## 1 Introduction
The software offers a quantitative estimation of the rigidity of the tissue in the region of interest. In order to aid in the evaluation, color coding can be used to display the rigidity values [10]. Unlike TE elastography, SWE elastography is incorporated into an ultrasound system. Thus, a conventional ultrasound can be performed at the same time to select an area of the parenchyma without blood vessels or focal changes [14]. Moreover, this method is not influenced by the respiratory stages and does not require the cooperation of children on breathing, which is quite challenging at a young age. Several papers have reported higher accuracy of SWE than TE and it is also less operator-dependent [14; 15]. Another great advantage is the fact that the color map can be stored and retrospectively analyzed as opposed to TE, making it especially suitable for children. SWE and TE are based on measuring the shear wave velocity propagating through the liver tissue. The liver stiffness in kPa is subsequently determined by converting the shearwave speed using [PERSON]'s elastic modulus (E = 3\(\rho\)v\({}^{2}\), where E is the stiffness, v is the shear wave velocity (m/s) and \(\rho\) is the density of tissue). The calculation is based on assumptions that the tissue is of constant density, homogenous, incompressible and with a linear elastic response. Thus, the conversion from velocity to stiffness may be inaccurate and it is preferable to report both measurements--velocity and stiffness [16]. Non-invasive serum markers have also been studied in various liver conditions in adults and children, yet fewer studies have been performed on non-invasive serum markers of liver fibrosis in postoperative Fontan and Glenn patients [17]. FibroTest investigation ( proprietary formula, Bipredictive, Paris, France) was developed by [PERSON] and colleagues [18] and is based on an algorithm that uses the results of several serum biochemical markers (alpha 2 macroglobulin, haptoglobin, apolipoprotein A1, total bilirubin, gammaglutamyl-transpeptidase) to evaluate the level of fibrosis and necro-inflammatory activity [19]. The Actitest estimates the degree of necro-inflammatory activity by associating ALT with the before-mentioned markers. These investigations have been validated in patients with chronic hepatitis B and C, toxic hepatopathy and non-alcoholic hepatic steatosis [20; 21; 22]. The AST to Platelet ratio index (APRI) was originally used to estimate the level of liver fibrosis in patients with chronic hepatitis C. The APRI values > 0.7 showed a sensitivity of 77% and a specificity of 72% for the detection of fibrosis. For the detection of severe fibrosis or cirrhosis of the liver, it has been shown that APRI > 1 has a sensitivity of 61% and a specificity of 64% [23]. The FIB-4 score, originally developed for HIV-HCV co-infected patients, uses age, AST, ALT and platelet count to estimate liver fibrosis [24]. A FIB-4 score > 3.25 has a specificity of 97% and a positive predictive value of 65% for advanced fibrosis. The aim of the study was to evaluate the degree of liver damage in patients that underwent Fontan or Glenn surgery compared to a group of healthy children using non-invasive methods, such as liver elastography and biochemical markers of fibrosis and liver inflammation. Secondly, we aimed to compare the serological biomarkers and elastography results of the Fontan group to the Glenn group.