Influence of portal vein occlusion on portal flow and liver elasticity in an animal model

Hepatic fibrosis causes an increase in liver stiffness, a parameter measured by elastography and widely used as a diagnosis method. The concomitant presence of portal vein thrombosis (PVT) implies a change in hepatic portal inflow that could also affect liver elasticity. The main objective of this study is to determine the extent to which the presence of portal occlusion can affect the mechanical properties of the liver and potentially lead to misdiagnosis of fibrosis and hepatic cirrhosis by elastography. Portal vein occlusion was generated by insertion and inflation of a balloon catheter in the portal vein of four swines. The portal flow parameters peak flow (PF) and peak velocity magnitude (PVM) and liver mechanical properties (shear modulus) were then investigated using 4D-flow MRI and MR elastography, respectively, for progressive obstructions of the portal vein. Experimental results indicate that the reduction of the intrahepatic venous blood flow (PF/PVM decreases of 29.3%/8.5%, 51.0%/32.3% and 83.3%/53.6%, respectively) measured with 50%, 80% and 100% obstruction of the portal vein section results in a decrease of liver stiffness by $0.8\%\pm0.1\%$, $7.7\%\pm0.4\%$ and $12.3\%\pm0.9\%$, respectively. While this vascular mechanism does not have sufficient influence on the elasticity of the liver to modify the diagnosis of severe fibrosis or cirrhosis (F4 METAVIR grade), it may be sufficient to attenuate the increase in stiffness due to moderate fibrosis (F2-F3 METAVIR grades) and consequently lead to false-negative diagnoses with elastography in the presence of PVT

In plane quantification of in vivo muscle elastic anisotropy factor by steered ultrasound pushing beams

Skeletal muscles are organized into distinct layers and exhibit anisotropic characteristics across various scales. Assessing the arrangement of skeletal muscles may provide valuable biomarkers for diagnosing muscle related pathologies and evaluating the efficacy of clinical interventions. In this study, we propose a novel ultrafast ultrasound sequence constituted of steered pushing beams was proposed for ultrasound elastography applications in transverse isotropic muscle. Based on the propagation of the shear wave vertical mode, it is possible to fit the experimental results to retrieve in the same imaging plane, the shear modulus parallel to fibers as well as the elastic anisotropy factor. The technique was demonstrated in vitro in phantoms and ex vivo in fusiform beef muscles. At last, the technique was applied in vivo on fusiform muscles (biceps braachi) and mono-penate muscles (gastrocnemius medialis) during stretching and contraction. This novel sequence provides access to new structural and mechanical biomarkers of muscle tissue, including the elastic anisotropy factor, within the same imaging plane. Additionally, it enables the investigation of multiples parameters during muscle active and passive length changes

Magnetic Resonance Elastography and Portal Hypertension: Influence of the Portal Venous Flow on the Liver Stiffness

International audienceThe invasive measurement of the hepatic venous pressure gradient is still considered as the reference method to assess the severity of portal hypertension. Even though previous studies have shown that the liver stiffness measured by elastography could predict portal hypertension in patients with chronic liver disease, the mechanisms behind remain today poorly understood. The main reason is that the liver stiffness is not specific to portal hypertension and is also influenced by concomitant pathologies, such as cirrhosis. Portal hypertension is also source of a vascular incidence, with a substantial diversion of portal venous blood to the systemic circulation, bypassing the liver. This study focuses on this vascular effect of portal hypertension. We propose to generate and control the portal venous flow (to isolate the modifications in the portal venous flow as single effect of portal hypertension) in an anesthetized pig and then to quantify its implications on liver stiffness by an original combination of MRE and 4D-Flow Magnetic Resonance Imaging (MRI). A catheter balloon is progressively inflated in the portal vein and the peak flow, peak velocity magnitude and liver stiffness are quantified in a 1.5T MRI scanner (AREA, Siemens Healthcare, Erlangen, Germany). A strong correlation is observed between the portal peak velocity magnitude, the portal peak flow or the liver stiffness and the portal vein intraluminal obstruction. Moreover, the comparison of mechanical and flow parameters highlights a correlation with the possibility of identifying linear relationships. These results give preliminary indications about how liver stiffness can be affected by portal venous flow and, by extension, by hypertension

Asymptotic Expansions for Stationary Distributions of Perturbed Semi-Markov Processes

New algorithms for computing of asymptotic expansions for stationary distributions of nonlinearly perturbed semi-Markov processes are presented. The algorithms are based on special techniques of sequential phase space reduction, which can be applied to processes with asymptotically coupled and uncoupled finite phase spaces.Comment: 83 page

Contribution à la compréhension des mécanismes de lésion cérébrale en situation de choc: Inclusion de l’anisotropie et de l’hétérogénéité par techniques d’imagerie médicale

Brain is the most commonly injured vital segment in case of road accident. Due to its complexity, intra-cerebral injuries remain still difficult to describe and diagnose. The link between mechanical head loadings and brain injuries is proposed by biomechanical analysis. Recent experimental studies have highlighted the importance of heterogeneity and anisotropy of the tissue mechanical response in case of head trauma. Main origin is imputed to axonal network. After a detailed state of the art about brain mechanical properties investigations, aim of this thesis is to improve the realism of behavior laws of Finite Element Models (FEM) by including information from recent medical imaging techniques. The influence of heterogeneity of brain tissue on its response to head impact is analyzed by developing heterogeneous brain FEM based on experimental in vivo Magnetic Resonance Elastography (MRE) data. Meanwhile, the anisotropy brain is highlighted by studying the deformation of axons in case of traumatic injury. The developed method is based on in vivo data from Diffusion Tensor Imaging (DTI) that inform on axons distribution into the brain. The overall results are summarized for the development of a viscohyperelastic anisotropic and heterogeneous human brain FEM based on in vivo MRE and DTI data. This new model provides a better predictability of diffuse axonal injury in case of head trauma.Le cerveau constitue le segment vital le plus fréquemment lésé en situation de choc traumatique. De par la complexité de cet organe, les lésions intracérébrales restent encore aujourd’hui difficiles tant à prédire qu’à décrire et à diagnostiquer. L’analyse biomécanique permet notamment de faire le lien entre les chargements mécaniques et les lésions subies par l’encéphale. De récentes études expérimentales ont mis en avant l’importance de l’hétérogénéité et de l’anisotropie du tissu mécanique sur sa réponse en cas de chargement mécanique, tout en en imputant l’origine aux réseaux d’axones structurant le segment céphalique. Après la mise en place d’un état de l’art relativement exhaustif, l’objectif de cette thèse est d’améliorer le réalisme des lois de comportements des modèles par éléments finis (MEF) en s’appuyant sur des techniques d’imagerie médicale récemment développées. L’influence de l’hétérogénéité du tissu cérébral sur sa réponse aux chocs traumatiques est analysée en développant des MEF hétérogènes de cerveau à partir de données expérimentales in vivo d’Elastographie par Résonance Magnétique (ERM). Parallèlement, l’anisotropie cérébrale est mise en évidence par l’étude de la déformation des axones lors de chocs traumatiques, en s’appuyant sur des données in vivo d’Imagerie du Tenseur de Diffusion (DTI). L’ensemble des résultats obtenus est synthétisé pour la mise en place d’un MEF visco-hyperélastique anisotrope et hétérogène de cerveau humain basé sur des données in vivo d’ERM et de DTI. Ce nouveau modèle permet une meilleure prédictibilité des lésions axonales diffuses en situation de choc traumatique

Contribution à la compréhension des mécanismes de lésion cérébrale en situation de choc (Inclusion de l'anisotropie et de l'hétérogénéité par techniques d'imagerie médicale)

Le cerveau constitue le segment vital le plus fréquemment lésé en situation de choc traumatique. De par la complexité de cet organe, les lésions intracérébrales restent encore aujourd hui difficiles tant à prédire qu à décrire et à diagnostiquer. L analyse biomécanique permet notamment de faire le lien entre les chargements mécaniques et les lésions subies par l encéphale. De récentes études expérimentales ont mis en avant l importance de l hétérogénéité et de l anisotropie du tissu mécanique sur sa réponse en cas de chargement mécanique, tout en en imputant l origine aux réseaux d axones structurant le segment céphalique. Après la mise en place d un état de l art relativement exhaustif, l objectif de cette thèse est d améliorer le réalisme des lois de comportements des modèles par éléments finis (MEF) en s appuyant sur des techniques d imagerie médicale récemment développées. L influence de l hétérogénéité du tissu cérébral sur sa réponse aux chocs traumatiques est analysée en développant des MEF hétérogènes de cerveau à partir de données expérimentales in vivo d Elastographie par Résonance Magnétique (ERM). Parallèlement, l anisotropie cérébrale est mise en évidence par l étude de la déformation des axones lors de chocs traumatiques, en s appuyant sur des données in vivo d Imagerie du Tenseur de Diffusion (DTI). L ensemble des résultats obtenus est synthétisé pour la mise en place d un MEF visco-hyperélastique anisotrope et hétérogène de cerveau humain basé sur des données in vivo d ERM et de DTI. Ce nouveau modèle permet une meilleure prédictibilité des lésions axonales diffuses en situation de choc traumatique.Brain is the most commonly injured vital segment in case of road accident. Due to its complexity, intra-cerebral injuries remain still difficult to describe and diagnose. The link between mechanical head loadings and brain injuries is proposed by biomechanical analysis. Recent experimental studies have highlighted the importance of heterogeneity and anisotropy of the tissue mechanical response in case of head trauma. Main origin is imputed to axonal network. After a detailed state of the art about brain mechanical properties investigations, aim of this thesis is to improve the realism of behavior laws of Finite Element Models (FEM) by including information from recent medical imaging techniques. The influence of heterogeneity of brain tissue on its response to head impact is analyzed by developing heterogeneous brain FEM based on experimental in vivo Magnetic Resonance Elastography (MRE) data. Meanwhile, the anisotropy brain is highlighted by studying the deformation of axons in case of traumatic injury. The developed method is based on in vivo data from Diffusion Tensor Imaging (DTI) that inform on axons distribution into the brain. The overall results are summarized for the development of a viscohyperelastic anisotropic and heterogeneous human brain FEM based on in vivo MRE and DTI data. This new model provides a better predictability of diffuse axonal injury in case of head trauma.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Non linear Biomechanical Model of the Liver: Hyperelastic Constitutive Laws for Finite Element Modeling

International audienceUnderstanding and modeling the liver biomechanics represent a significant challenge due to its complex nature. While many studies have been performed to fit hyperelastic constitutive laws on rheological experiments, they tend to agree about the importance of strain rate in the liver mechanical behavior. Furthermore, as the liver is heavily perfused with blood, its constitutive behavior is greatly porous. Supported by these observations, we developed a porous visco-hyperelastic model as a liver parenchyma material. More precisely, visco-hyperelasticity is obtained through Prony series while the mechanical effect of liver perfusion is represented with a linear Darcy's law. Since this mechanical model is developed in the context of real time surgery simulation, a compromise between biomechanical accuracy and computational efficiency must be found. We propose the Multiplicative Jacobian Energy Decomposition method (MJED) to obtain a fast assembly of stiffness matrices on linear tetrahedral elements