## 2 Current Elastography Methods
The results range between 100-400 dB/m depending on the amount of fat tissue within the liver. Three types of probes are available for this system: the S probe, designed for children since their thoracic diameters is <75 cm, for measurements between 1.5-5 cm, with a frequency of 5.0 MHz; the M probe designed for measurements up to 6.5 cm from the skin, with a frequency of 3.5 MHz; and the XL probe designed for a distance >2.5 cm between the skin and liver capsule, with a frequency of 2.5 MHz, for measurements ranging between 3.5-7.5 cm. As of today, CAP has the ability to display only valid measurements, being computed as a result of the ultrasound signals used for acquiring liver stiffness measurements [35]. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Author, Year, and Country** & **Study Type/No of** & **Age (Years)** & \multicolumn{2}{c}{**MRE (GPA)**} \\
**Patients** & **Mean/Median \({}^{*}\)** & **Normal Weight Children** & **NAFLD/NASH Children** & **Observations** \\ \hline [PERSON] et al., 2020, USA [33] & Prospective/81 & \(12.6\pm 2.6\) & \(2.45\pm 0.35\) & - & The values are significant higher than in adults (\(21.0\pm 0.23\) Hz) (\(p<0.001\)) \\ \hline [PERSON] et al., 2020, USA [34] & Prospective/71 & \(12\,^{*}\) & \(2.1\pm 1.96\) & - & P95 for normal liver stiffness was 2.8 kPa. \\  & & & & Liver stiffness was independent of sex, age, or BMI \\ \hline [PERSON] et al., 2020, USA [54] & Retrospective/122 & \(14.7\pm 3.5\) & - & \(2.6\pm 0.7\) kPa for boys & There are stiffness variations within the 8 \\ USA [54] & Retrospective/202 & \(13.4\pm 2.9\) & - & \(2.6\pm 0.6\) & Covinaud segments of the liver in children \\ \[PERSON] et al., 2018, USA [50] & Prospective/202 & \(13.4\pm 2.9\) & - & \(2.6\pm 0.6\) & Relationships between stiffness, and organ volume and open patient-specific factors, including sex, age, BMI, serum ALT, and diabetic status \\ \[PERSON] et al., 2018, USA [55] & Prospective/86 & \(14.2\,^{*}\) & \(0.51\); \(\mathrm{F2.36\pm 1.12\,F3.45}\) & \(0.63\); \(\mathrm{F2.27\pm 0.59\,F3.74}\) &
\begin{tabular}{l} MR elastography performs significantly \\ better for distinguishing stage 0-1 versus \\ stage 20 \(\,\) higher fibrosis in patients without \\ stenosis than in those with stenosis. \\ \hline [PERSON] et al., 2017, USA [56] & Retrospective/372 & \(12.6\pm 3.7\) & - & \(2.94\pm 1.11\) kPa & Mean patient BMI was \(29.4\pm 1.01\) kg/m\({}^{2}\) \\ \hline [PERSON] et al., 2017, USA [57] & Prospective/90 & \(13.1\pm 2.4\) & - & \(2.35\) kPa & Correlation with fibrosis (by biopsy) was 72.2\% \\ \hline \multicolumn{5}{c}{_Calibration: At 28 Hz: \(1.2\pm 2.4\)_} \\
02/At 56 Hz: \(2.2\pm 0.37\) At 84 &
\begin{tabular}{l} Liver stiffness values are lower and vary less \\ _Advances: At 28 Hz: \(1.3\pm 0.3\)_/At 84 \\ 0.3/At 56 Hz: \(2.2\pm 0.2\)/At 84 \\ \end{tabular} & Liver stiffness values are lower and vary less \\ \multicolumn{5}{c}{_Advances: At 28 Hz: \(1.3\pm 0.3\)_} \\
03/At 56 Hz: \(6.5\pm 1.2\) &
\begin{tabular}{l} Liver stiffness values are lower and vary less \\ with frequency in children and adolescents \\ than in adults. \\ \end{tabular} \\ \hline [PERSON] et al., 2014, USA [58] & Prospective/35 & \(13\,^{*}\) & - & F1: 2: 2 kPa; F3. 3.6 kPa; Fk: & Cut-off = 2.71 kPa=8\% sensitivity, 85\% specificity for F2 with an AUROC of 0.92 \\  & & & \(4.9\) kPa & (59\% C, 0.79-10.0\(p=0.02\)) \\ \hline \multicolumn{5}{c}{**Legend: MRE–magnetic resonance elastography, y–yours:** Mean age calculated from mean of the three age groups in the study.} \\ \end{tabular}
\end{table} 
Table 1: Magnetic resonance elastography (MRE) values in normal weight and obese children. ARFI techniques are available on both linear and curvilinear transducers and as aforementioned, they rely on the generation of shear waves from the push pulse of the ultrasound beam [59]. In practice, an initial push pulse is applied to the tissue, inducing shear waves perpendicular to the ultrasound beam.