhosis or fibrosis | | Focal sparing (segment 4, hilum) | Variable | Can mimic mass; benign variant | | Hepatomegaly | Non-specific | Common but seen in many conditions | **High-Yield:** Steatosis is diagnosed when the liver parenchyma becomes hypoattenuating relative to the spleen. This is because fat has lower density than normal hepatic tissue. The threshold is typically liver attenuation 10–20 HU lower than spleen. **Mnemonic:** **FAT** = **F**at decreases **A**ttenuation, **T**hus liver < spleen. ### Differential Features - **Cirrhosis:** Liver may be hypoattenuating, but portal hypertension, ascites, and delayed enhancement are present. - **Hepatitis:** Liver may be hypoattenuating acutely, but periportal edema and ascites are more prominent. - **Focal lesion:** Steatosis is diffuse and bilateral; focal areas suggest mass or focal sparing. **Clinical Pearl:** Focal fatty sparing (especially in segment 4 near the hilum) is a benign variant and should not be mistaken for a mass. It follows vascular territories and has normal enhancement. **Warning:** A single low liver attenuation value without comparison to spleen is non-specific. Always compare liver-to-spleen ratio; this is the gold standard CT sign for steatosis. [cite:Robbins & Cotran Pathologic Basis of Disease 10e Ch 18; Grainger & Allison's Diagnostic Radiology 6e]