## 2 Current Elastography Methods  
Elastography methods are defined by the non-invasive assessment of tissue elasticity and its mechanical properties resulting from the application of external forces. Two major terms that are commonly confounded in practice must be differentiated: tissue elasticity, i.e., the tissue ability to deform and resume its normal shape in the setting of an applied stress dependently of tissue stiffness, the second term, which indicates tissue composition and structure [26]. Thus, when referring to elastography, the term liver stiffness seems to be more appropriate. Liver elastography has gained a lot of interest in the context of NAFLD, a current global public health problem in all age groups. Thus, liver elastography aims to objectively characterize diffuse liver disease and it might reflect pathologic processes such as fibrosis, inflammation or congestion [27; 28; 29; 30]. Taking into account the increased rates of both morbidity and mortality in patients with diffuse liver disease [31], a proper management is essential for the best outcome in these patients. Furthermore, the management depends mostly on the accurate staging of liver fibrosis. Liver elastography seems to be a promising diagnostic tool in the assessment of liver fibrosis irrespectively of the age and type of chronic liver disease. Current elastography methods for liver assessment encompass magnetic resonance elastography (MRE), and three main ultrasound-based elastography techniques, i.e., transient elastography (TE, Figure 1), acoustic radiation force impulse (ARFI) divided into point shear wave elastography (p-SWE), and 2-dimensional shear wave elastography (2D SWE, Figure 2), and strain elastography. The latter one is not used for liver stiffness measurement and therefore we will not focus on its description since it does not fulfill the aim of this review. Liver fibrosis is a dynamic process consisting of excessive extracellular matrix accumulation as a result to injury and inflammation, followed by degradation of this excessive matrix and consequent remodeling [25]. Fibroblasts and myofiblabats also contribute to hepatic fibrosis [25]. Independently of its continuous dynamics, at some point hepatic fibrosis exceeds degradation, leading to cirrhosis and related-changes in vascular architecture. Moreover, both excessive extracellular matrix in the setting of liver fibrosis and vascular architecture changes result in increased liver stiffness [25]. Figure 1: Liver transient elastography (TE) in normal weight healthy children. MRE of liver uses mechanically generated shear waves, which are propagated into the liver and their images are obtained using a modified phase contrast MRI sequence, further converted into tissue stiffness maps or elastograms based on an inversion algorithm [32; 33]. It is important to mention that the propagation of these shear waves occurs faster in stiffer tissue than in softer tissue, i.e., in cirrhotic tissue as compared to normal liver. Clinical liver MRE set up usually consists of an active driver, a passive driver positioned in the inferior part of the right chest wall closer to the liver, and a 25-foot-long plastic tube connecting these drivers. The active driver is represented by an acoustic driving system that generates 60 Hz shear waves in tissue from outside the scanner room. The passive driver is activated as a result of varying acoustic pressure triggered from the active driver via the plastic tube. Patients must rest in a supine position during the exam. Tissue displacement caused by the propagation of shear waves is determined by a modified phase contrast imaging sequence, which uses different conventional MR sequences, such as gradient recalled echo, echoplanar imaging, spin echo, or balanced steady state free precision. MRE has the capacity to spatially map and quantify displacement patterns, which are useful for the calculation of certain mechanical characteristics related to wave propagation.