The paper deals with modeling the liver perfusion intended to improve quantitative analysis of the tissue scans provided by the contrast-enhanced computed tomography (CT). For this purpose, we developed a model of dynamic transport of the contrast fluid through the hierarchies of the perfusion trees. Conceptually, computed time-space distributions of the so-called tissue density can be compared with the measured data obtained from CT; such a modeling feedback can be used for model parameter identification. The blood flow is characterized at several scales for which different models are used. Flows in upper hierarchies represented by larger branching vessels are described using simple 1D models based on the Bernoulli equation extended by correction terms to respect the local pressure losses. To describe flows in smaller vessels and in the tissue parenchyma, we propose a 3D continuum model of porous medium defined in terms of hierarchically matched compartments characterized by hydraulic permeabilities. The 1D models corresponding to the portal and hepatic veins are coupled with the 3D model through point sources, or sinks. The contrast fluid saturation is governed by transport equations adapted for the 1D and 3D flow models. The complex perfusion model has been implemented using the finite element and finite volume methods. We report numerical examples computed for anatomically relevant geometries of the liver organ and of the principal vascular trees. The simulated tissue density corresponding to the CT examination output reflects a pathology modeled as a localized permeability deficiency.

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Considering the fact that hemodynamics plays an important role in the patency and overall performance of implanted bypass grafts, this work presents a numerical investigation of pulsatile non-Newtonian blood flow in three different patient-specific aorto-coronary bypasses. The three bypass models are distinguished from each other by the number of distal side-to-side and end-to-side anastomoses and denoted as single, double and triple bypasses. The mathematical model in the form of time-dependent nonlinear system of incompressible Navier-Stokes equations is coupled with the Carreau-Yasuda model describing the shear-thinning property of human blood and numerically solved using the principle of the SIMPLE algorithm and cell-centred finite volume method formulated for hybrid unstructured tetrahedral grids. The numerical results computed for non-Newtonian and Newtonian blood flow in the three aorto-coronary bypasses are compared and analysed with emphasis placed on the distribution of cycle-averaged wall shear stress and oscillatory shear index. As shown in this study, the non-Newtonian blood flow in all of the considered bypass models does not significantly differ from the Newtonian one. Our observations further suggest that, especially in the case of sequential grafts, the resulting flow field and shear stimulation are strongly influenced by the diameter of the vessels involved in the bypassing.

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Background: In clinical medicine, little is known about the use of allografts for portal vein (PV) reconstruction after pancreaticoduodenectomy (PD). Portal and caval systems are physiologically different, therefore the properties of allografts from caval and portal systems were studied here in a pig model. Materials and Methods: PD with PV reconstruction with allogeneic venous graft from PV or inferior vena cava (IVC) was performed in 26 pigs. Biochemical analysis and ultrasonography measurements were performed during a 4-week monitoring period. Computer simulations were used to evaluate haemodynamics in reconstructed PV and explanted allografts were histologically examined. Results: The native PV and IVC grafts varied in histological structure but were able to adapt morphologically after transplantation. Computer simulation suggested PV grafts to be more susceptible to thrombosis development. Thrombosis of reconstructed PV occurred in four out of five cases in PV group. Conclusion: This study supports the use of allografts from caval system for PV reconstruction in clinical medicine when needed.Pancreatic cancer is one of the leading causes of cancer mortality in developed countries; its incidence has been rising over last decades and without a breakthrough in therapy this trend is expected to continue (1, 2). The incidence of pancreatic cancer is almost equal to its mortality rate and the estimated 5-year survival rate is only 5% (1, 3).Nowadays the only potential curative therapy of this disease is radical surgical resection, which can help to increase the 5year survival rate up to 25% (4). The standard surgical method for curative resection of tumours arising in the head of the pancreas is pylorus-preserving pancreaticoduodenectomy (PPPD) (5). However, the resectability of pancreatic cancer is limited by vessel infiltration. In the presence of tumour ingrowth into adjacent arteries, surgical resection is not generally recommended due to high postoperative morbidity and mortality (4, 6). However, tumours infiltrating venous structures [such as the superior mesenteric vein (SMV), and portal vein (PV)] can be safely resected together with involved part of the vein, resulting in morbidity and mortality comparable with standard PPPD (4, 7).Resection of either PV or SMV during PPPD requires a suitable reconstruction of the involved vessel. In cases when only a tangential resection or short segmental resection is performed, the vein can be reconstructed by venorrhaphy, patch plasty or primary anastomosis. When a longer venous 6603

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Physiologically realistic results are the aim of every blood flow simulation. This is not different in aorto‐coronary bypasses where the properties of the coronary circulation may significantly affect the relevance of the performed simulations. By considering three patient‐specific bypass geometries, the present article focuses on two aspects of the coronary blood flow – its phasic flow pattern and its behaviour affected by blood rheology. For the phasic flow property, a multiscale modelling approach is chosen as a means to assess the ability of five different types of coronary boundary conditions (mean arterial pressure, Windkessel model and three lumped parameter models) to attain realistic coronary haemodynamics. From the analysed variants of boundary conditions, the best option in terms of physiological characteristics and its potential for use in patient‐based simulations, is utilised to account for the effect of shear‐dependent viscosity on the resulting haemodynamics and wall shear stress stimulation. Aside from the Newtonian model, the blood rheology is approximated by two non‐Newtonian models in order to determine whether the choice of a viscosity model is important in simulations involving coronary circulation. A comprehensive analysis of obtained results demonstrated notable superiority of all lumped parameter models, especially in comparison to the constant outlet pressure, which regardless of bypass type gave overestimated and physiologically misleading results. In terms of rheology, it was noted that blood in undamaged coronary arteries behaves as a Newtonian fluid, whereas in vessels with atypical lumen geometry, such as that of anastomosis or stenosis, its shear‐thinning behaviour should not be ignored.