STEATOSIS FATTY INFILTRATION LIVER

DIFFUSE LIVER CONDITIONS
Diseases which diffusely affect the liver may have very non-specific ultrasound appearances. Suspicion is usually raised following altered LFTs and the diagnosis made histologically. A number of diffuse liver conditions can cause hepatocellular (or non-obstructive) jaundice which is associated with increased levels of unconjugated bile in the blood. Many of these can be demonstrated with ultrasound, others cannot. The main role of ultrasound in the jaundiced patient is to exclude any obstructive cause (by the presence or absence of biliary duct dilatation) and to search for liver metastases or signs of a diffuse liver condition.

Fatty infiltration (steatosis)
The process of accumulation of fat within the hepatic cells may be either focal (see above) or diffuse. Related to various conditions such as alcoholism, obesity and diabetes, it is associated with any process which alters liver metabolism and it is reversible in many circumstances. The acoustic properties of fat differ from those of normal liver tissue. The liver appears hyperechoic as the fat globules provide interfaces which are highly reflective.

As the level of fat deposition increases, the level of echogenicity may reach that of the highly reflective portal tract walls. This has the effect of reducing the prominence of the portal tracts and making the liver appear smooth and homogeneous, with closely packed, fine echoes. The contrast between the liver and parenchyma of the right kidney is therefore increased (a particularly useful sign confirming that the correct gain settings have been used). Hepatomegaly is also a feature, though not invariably.

Finally, the attenuation of fat is greater than that of normal liver tissue; this has the effect of reduced penetration in the far field, rather as if the time gain compensation (TGC) paddles or slope control had been incorrectly set. In severe cases of infiltration, most of the sound is reflected back to the transducer in the first few centimetres, creating a highly reflective near-field band through which the sound is unable to penetrate.

Fatty infiltration itself is not usually a significant finding; however it often occurs in conjunction with other significant diffuse processes such as cirrhosis. Its increased attenuation reduces the ability of ultrasound to exclude other disease or focal lesions and therefore CT is often a useful adjunct.