Transient elastography (TE)

• liver fibrosis is the natural wound-healing response to parenchymal injury in chronic liver diseases
  • simple steatosis is a usually reversible and benign condition, but steatosis may be part of the more sinister condition in the setting of steatohepatitis where inflammation and hepatocyte changes co-exist
  • both liver fibrosis and steatohepatitis may eventually result in liver cirrhosis and its various complications
    • sensitive detection and accurate staging of liver fibrosis and steatosis is now essentially indispensable in the decision process of treatment in chronic viral hepatitis as well as predicting disease prognosis
    • also vital to monitor disease progression and response to treatment
  • diagnostic accuracy of liver biopsy is limited by the sampling variability
    • limited by its invasiveness and patient acceptability.
    • average size of biopsy is 15 mm in length, which represents 1/50000 the size of the entire liver. There is significant variability in the histologic assessment of two readings of the same biopsy by the same pathologist, and between two pathologists, even among those who are highly specialized
      
      
• Transient elastography (TE, Fibroscan®) is a non-invasive tool with satisfactory accuracy to estimate liver fibrosis and steatosis
  • liver stiffness measurement (LSM) with TE has been well validated to detect advanced fibrosis in most liver diseases. LSM is useful in predicting hepatocellular carcinoma (HCC), portal hypertension, post-operative complications in HCC patients, and survival
  • works by measuring shear wave velocity. In this technique, a 50-MHz wave is passed into the liver from a small transducer on the end of an ultrasound probe
    • probe also has a transducer on the end that can measure the velocity of the shear wave (in meters per second) as this wave passes through the liver. The shear wave velocity can then be converted into liver stiffness, which is expressed in kilopascals. Essentially, the technology measures the velocity of the sound wave passing through the liver and then converts that measurement into a liver stiffness measurement; the entire process is often referred to as liver ultrasonographic elastography
    • unfortunately, failed acquisition of TE is common in obese patients. Furthermore, obese patients may have higher LSM results even in the same stage of liver fibrosis. The new XL probe, a larger probe with lower ultrasound frequency and deeper penetration, increases the success rate of TE in obese patients
      • development of S and XL probes aim to cater for different population groups of various body-build types
        • S probe contains a higher frequency ultrasonic transducer and shallower measurements below the skin surface, which suit pediatric subjects and those with small body build
        • XL probe contains a lower frequency and a more sensitive transducer, a deeper focal length, larger vibration amplitude and a higher depth of measurements below the skin surface - probe serves obese subjects

Notes:

• biomarker tests and radiologic tests are not mutually exclusive, and many guidelines now recommend that clinicians perform both a serum test and a Fibroscan 
  • when both tests indicate mild or no disease, then the combined result is both sensitive and specific, and clinicians can be confident in this result. Likewise, when both tests indicate advanced or significant disease, this result has high sensitivity, high specificity, and a high predictive value

Reference:

• 1) Afdhal NH. Fibroscan (transient elastography) for the measurement of liver fibrosis.Gastroenterol Hepatol (N Y). 2012 Sep;8(9):605-7
• 2) Wong GL. Transient elastography: Kill two birds with one stone? World J Hepatol. 2013 May 27;5(5):264-74