Physicians interested in non-alcoholic fatty liver disease (NAFLD) in pediatric patients.The aim of the study is to use multifrequency MR elastography (MMRE) to detect changes in liver stiffness associated with NAFLD and to discriminate benign fatty liver from mild and advanced fibrosis in children.32 patients (age range 10-17 years, 9 females) who are overweight or obese (average BMI: 35.2 kg/m²) and exhibit prolonged elevation of serum alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) (>50 U/l for at least 3 months) were recruited. MMRE (1) was conducted in a 1.5 T scanner (Siemens, Magnetom Sonata) using 7 harmonic frequencies (30 to 60 Hz, 5 Hz increment) as detailed in (2). The full 3D wave field was recorded using a single-shot EPI sequence with motion-encoding gradients. Total acquisition time for 9 consecutive slices of 2.7 × 2.7 × 5 mm³ resolution, 7 frequencies, and 8 wave dynamics was 5 minutes and 8 seconds. MRE wave data was reconstructed using multifrequency dual elasto visco (MDEV) inversion as detailed in (3), yielding parameter maps of the magnitude of the complex shear modulus |G^*|. Hepatic fat fraction (HFF) was estimated by the Dixon method. Liver biopsy was performed in all subjects for fibrosis grading according to the METAVIR score and serum biomarkers such as ALT and AST were obtained. Transient elastography (TE) was also applied to all the patients to assess liver stiffness.Based on histological staging, 23 subjects had no or early fibrosis (4 with F0, 14 with F1 and 5 with F2). 9 subjects had advanced fibrosis with F3. Fig.1 shows MRE magnitude image, shear wave image at 50 Hz drive frequency, HFF map and elastogram (|G^*| map) in the central slice of one patient with F2 fibrosis. The patients were divided into two groups (F0-2 and F3) based on biopsy proven fibrosis stage, group-mean values of |G^*| and HFF are shown in Fig.2 based on pooling groups into mild (F0-F2) and severe fibrosis (F3). Mann–Whitney U test revealed higher |G^*|-values in F3 (3.2±0.5 kPa) than in F0-2 (2.4±0.4 kPa, P <0.001). Values for sensitivity (100%) and specificity (91%) were obtained by a cutoff of 2.71 kPa for detecting severe fibrosis with an AUROC of 0.93 (95% CI 0.84-1.03; p < 0.001). HFF is lower in F0-2 (22±13 %) compared to that of F3 (31±8 %, P <0.05). Additionally, a positive correlation between the liver stiffness and ALT (Person r = 0.64, P = 0.0002, Fig 2c) was obtained. For separating these two patient groups, TE has an unsatisfactory diagnostic accuracy with an AUROC of 0.55.MRE has its advantages over TE when examining obese patients as abdominal wall fat deposition could be a limiting factor for TE (4). For this reason, we achieved a higher diagnostic precession by MRE as compared to TE. Our results are consistent with previous reports of MRE in children with chronic liver disease (5) and adults with NAFLD (6, 7). An increase of HFF was also observed in the F3 group, however, compared to hepatic stiffness measured by MRE, it is less sensitive in differentiating F0-2 and F3. Additionally, we found that liver stiffness is positively correlated with ALT which is related to liver injury. No correlation was found between liver stiffness with AST-to-ALT ratio which is commonly used to identify adult patients with advanced fibrosis. However, recent data suggested that the adult scores may not be accurate for predicting advanced fibrosis in children (4). Our study is limited by small sample size in particular for F2 precluding the separation of F1 and F2. This will be addressed by more patients who are enrolled in a current study.The MMRE-measured shear modulus |G^*| of the liver is sensitive to differentiate F0-2 and F3 in pediatric patients with NAFLD without limitations due to obesity. Liver stiffness was positively correlated with ALT and can potentially serve as a quantitative imaging marker for the noninvasive assessment of liver fibrosis in patients with NAFLD.