Diffuse hepatic steatosis

Diffuse hepatic steatosis, also known as fatty liver, is a common imaging finding and can lead to difficulties assessing the liver appearances, especially when associated with focal fatty sparing.

Epidemiology

Diffuse hepatic steatosis is common, affecting ~25% of the population. 

Pathology

Hepatic steatosis is due to the abnormal accumulation of lipids, particularly triglycerides within hepatocytes ^3-4. These are found in both small and large vesicles. Macroscopically, the liver is enlarged, yellow and greasy. Steatosis can lead to fibrosis and cirrhosis. 

Aetiology 

Diffuse hepatic steatosis is often idiopathic. However, it may be associated with¹:

• alcohol abuse
• nonalcoholic fatty liver disease (NAFLD) ⁴, which is associated with
  • insulin resistance / diabetes mellitus
  • obesity
  • dyslipidaemia
• exogenous steroid intake
• drugs (amiodarone, methotrexate, chemotherapy)
• IV hyperalimentation
• chronic hepatitis
• pregnancy: acute fatty liver of pregnancy (AFLP) ⁴
• metabolic disorders
  • glycogen storage diseases
• radiation

Radiographic features

General features include:

• mild hepatomegaly (in ~75%) ⁵
• attenuation/signal of liver shifted towards that of fat 
• focal fatty sparing
  • islands of normal liver tissue within sea of hepatic steatosis 
  • possibly occur due to regional perfusion differences ² 
  • importantly, compared to intrahepatic masses, fatty sparing has no mass effect with no distortion of vessels 
  • see alsofocal hepatic steatosis

Conventional radiography

Radiolucent liver sign: liver soft-tissue outline difficult to appreciate ⁵.

Ultrasound

Steatosis manifests as increased echogenicity and beam attenuation ^2,12. This results in:

• renal cortex appearing relatively hypoechoic compared to the liver parenchyma (normally liver and renal cortex are of a similar echogenicity)
  • increased echogenicity relative to the spleen if question of medical renal disease
• absence of the normal echogenic walls of the portal veins and hepatic veins
  • important not to assess vessels running perpendicular to the beam, as these produce direct reflection and can appear echogenic even in a fatty liver
• poor visualisation of deep portions of the liver
• poor visualisation of the diaphragm

Sonoelastography: can assess the degree of accompanying fibrosis by measuring tissue stiffness (Fibroscan, Acoustic Radiation Force Impulse) ¹⁰.

Grading

• see: grading of diffuse hepatic steatosis

CT

Steatosis causes reduced liver attenuation. This results in:

•  low hepatic attenuation compared with spleen on non-contrast imaging imaging
  • non-fatty liver is normally 6-12 HU greater density than spleen ⁵
  • arterial phase or portal venous phase scans should not be used as the the spleen enhances earlierthan the liver due to predominant systemic arterial supply. Fatty liver can be diagnosed by contrast-enhanced CT if absolute attenuation is less than 40 HU, but this threshold has limited sensitivity ¹¹
• relatively hyperattenuating intrahepatic vessels

MRI

Requires both in- and out-of-phase imaging to to be adequately assessed¹. Fatty liver appears:

• T1: hyperintense
• T2: mildly hyperintense
• out-of-phase imaging
  • signal drop out in fatty liver in out of phase greater than 15%

Of note, on in- and out-of-phase imaging, the maximum signal loss occurs when there is 50% fatty infiltration of the liver. In situations in which there is >50% fatty infiltration, the out-of-phase sequence paradoxically becomesless hypointense than at 50%. This happens because there are relatively fewer fewer water molecules to cancel out the fat signal. Chemical shift artifact at the parenchyma-vessel interface aids in detecting this situation ¹³.

Other MR uses:

• MR spectroscopy: accurate quantitative non-invasive assessment of hepatic steatosis ⁸
• MR elastography: shows promise as a method for assessing accompanying hepatic fibrosis ⁹

Nuclear medicine

• Tc⁹⁹m sulfur colloid 
  • uptake is reduced in fatty liver ⁵
  • reduced hepatic uptake relative to spleen (reversal of normal liver: spleen uptake)
  • focal fatty area can simulate a hepatic mass ⁷
• Xenon¹³³: accurate quantitative non non-invasive assessment of hepatic steatosis⁸
• FDG-PET: liver uptake is not altered by the presence of steatosis ⁶

Treatment and prognosis

As long as hepatic fibrosis and cirrhosis have not developed, fatty change is reversible with modification of the underlying causative factor, e.g. alcohol, pregnancy, obesity, diet.

Practical points

There is potential for missing mild hepatic steatosis on ultrasound if there is concurrent chronic renal disease, which increases the echogenicity of the kidneys. If there is any question that the patient may have chronic renal disease, comparison of the left kidney with the spleen may be useful. A greater echogenicity difference between the right kidney and the liver than between the left kidney and the spleen is indicative of hepatic steatosis ¹².

See also

• hepatic attenuation on CT
• in- and out-of-phase MR sequence