Hepatic steatosis, or fatty liver disease, is a condition characterized by the accumulation of excess fat within liver cells (hepatocytes). This widespread condition is frequently linked to metabolic conditions like obesity and diabetes mellitus, though it can also be associated with alcohol use. Ultrasound imaging is a common, non-invasive method used to detect this fat buildup. It provides a safe and accessible first step for doctors to visualize the liver’s condition.
The Ultrasound Examination Process
An abdominal ultrasound is a straightforward and painless procedure. To prepare, a patient is often asked to fast for several hours beforehand. This helps to reduce the amount of gas in the intestines, which can interfere with the clarity of the ultrasound images. During the examination, the patient lies comfortably on a table, usually on their back.
A sonographer will apply a water-based gel to the skin over the abdomen. This gel helps to eliminate air pockets between the skin and the ultrasound transducer, ensuring good transmission of the sound waves. The sonographer then gently presses the transducer against the skin and moves it across the abdomen to view the liver and surrounding organs from different angles. The entire process uses high-frequency sound waves to create live images and does not involve radiation.
Interpreting Ultrasound Findings
When a radiologist examines liver ultrasound images, they are primarily looking for changes in echogenicity, which refers to the way sound waves reflect off the tissue. A healthy liver has an echogenicity that is very similar to the outer layer, or cortex, of the adjacent right kidney. In a liver with hepatic steatosis, the accumulated fat droplets cause the sound waves to scatter more, making the liver appear brighter on the screen. This is described as increased echogenicity or a “hyperechoic” liver.
Another sign of fatty liver disease is the obscuring of internal structures. The increased fat content can cause the ultrasound beam to weaken as it travels deeper into the tissue, a phenomenon known as posterior attenuation. This can make it difficult to clearly see the walls of blood vessels, such as the portal vein, or the diaphragm, which sits just behind the liver. In some cases, the liver itself may be enlarged, a condition called hepatomegaly.
Grading the Severity of Steatosis
To provide a more detailed assessment, radiologists use a grading system to classify the severity of hepatic steatosis from ultrasound images. This qualitative scale helps standardize reporting and categorizes the condition into three main levels.
A diagnosis of Grade 1, or mild steatosis, is given when there is a slight and diffuse increase in the liver’s echogenicity. At this stage, the diaphragm and the borders of the intrahepatic blood vessels are still clearly visible.
Grade 2, or moderate steatosis, is characterized by a more pronounced increase in echogenicity, which begins to obscure the view of the portal vein walls. The diaphragm, however, can still be adequately visualized.
In cases of Grade 3, or severe steatosis, the liver’s echogenicity is so significantly increased that it results in poor or no visualization of the portal vein walls and the diaphragm.
Limitations and Subsequent Steps
Ultrasound is an effective initial tool, but it has limitations. The interpretation of the images can be operator-dependent, and its accuracy may be reduced in patients with significant obesity. A significant limitation is that a standard ultrasound cannot distinguish between simple fat accumulation (steatosis) and the more serious condition of non-alcoholic steatohepatitis (NASH), which involves both fat and inflammation. It also cannot quantify the amount of fibrosis, or scarring, in the liver.
If an ultrasound indicates fatty liver, a doctor may recommend further tests to get a more complete picture of liver health. This often includes blood tests to measure liver enzymes like alanine aminotransferase (ALT) and aspartate aminotransferase (AST), which can indicate inflammation. To assess for fibrosis, another non-invasive test called transient elastography may be ordered. This technology, known by the brand name FibroScan, uses sound waves to measure the stiffness of the liver, which correlates with the degree of scarring.