Liver

Recognizing the "bright" liver: role of ultrasound in the evaluation of the fatty liver

KEY POINTS

  • Fatty liver, also known as hepatic steatosis, is a condition characterized by triglyceride (neutral) fat accumulation within the cytoplasm of hepatocytes (Fig. 1). Macroscopically the steatotic liver has a yellowish mosaic coloration (Fig. 2).
  • It is a benign process and a reversible condition with lifestyle modifications, although some patients may progress to liver disease and cirrhosis.
  • Fatty infiltration of the liver is a manifestation of hepatocelular injury. Many conditions are associated with fatty liver disease (Table 1).
  • Patient is often asymptomatic, althoug vague abdominal pain and elevated liver enzymes may be present.
  • Severe fatty infiltration usually results in an enlarged liver (hepatomegaly).
  • Liver biopsy and histologic analysis is considered the gold standard for the assesment of fatty liver. However, fatty liver also can be diagnosed with the use of cross-sectional imaging, commonly starting with ultrasound.​

Fig.1  Photomicrograph of early stage of a steatotic liver depicted by numerous large lipid vacuoles (asterisks) displacing the nuclei of hepatocytes to the periphery of cytoplasm. 

Fig.2 Steatotic liver with a yellowish mosaic coloration 

Table 1. Causes of liver steatosis


ULTRASOUND AND LIVER STEATOSIS

    Ultrasound is the simplest and most commonly used imaging method to detect hepatic steatosis. However, it is an operator dependent modality and has not been applicable its use to quantitative assesment of hepatic steatosis on clinical practice. Thus, ultrasound evaluation of fatty liver disease is mainly qualitative.

NORMAL ECHOGENICITY OF LIVER AND SPLEEN

    The normal echogenicity of the liver is determined by comparing the liver echogenicity with that of the cortex of the right kidney. Compared to renal cortex, the liver usually has the same or a slightly greater echogenicity than the right kidney.

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Sonographically speaking, the normal liver is isoechoic to discreetly hyperechoic compared to right kidney (Fig. 3)

Fig.3 Right midclavicular longitudinal view demonstrates the isoechogenicity between liver (L) and right kidney (RK).  

    To report a hyperchogenic liver we use to use the relative echogenicity of the kidneys compared with the spleen and liver. The normal spleen is slightly more echogenic than the left kidney and the normal liver (Fig. 4)

Fig.4  High left intercostal view demonstrates the slightly hyperechoic spleen (S) compared with the left kidney (LK)  

LEARNING POINT

    If the difference in echogenicity between the liver and right kidney is GREATER than the difference between the spleen and left kidney, the liver parenchyma has abnormally increased echogenicity (Fig. 5)

Fig. 5 Relative comparison between liver and spleen echogenicity with the ipsilateral kidneys:Observe that the difference in echogenicity between the liver (L)  and right kidney (RK) is higher than the difference between the spleen (S) and left kidney (LK). 

    We have assumed that this approach is correct only when using the same machine settings in evaluating relative echogenicity between liver and spleen and also that both kidneys have NORMAL AND EQUAL echogenicity.

ULTRASOUND DIAGNOSIS OF FATTY LIVER

Three sonographic parameters are the hallmark of fatty liver (they go hand in hand with each other):

1. Diffusely increased parenchymal echogenicity (the so-called "bright liver")

2. Increased attenuation of ultrasound beam

3. Suboptimal visualization of diaphragm and hepatic vessels

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     Because the normal liver may be slightly more echogenic than the right kidney and to avoid false-positive diagnosis,  careful interpretation should be considered if only one of these criteria are present.

Table 2. Ultrasound features of liver steatosis

SONOGRAPHIC GRADING FOR DIFFUSE STEATOSIS

  Because of its "operator dependent method" nature, sonography grading of steatosis is subjective and prone to interobserver variation. The following three grades have been described for ultrasound classification of fatty infiltration:


Mild: minimally increased parenchymal echogenicity with normal visualization of intrahepatic vessels and diaphragm

Moderate: increased echogenicity with impaired visualization of diaphragm and intraheptic vessels walls.

Severe: marked increase in echogenicity of liver parenchyma with poor visualization of intrahepatic vessels and diaphragm

Fig 6.  A- Normal liver slightly hiperchoic compared with right kidney. Note intrahepatic vessels (red arrow) sharply demarcated and liver posterior aspect - diaphragm (white arrow)- well depicted. B,C and D - Increasing in liver echogenicity with progressive worsening of hepatic vessels (red arrow) and diaphragm (white arrow) visualization.    

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    After all, even if the diagnosis is still uncertain, it is possible to detect the caracteristics areas of focal fatty sparing, which allows for a more confident diagnosis of steatosis (Fig. 7).

Fig 7. Common areas of fatty sparing - "Bright" liver showing a hypoechoic area (arrow) with angulated margins at the portal vein bifurcation.  B - Hyperechoic liver with a hypoechoic lesion (arow) at the anterior surface of gallbladder. Note that focal fatty sparing has no mass effect or architectural distortion. 

REFERENCES:

1. anatpat.unicamp.br (Pictures 1 and 2)

2. Heller, Matthew T., and Mitchell E. Tublin. "The role of ultrasonography in the evaluation of diffuse liver disease." Radiologic Clinics 52.6 (2014): 1163-1175.

3. El-Kader, Shehab M. Abd, and Eman M. Salah El-Den Ashmawy. "Non-alcoholic fatty liver disease: The diagnosis and management." World journal of hepatology 7.6 (2015): 846.

4. Hamer, Okka W., et al. "Fatty liver: imaging patterns and pitfalls." Radiographics 26.6 (2006): 1637-1653.

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Written by Dr. Augusto César
Board certified Brazilian radiologist, co-founder of the Salus Sonography project and Medultra smartphone app. Currently works as an interventional and general radiologist in private practice, São Paulo, Brazil.

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