Transient elastography

Transient elastography known by the brandname FibroScan is a non-invasive test to quantify liver fibrosis. It also detects and quantifies steatosis simultaneously using the Controlled Attenuation Parameter (CAP). It is a type of transient elastography.

Technology

Transient elastography has been validated by studies. The French company Echosens created transient elastography as a product of French government-funded research. Echosens received approval from Health Canada for transient elastography and its 3 probes in September 2009. The full exam requires for the operator to obtain 10 measurements of stiffness (in kPa) from the patient by pressing a probe in between the ribs. From the ten results, the median number is obtained and compared with the designated values on the FibroScan Scoring Card. The scoring card results range from 1.5 kPa to 75 kPa. Between 90–95% of healthy people without liver disease will have a liver scarring measurement less than 7.0 kPa (median is 5.3 kPa).

Probes

The device can be used with three types of probes:

• The standard or M probe for most patients
• The paediatric or S probe for children or adults with narrow rib cages
• The XL probe for obese patients with BMI over 30 kg/m².

Physicians compare the resulting median fibrosis value in kPa and compare with the aligning liver condition on the scoring card to diagnose the stage of fibrosis

Measuring stiffness and fat

Although the transient elastography system uses ultrasound, its ultrasound elastography technique is more similar to seismology than to ultrasound imaging. The function of the unit’s ultrasound is only to display a one-dimensional image which assists the operator in selecting the measurement zone.

Controlled attenuation parameter

The Controlled Attenuation Parameter (CAP) is a measurement of the ultrasound attenuation. It is correlated to the decrease in amplitude of ultrasound waves as they propagate through the liver. CAP has been designed on the assumption that fat affects ultrasound propagation. Therefore, the more steatosis, or liver fat there is, the higher the CAP result will be.

Comparison to liver biopsy

Fibrosis of the liver, which is associated with significant morbidity and mortality, occurs as a result of an exaggerated healing response triggered by chronic insults to the liver such as injury, infection, and inflammation. Fibrosis of the liver develops when scar tissue replaces normal liver tissue. It can be localized or diffused, and it can progress to cirrhosis, which increases the risk of developing liver cancer. Assessing the degree of fibrosis is important when making decisions about starting treatment, assessing treatment response, and screening for liver cancer. The current gold standard for assessing the condition of the liver is by performing a liver biopsy. Liver biopsies are useful for distinguishing between viruses affecting the liver, whereas FibroScan can only test the stiffness and steatosis. However many patients refuse biopsies due to discomfort. FibroScan will measure a liver volume of 3 cm², up to 200 times larger than does a liver biopsy, since a larger volume of biopsy specimen could be harmful for patients.