Ultrasound and CT in the diagnosis of diffuse steatosis
In the diffuse form of fatty infiltration, ultrasound demonstrates abnormally high parenchymal echogenicity. The adjacent cortex of the right kidney is typically used for comparison—normally their echogenicity is comparable, but in steatosis, the liver becomes significantly brighter.
On unenhanced computed tomography (CT), the main finding is a marked decrease in hepatic X-ray attenuation. This alters the normal attenuation gradient between the liver and spleen, while the intrahepatic vessels appear hyperdense ("bright") against the "dark" parenchyma. At the same time, the overall organ shape and vascular architecture remain preserved.
MRI — the gold standard
MRI is recognized as the most sensitive and specific tool for detecting steatosis. The method allows not only the detection of fat but also its precise quantification.
The key technique is chemical shift imaging based on Dixon's method. In-phase and out-of-phase images are compared. A reliable sign of fat presence is a significant signal drop on out-of-phase images compared to in-phase images. This is especially important for diagnosing focal heterogeneous fatty infiltration.
Focal fat deposition and areas of intact parenchyma
Focal fat deposition (focal fat deposition) and focal areas of healthy tissue against a background of diffuse steatosis (focal fat sparing) often present diagnostic challenges. They can mimic solid tumors: adenomas, focal nodular hyperplasia, or hepatocellular carcinoma.
Features of focal lesions:
- Localization: They are most frequently found in segment IV of the liver (medial segment of the left lobe), typically anteriorly near the falciform ligament or posteriorly on the visceral surface.
- Ultrasound: The lesions have clear margins, but unlike tumors, they do not create a mass effect and do not deform the liver capsule.
- MRI: This is the definitive method to confirm the nature of the lesion. Standard T2-weighted fat-suppressed images will show signal loss from the fatty areas, while gradient-echo in-phase/out-of-phase sequences will confirm the presence of normal liver tissue in the spared zones.
Briefly: answers to topical questions
What are the normal liver attenuation values in Hounsfield units (HU) and how do they change in steatosis on CT?
Normally, the X-ray attenuation of the liver is 54–60 Hounsfield units (HU). In diffuse steatosis, CT shows a marked decrease in parenchymal attenuation.
| Condition | X-ray attenuation on CT |
|---|---|
| Normal | 54–60 HU (8–10 HU higher than splenic attenuation) |
| Steatosis | Marked decrease in attenuation (lower than splenic attenuation) |
Due to the altered normal attenuation gradient on unenhanced CT (unenhanced CT), blood vessels appear hyperdense against the "dark" liver. In the portal venous phase, parenchymal attenuation also remains lower than that of the spleen.
What hepatospecific contrast agents are used in MRI for the differential diagnosis of focal steatosis and HCC?
Gadoxetic acid (gadoxetic acid) is used as a hepatospecific contrast agent in MRI. Focal fat deposition or areas of intact parenchyma can mimic hepatocellular carcinoma (hepatocellular carcinoma), so a contrast-enhanced study is required to rule out a neoplastic process. In the hepatocyte phase (hepatocyte phase) following gadoxetic acid administration, hepatocellular carcinoma demonstrates a typical sign—low signal intensity against the contrast-enhanced healthy liver parenchyma.
Class and exam questions
Why is the echogenicity of the liver compared to the right kidney during a liver ultrasound?
Normally, the echogenicity of the liver parenchyma and the renal cortex is approximately equal. In steatosis, liver echogenicity increases significantly, making it noticeably brighter ("whiter") than the kidney.
How to distinguish focal steatosis from a liver tumor?
Focal steatosis does not cause a mass effect (it does not displace vessels or deform the capsule). Chemical shift MRI or contrast-enhanced studies are used to reliably rule out malignancy.
What happens to liver attenuation on unenhanced CT in fatty liver disease?
The attenuation of the liver parenchyma is markedly reduced, making it appear darker than the spleen, while blood vessels stand out as contrast-enhanced (hyperdense) against it.
What else to study on this topic
- Ultrasound and elastographic signs of liver cirrhosis.
- Changes in hepatic hemodynamics and the steal syndrome in portal hypertension.
- MRI findings in hepatic iron overload (hemochromatosis).
- Focal confluent fibrosis in cirrhosis: CT and MRI semiotics.
- Differential diagnosis of atypical regenerative nodules and hepatocellular carcinoma.
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