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

Characterization of susceptibility artifacts in magnetic resonance thermometry images during laser interstitial thermal therapy: dimension analysis and temperature error estimation

Objective: Laser interstitial thermal therapy (LITT) is a minimally invasive procedure used to treat a lesion through light irradiation and consequent temperature increase. Magnetic Resonance Thermometry Imaging (MRTI) provides a multidimensional measurement of the temperature inside the target thus enabling accurate monitoring of the zone of damage during the procedure. In proton resonance frequency shift-based thermometry, artifacts in the images may strongly interfere with the estimated temperature maps. In our work, after noticing the formation of the dipolar-behavior artifact linkable to magnetic susceptibility changes during in vivo LITT, an investigation of susceptibility artifacts in tissue-mimicking phantoms was implemented. Approach: The artifact was characterized: (i) by measuring the area and total volume of error regions and their evolution during the treatment; and (ii) by comparison with temperature reference provided by three temperature sensing needles. Lastly, a strategy to avoid artifacts formation was devised by using the temperature-sensing needles to implement a temperature-controlled LITT. Main results: The artifact appearance was associated with gas bubble formation and with unwanted treatment effects producing magnetic susceptibility changes when 2 W laser power was set. The analysis of the artifact's dimension demonstrated that in the sagittal plane the dipolar-shape artifact may consistently spread following the temperature trend until reaching a volume 8 times bigger than the ablated one. Also, the artifact shape is quite symmetric with respect to the laser tip. An absolute temperature error showing a negative Gaussian profile in the area of susceptibility artifact with values up to 64.4 °C was estimated. Conversely, a maximum error of 2.8 °C is measured in the area not-affected by artifacts and far from the applicator tip. Finally, by regulating laser power, susceptibility artifacts formation was avoided, and appreciable thermal damage was induced. Significance: Such findings may help in improving the MRTI-based guidance of thermal therapies

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

Analysis of cavitation artifacts in Magnetic Resonance Imaging Thermometry during laser ablation monitoring

: Magnetic Resonance Thermometry Imaging (MRTI) holds great potential in laser ablation (LA) monitoring. It provides the real-time multidimensional visualization of the treatment effect inside the body, thus enabling accurate intraoperative prediction of the thermal damage induced. Despite its great potential., thermal maps obtained with MRTI may be affected by numerous artifacts. Among the sources of error producing artifacts in the images., the cavitation phenomena which could occur in the tissue during LA induces dipole-structured artifacts. In this work., an analysis of the cavitation artifacts occurring during LA in a gelatin phantom in terms of symmetry in space and symmetry of temperature values was performed. Results of 2 Wand 4 W laser power were compared finding higher symmetry for the 2 W case in terms of both dimensions of artifact-lobes and difference in temperature values extracted in specular pixels in the image. This preliminary investigation of artifact features may provide a step forward in the identification of the best strategy to correct and avoid artifact occurrence during thermal therapy monitoring. Clinical Relevance- This work presents an analysis of cavitation artifacts in MRTI from LA which must be corrected to avoid error in the prediction of thermal damage during LA monitoring

Modélisation de la dynamique des compartiments liquidiens intracraniens (de l'approche globale ad hoc à la description physique a priori)

Cette thèse est une contribution à la modélisation et la compréhension des mécanismes biophysiques régissant la dynamique intracrânienne. Les connaissances anatomiques et fonctionnelles sur le système intracrânien proviennent essentiellement d une technique d exploration très performante, l imagerie par résonance magnétique (IRM) de flux. Le système intracrânien est soumis à une dynamique complexe régissant à la fois la circulation du LCS entre les divers compartiments le constituant, l évolution temporelle des pressions dans ces compartiments et leurs réponses viscoélastiques aux pulsations cardiaques. L une des pathologies les plus graves de ce système est l hydrocéphalie, elle se manifeste par une dilatation anormale des ventricules cérébraux. Notre objectif est double : (i) Construire un modèle global de la dynamique intracrânienne s appuyant sur des analogies électriques traduisant son caractère hautement dissipatif (résistances) et élastique ou compliant (capacités) et sur les données IRM de flux issue de sujets sains ; (ii) Relier la dilatation ventriculaire aux caractéristiques de la dynamique régissant les processus intracrâniens. Un modèle global de la dynamique intracrânienne est un modèle multi-compartimental dont l architecture reflète fidèlement la structure anatomique du système. Notre modèle rendre compte des données d observation IRM des flux sanguin et LCS. La construction d un modèle biophysique global du système intracrânien selon la voie analytique s avère difficile : nous nous heurtons très vite à l étape incontournable de la réduction de la complexité du système. Il est donc nécessaire de construire pas à pas les concepts clés, absorbant cette complexité et qui constitueront les ingrédients du modèle complet. Il nous est donc apparu plus pertinent de modéliser les compartiments en présence individuellement . Cette procédure a été appliquée en premier lieu à la dynamique des ventricules. Ce modèle retient comme processus de régulation essentiel, celui du volume des ventricules et donc celui de la pression intra-ventriculaire. Cette régulation est assurée par les propriétés viscoélastiques d une membrane tapissant l intérieur des ventricules, l épendyme. Selon notre approche, l hydrocéphalie pourrait être décrite comme une instabilité dynamique du système ventriculaire.AMIENS-BU Santé (800212102) / SudocSudocFranceF

Quantification and mapping of cerebral hemodynamics before and after carotid endarterectomy, using four-dimensional flow magnetic resonance imaging

Background and purpose: A carotid stenosis can have a profound impact on the cerebral hemodynamics that cannot be inferred from the degree of stenosis by itself. We aimed to quantify and map the distribution of blood flow rate (BFR) in cerebral arteries before and after carotid endarterectomy (CEA), using four-dimensional phase-contrast magnetic resonance imaging (4D PCMRI). Methods: Nineteen patients (71±6 years, 2 women) with symptomatic carotid stenosis (≥50%)undergoing CEA were investigated using 4D PCMRI before and after surgery. BFR was measured in 17 cerebral arteries and in the ophthalmic arteries (OA). Collateral recruitment through the anterior and posterior communicating arteries, OA and the leptomeningeal arterial route was identified and quantified. BFR laterality was described as contralateral BFR minus ipsilateral BFR in paired arteries. Results: Total cerebral blood flow increased by 15% (p&lt;0.01) after CEA. On the ipsilateral side, increased BFR was found after CEA in internal carotid artery (ICA) (246±62mL/min vs. 135±80mL/min; p&lt;0.001), anterior cerebral artery (87±mL/min vs. 38±58mL/min; p&lt;0.01) and middle cerebral artery (MCA) (149±43mL/min vs. 119±34mL/min; p&lt;0.01), resulting in a postoperative BFR distribution without signs of laterality. In patients with preoperatively recruited collaterals (n=9), BFR laterality was found in MCA before, but not after, CEA (p&lt;0.01). This laterality was not found in patients without collateral recruitment (n=10) (p=0.2). The degree of stenosis did not differ between the groups with vs. without collateral recruitment (p=0.85).  Conclusion: 4D PCMRI is a useful technique to quantify cerebral hemodynamic changes seen in patients with carotid stenosis before and after CEA. MCA laterality, seen in patients with collateral recruitment before CEA, pointed towards a hemodynamic disturbance in MCA territory for those patients. This study introduces a new and non-invasive way to evaluate cerebral hemodynamics due to carotid stenosis prior to and after CEA.Originally included in thesis in manuscript form with title "Quantification and mapping of cerebral hemodynamics before and after carotid endarterectomy: a 4D PCMRI study"</p

MAGNETIC RESONANCE ELASTOGRAPHY: INFLUENCE OF THE PORTAL VENOUS FLOW ON THE LIVER STIFFNESS

International audienceBackground: This presentation aims not only to present the base concepts of Magnetic Resonance Elastography (MRE), but also to present a specific in vivo application to link the liver elasticity measurement to the hepatic vascular flow (as described below). Aims: The invasive measurement of the hepatic venous pressure gradient (HVPG) is still considered as the reference method to assess the degree of Portal Hypertension (PH) [Boleslawski et al. 2012]. Even though previous studies have shown that the liver stiffness (LS) measured by elastography could predict PH in patients with chronic liver disease [Ipek-Ugay et al. 2016] the mechanisms behind remain today poorly understood. The main reason is that the LS is not specific to PH and is also influenced by concomitant pathologies, such as cirrhosis [Bavu et al. 2011; Ferraioli et al. 2012]. PH is also source of a substantial diversion of portal venous blood to the systemic circulation, bypassing the liver [Berzigotti et al. 2013]. This study focuses on this effect of PH. Methods: We propose to generate and control the portal venous flow (PVF) (to isolate the modifications in the PVF as single effect of PH) in two pigs and then to quantify its implications on LS by an original combination of MRE and 4D-Flow Magnetic Resonance Imaging (MRI), respectively. A catheter balloon is progressively inflated in the portal vein (Fig.(A)) and the following parameters are quantified in a 1.5T MRI scanner (AREA, Siemens Healthcare, Erlangen, Germany): (1) the peak flow (PF) and the peak velocity magnitude (PVM) (4D-Flow, Fig.(B)); (2) the LS µ (MRE, Fig.(C)). Results: A strong correlation appears between both the PVM, PF or the LS and the inflation state of the balloon. Moreover, the comparison of LS with PVM or PF highlights a strong correlation of these parameters with the possibility of identifying linear relationships (Fig.(D)). Conclusions: These results are thought to give indications about how LS can be affected by PH. Otherwise, this study provides unprecedented information for the mechanical modeling of the liver (including mechanical properties and vascular flows) dedicated to computer-assisted surgery. Fig. A catheter balloon is successively inflated in the portal vein (A). The flow (B) and mechanical (C) parameters are successively measured. LS is shown as correlated to PVM and PF (D). Acknowledgements: This work has benefitted from support of the ANR (Agence Nationale de la Recherche) by the French national program "Investissements d'Avenir" (IHU, ANR-10-IAHU-02)

A Stereotactic Probabilistic Atlas for the Major Cerebral Arteries

Improved whole brain angiographic and velocity-sensitive MRI is pushing the boundaries of noninvasively obtained cerebral vascular flow information. The complexity of the information contained in such datasets calls for automated algorithms and pipelines, thus reducing the need of manual analyses by trained radiologists. The objective of this work was to lay the foundation for such automated pipelining by constructing and evaluating a probabilistic atlas describing the shape and location of the major cerebral arteries. Specifically, we investigated how the implementation of a non-linear normalization into Montreal Neurological Institute (MNI) space improved the alignment of individual arterial branches. In a population-based cohort of 167 subjects, age 64-68 years, we performed 4D flow MRI with whole brain volumetric coverage, yielding both angiographic and anatomical data. For each subject, sixteen cerebral arteries were manually labeled to construct the atlas. Angiographic data were normalized to MNI space using both rigid-body and non-linear transformations obtained from anatomical images. The alignment of arterial branches was significantly improved by the non-linear normalization (p &lt; 0.001). Validation of the atlas was based on its applicability in automatic arterial labeling. A leave-one-out validation scheme revealed a labeling accuracy of 96 %. Arterial labeling was also performed in a separate clinical sample (n = 10) with an accuracy of 92.5 %. In conclusion, using non-linear spatial normalization we constructed an artery-specific probabilistic atlas, useful for cerebral arterial labeling