## 3 The Opportunities and Challenges of Ultrasound-Based Elastography in Children NAFLD
Contrarily, in terms of supersonic shear imaging, the authors found a median cutoff value of 7.9 kPa in case of stage 1, 9.4 kPa in stage 2, 14.2 kPa for stage 3, and 23.94 kPa for stage 4 [110], although for ARFI the results are provided in m/s. For supersonic shear imaging such as [PERSON]'s modulus in kPa, both elastography methods reveal increasing values as liver fibrosis advances. Similar trends were provided by [PERSON] et al., who reported the results of 2D-SWE as both m/s and [PERSON]'s modulus in kPa [19]. Based on these findings we might assume that the values liver stiffness assessed in m/s or in kPa are directly related since an increase in both in noticed in stiffer livers. Another study which assessed the differences between ARFI and TE on children with chronic liver disease in the setting of metabolic disorders, as well as those with normal liver, suggested that ARFI might be more accurate differentiating between the stages of fibrosis [111]. Similar findings were reported by [PERSON] et al., concluding that 2D-SWE might be considered reliable in detecting advanced liver fibrosis in children with NASH [112]. Contrarily, [PERSON] et al., who assessed liver stiffness in children with obesity using both TE and 2D-SWE, pointed out that TE and velocity measured on 2D-SWE might represent useful tools for detecting NAFLD [19]. Overall, [PERSON] et al. concluded in a comparative study which assessed the diagnostic performance of supersonic shear imaging, TE and ARFI in NAFLD staging that all these methods have similar diagnostic performance for staging fibrosis in the setting of NAFLD [113]. A major limitation of elastography methods is represented by a proper differentiation between mild fibrosis and normal liver tissue. Thus, pSWE was found to have a good accuracy in distinguishing severe from mild fibrosis, but not between normal liver tissue and low-grade fibrosis in children with NAFLD [78]. The increased risk of mortality in patients with NAFLD is mostly related to liver fibrosis itself [114] and its proper diagnosis, as well as its early staging are crucial for improving the long-term prognosis in children with overweight and obesity. A very recent study that compared liver stiffness values on 2D SWE and TE proved higher values in children with obesity as compared to normal ones, suggesting that these methods might be extremely useful in clinical practice for detecting early stages of fibrosis associated to NAFLD [19]. Furthermore, in order to increase the reliability of TE, [PERSON] et al. combined this technique with pediatric NAFLD index and showed that this combination might identify patients that require liver biopsy for advanced fibrosis confirmation and closer follow-up for a timely diagnosis of cirrhosis-related complications [115]. The most relevant studies that compared the results between liver elastography and biopsy findings in children NAFLD were summarized in Table 3.  
In terms of pediatric NAFLD staging, studies that assessed the role of elastography are promising although its precision is not perhaps optimal. Multiple studies aimed to identify if elastography is able to replace liver biopsy in order to diagnose this condition, an emerging concern worldwide. Two histology scoring systems were used for assessing liver fibrosis: Brunt classification [116] defined as grade 0 for no fibrosis, grade 1 for zone 3 perisinusoidal and/or pericellular fibrosis, grade 2--the changes identified in grade 1 plus focal or extensive periodst fibrosis, grade 3 as in grade 2 plus focal or extensive bridging fibrosis and grade 4 defining cirrhosis; and Kleiner activity scoring system with 0 for no fibrosis, 1 for periodst or perisinusoidal fibrosis, 2 for perisinusoidal and portal or periportal fibrosis, 3 for bridging fibrosis, and 4 when cirrhosis occurs [117]. [PERSON] et al.