Gastrointestinal Imaging

Fatty infiltration, hepatic

(also called fatty metamorphosis of the liver and steatosis of the liver), an excessive, but reversible, accumulation of triglycerides in the cytoplasma of the hepatocytes, exceeding the normal 5% of liver weight. Fatty liver infiltration is associated with a variety of conditions and diseases. Most common are excessive alcohol consumption, obesity, parenteral nutrition and steroids administration or excessive endogeneous production of steroids.

Alteration or disruption of one or more steps in hepatic lipid metabolism may result in abnormal accumulation of triglycerides in the hepatocytes. Fatty infiltration of the liver may be diffuse affecting the whole liver but is commonly nonuniform or focal. It is assumed that focal steatosis is related to regional differences or disturbances in hepatic blood flow. Focal steatosis is usually observed in the liver tissue adjacent to the fissure of the ligamentum teres. Focal sparing, which is defined as a small area of normal liver parenchyma spared by the process of fatty infiltration within a liver that is globally steatotic, is frequently observed. Focal sparing occurs usually in areas such as the liver parenchyma adjacent to the gallbladder bed, the medial segment of the left liver lobe, the liver hilum ventral to the bifurcation of the main branch of the portal vein and the subcapsular areas.

Fatty infiltration of the liver usually will not cause any clinical symptomatology, although in some persons, vague upper quadrant pain may be reported. On clinical examination hepatomegaly may be noted and an elevation of enzymes such as transaminases alkaline phosphatases may be present.

Fatty infiltration can be well visualized by radiological imaging and is a common finding. Ultrasonography will demonstrate the following typical features in patients with diffuse fatty infiltration:

  • global increased echogenicity of the liver parenchyma;

  • suboptimal display of the intrahepatic vasculature; and

  • poor visualization of the posterior portions of the liver because of the increased attenuation of the ultrasound beam by the fatty infiltrated liver.

    Focal fatty liver infiltration is identified by ultrasound and CT on the base of its geometric or wedge shape (segmental or subsegmental involvement) (Fig.1). More rarely the areas of focal fatty infiltration have a round or ovoid configuration which may suggest the presence of a space-occupying lesion (Fig.2). However, the presence of nondistorted nor displaced vessels coursing through the steatotic area, as well as the fact that the findings are seen at the sites of predilection for focal steatosis will help in the diagnosis.

    • CT will display in the generalized form of fatty infiltration, a decrease in mean hepatic attenuation values proportional to the degree of increase of the hepatic triglycerids, and is thus a very reliable method to assess the degree of fatty infiltration in the individual patient. In addition, the intrahepatic vessels are more clearly distinguished as hyperattenuating structures from the surrounding liver parenchyma because of the decreased attenuation of the liver parenchyma. Whereas in normal individuals the attenuation value of the liver is slightly higher than that of the spleen, in fatty liver infiltration the attenuation values of both organs tend to be equal or the ratios may be reversed. Focal fatty infiltration is distinguished on CT from space occupying lesions by the fact that focal fatty infiltration does not cause any mass effect nor contour deformation of the organ. Moreover, following intravenous contrast medium administration during the vascular phase of enhancement, it is noted that the intrahepatic vessels follow their normal course through the lesion without deformity. Finally, the typical location of the anomaly also points to focal fatty infiltration. Focal sparing is identified on ultrasound (Fig.3a) as hypoechoic areas within a hyperechoic liver. These areas have an ovoid, round or even linear shape. Their location at typical sites is also helpful. Focal sparing is recognized on CT (Fig. 3b) as a geographic area of relatively higher attenuation seen in a typical location. Diffuse fatty liver infiltration on CT can contribute to render liver metastasis and dilated intrahepatic bile ducts less conspicuous because the attenuation difference between liver and lesion or bile ducts decreases. On the other hand hyperattenuating lesions following intravenous contrast medium administration will become more conspicuous because of the increase of attenuation difference with the lower enhancing steatotic liver.

    MRI can greatly contribute to the correct diagnosis in doubtful cases in order to differentiate focal fatty infiltration or focal sparing from focal tumoural processes. On T1-weighted images, areas of fatty infiltration will show an increased signal intensity. Chemical shift imaging (CSI), which exploits the difference in resonance frequency between the protons present in the fatty acids and protons present in water will readily allow a correct diagnosis.

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    Fig.1

    Focal fatty infiltration, hepatic. CT of the liver without contrast medium administration. There is a segmental area of markedly decreased attenuation in the right liver lobe.
    Fatty infiltration, hepatic, Fig.1
    Fatty infiltration, hepatic, Fig.2 (a)
    Fatty infiltration, hepatic, Fig.2 (b)
    Fatty infiltration, hepatic, Fig.3 (a)
    Fatty infiltration, hepatic, Fig.3 (b)