*3.2. Ultrasound-Based Elastography Techniques*

In 2005, it was demonstrated that the propagation and velocity of low-frequency pulsed shear waves in real time in biological tissue is directly correlated to the amount of extracellular matrix [107]. Respective methods relying on the velocity of shear waves, introduced as "transient elastography", "vibration-controlled transient elastography (VCTE)" or "real-time shear wave elastography (SWE)" are particularly suited to detect liver cirrhosis by high stiffness in which it has a diagnostic specificity of 99% [108]. It is noteworthy, that a VCTE device such as the FibroScan introduced by Echosens SA can be performed as a point-of-care (POCT) test at the place of patient care enabling test results to be immediately shared instantly with care providers or patients. Unfortunately, the device has potential technical limitations in patients suffering from ascites, high quantities of chest wall fat, and in individuals who are morbidly obese [109].

In acoustic radiation force impulse elastography (ARFI) measuring beam passes over a standardized region of interest of the liver, obesity as defined by a BMI of larger 30 kg/m2 or ascites are not considered obstacles [110]. However, ARFI imaging was found to have a poor diagnostic performance in patients with BMI larger than 35 kg/m2 [111]. Other ultrasound real-time imaging modalities for soft tissue elasticity mapping are SWE or supersonic shear imaging (SSI) [104]. In these measurements, the propagation of the waves is stored in small video clips from which the elasticity of the analyzed tissue can be mapped quantitatively [112]. Although, these methods have good to excellent performance for the noninvasive staging of fibrosis in hepatitis B infected patients, data on other liver disease cohorts are still needs to be established [113].

However, in some disease conditions, the combination of elastography techniques with special NAFLD scores composed of simple measures of patient characteristics and clinical chemistry parameters increase the overall accuracy in assessment of clinically significant liver fibrosis in NAFLD [114]. The combination of various methods is therefore potentially relevant to better identify patients in which a liver biopsy is appropriately indicated.
