Liver elastography is a non-invasive diagnostic technique used to quickly and safely assess the physical condition of the liver. It serves as an alternative to the traditional liver biopsy, which is an invasive surgical procedure with potential complications. Elastography measures the stiffness of liver tissue, a property that correlates directly with the amount of scarring, or fibrosis, that has developed. This assessment is particularly useful for individuals managing chronic liver conditions, such as Hepatitis C, Hepatitis B, and Non-Alcoholic Fatty Liver Disease (NAFLD) or Non-Alcoholic Steatohepatitis (NASH). By quantifying stiffness, the test provides physicians with a numerical value to determine the extent of liver damage and monitor disease progression.
How Liver Stiffness is Measured
The fundamental principle behind liver elastography is measuring how quickly a mechanical wave travels through the liver tissue. The machine generates a low-frequency shear wave, and its propagation speed is directly proportional to the stiffness of the material. A softer, healthy liver allows the wave to travel slowly, while a stiff, scarred liver causes the wave to travel faster, yielding a higher measurement.
Two main technologies are used for this assessment: Vibration-Controlled Transient Elastography (VCTE), known as FibroScan, and Magnetic Resonance Elastography (MRE). VCTE uses an ultrasound probe to deliver a mechanical vibration, generating a shock wave tracked by the same probe. This procedure is quick, painless, and typically performed in an outpatient clinic.
MRE is a more specialized technique integrating mechanical waves with Magnetic Resonance Imaging (MRI) technology. The MRI scanner captures images of the shear wave as it moves through the organ, creating a visual map of stiffness across a larger area of the liver. MRE is less affected by patient factors like obesity and provides a high-quality, comprehensive measurement.
Understanding the Standard Interpretation Ranges
Liver elastography results are expressed in kilopascals (kPa), a unit of pressure quantifying the measured stiffness of the tissue. For a healthy liver with minimal scarring, the normal stiffness measurement typically falls between 2 and 7 kPa. Results in this lower range usually correspond to minimal or no fibrosis, designated as F0 or F1 in the METAVIR staging system used to grade scarring severity.
As the kPa value increases, it correlates with higher stages of chronic liver scarring. A reading between 7 and 11 kPa often indicates significant fibrosis (stage F2). This stage suggests scarring is present and beginning to extend, but it is not yet advanced. Measurements between 11 and 14 kPa typically suggest severe fibrosis (stage F3), where the scarring is widespread and forming connections between different liver areas.
The most significant finding is cirrhosis (stage F4), indicated by a stiffness measurement often at or above 14 kPa, depending on the underlying cause. Cirrhosis represents advanced scarring that has severely altered the liver’s architecture.
The specific kPa cut-off values defining each fibrosis stage vary depending on the underlying cause of the liver disease, such as Hepatitis C or Non-Alcoholic Fatty Liver Disease. For example, the threshold for diagnosing cirrhosis (F4) in Hepatitis C may be around 14 kPa, but it might be 19 kPa for alcohol-related disease. Physicians use these disease-specific cut-offs, along with the patient’s medical history, to accurately interpret the kPa value and assign the correct fibrosis stage.
Situations That Influence Results
Several factors unrelated to chronic scarring can temporarily increase the liver stiffness measurement, potentially leading to an overestimated fibrosis result. Acute inflammation, often called a hepatitis flare, causes the liver tissue to swell and stiffen, resulting in a transiently elevated kPa reading. This effect is noticeable when transaminase levels, markers of liver cell damage, are high. Severe liver congestion, often a complication of right-sided heart failure, also causes a false elevation due to increased blood volume.
Similarly, extrahepatic cholestasis (blockage of bile flow outside the liver) can temporarily raise the elastography score. These conditions cause tissue tension that the machine registers as increased stiffness, even without worsening chronic fibrosis. Patient preparation is also a factor, as liver stiffness values can increase immediately after a meal. For this reason, the procedure is typically recommended after a period of fasting, usually two to three hours. When an unexpectedly high result is obtained, physicians must consider these acute and temporary factors before concluding that chronic fibrosis has progressed.