## 2 Current Elastography Methods
Several MRE algorithms are available for the inversion of wave information into stiffness maps such as direct inversion of differential equations of motion, spatial frequency measurement, or iterative method based on the finite element model. These algorithms are used for measuring an important mechanical tissue characteristic, i.e., 'the magnitude of the complex shear modulus' accounting for both elasticity and viscosity, expressed in kilopascals [34; 35; 36]. It is well-documented that MRE is more accurate for both detecting and staging of liver fibrosis [32; 37] as compared to ultrasound-based elastography methods, but with definitely higher associated costs. MRE technical performance reaches approximately 95% and the most common cause of failure is iron overload [25]. Additionally, a study that assessed 781 liver MREs pointed out other independent factors related to failure, among which were ascites and increased body mass index [38]. Nevertheless, a more recent study performed on 71 healthy pediatric volunteers revealed a mean liver stiffness value of 2.1 kPa, pointing out that sex, age, or BMI had no impact on liver stiffness measurement [34]. Conflicting results might be explained by technical or platform-related differences in terms of the magnitude of the relative stiffness of the individual individual MREs. Figure 2: Liver 2-dimensional shear wave elastography(2D-SWE) in normal weight healthy children. of strength, driver frequency used for MRE, MR scanner vendor, or they might reflect the differences between the populations included in the study [34]. Liver thresholds for MRE defined for healthy children differ from those in healthy adults [39]. Thus, [PERSON] et al. revealed that healthy children present lower liver stiffness values as compared to healthy adults [40]. These findings came in contradiction to those of [PERSON] et al., who recently found that normal liver stiffness values were higher in children than previously reported for adults [33]. In terms of liver fibrosis, different studies pointed out that the cut-off values range between 2.4-2.93 kPa [41; 42; 43; 44; 45]. A strong correlation has been identified between MRE liver stiffness measurements and the amount of fibrosis in the liver biopsy samples suggesting that MRE has the same accuracy as biopsy for liver fibrosis staging [46]. The accuracy of MRE for detecting liver fibrosis increases with the stage of fibrosis, but with an excellent overall performance [37]. Recent studies underlined that MRE has only modest accuracy in distinguish between NASH and simple steatosis [47; 48; 49], a large study involving pediatric patients pointing out that liver fat may express a mild softening effect in the setting of MRE assessment [50]. Therefore, a considerably improved alternative method consists of a comprehensive liver MRI study defined by fat quantification, detection of inflammation and staging of liver fibrosis [51], with an obvious higher accuracy, but considerable associated costs. Nevertheless, [PERSON] et al. pointed out that MRE has a 94% sensitivity and a 73% specificity with an accuracy of 0.93 in discriminating between simple steatosis and NASH [45]. In terms of NAFLD, the MRE specificity and sensitivity rates were proven to be 93% and 85% in detecting advanced fibrosis [52]. The most important findings regarding MRE in children are summarized in Table 1. Altogether, ultrasound based elastography methods might be a more available option with lower associated costs and similar result. The shear wave-based methods, i.e., TE and ARFI, assess the speed of these waves in tissue depending on tissue stiffness, the waves being triggered by an external mechanical force in case of TE and the push pulse of a focused ultrasound beam in case of ARFI [53]. TE is a 1D technique provided by the FibroScan system (Echosens, Paris, France) and its latest version uses the controlled attenuation parameter (CAP) tool in order to measure the decrease in amplitude of ultrasound signal in the liver.