Analyzing the human liver vascular architecture by combining vascular corrosion casting and micro-CT scanning: a feasibility study

Magnetic-Controlled Microrobot: Real-Time Detection and Tracking through Deep Learning Approaches

As one of the most significant research topics in robotics, microrobots hold great promise in biomedicine for applications such as targeted diagnosis, targeted drug delivery, and minimally invasive treatment. This paper proposes an enhanced YOLOv5 (You Only Look Once version 5) microrobot detection and tracking system (MDTS), incorporating a visual tracking algorithm to elevate the precision of small-target detection and tracking. The improved YOLOv5 network structure is used to take magnetic bodies with sizes of 3 mm and 1 mm and a magnetic microrobot with a length of 2 mm as the pretraining targets, and the training weight model is used to obtain the position information and motion information of the microrobot in real time. The experimental results show that the accuracy of the improved network model for magnetic bodies with a size of 3 mm is 95.81%, representing an increase of 2.1%; for magnetic bodies with a size of 1 mm, the accuracy is 91.03%, representing an increase of 1.33%; and for microrobots with a length of 2 mm, the accuracy is 91.7%, representing an increase of 1.5%. The combination of the improved YOLOv5 network model and the vision algorithm can effectively realize the real-time detection and tracking of magnetically controlled microrobots. Finally, 2D and 3D detection and tracking experiments relating to microrobots are designed to verify the robustness and effectiveness of the system, which provides strong support for the operation and control of microrobots in an in vivo environment.

Cirrhotic hepatocellular carcinoma-based decellularized liver cancer model for local chemoembolization evaluation

Transarterial chemoembolization (TACE) is a common treatment for unresectable intermediate stage hepatocellular carcinoma (HCC) and involves the combination of chemotherapy agents and embolic materials to target and block the blood supply to the tumor, leading to localized treatment. However, the selection of clinical chemoembolization agents remains limited, and the effectiveness of various agents is still under investigation. Meanwhile, replicating the complex vasculature and extracellular matrix (ECM) circumstances of HCC in in vitro models for evaluating embolic agents proves to be challenging. Herein, we developed a decellularized cancerous liver model with translucent appearance, a complicated hepatic vascular system and tissue-specific ECM for the evaluation of embolic agents. Inkpad oil and microparticles were used to illustrate different systems of vascular structures between healthy and HCC rats' livers. Quantitative analysis with AngioTool revealed significant differences in vessel density and lacunarity between the two groups. Proteomics showed higher secretion of collagens in the HCC rat liver models than in healthy livers. Utilizing this in vitro model, we investigated the impact of tumor-specific vascular structure and ECM composition on chemoembolization performance, the two key factors inaccessible by currently available drug release testing platforms. Our findings revealed that the presence of an aberrant vascular system and the distorted ECM within the model led to drug retention. This preclinical model holds great promise as a valuable tool for evaluating embolic agents and studying their performance in the tumor microenvironment. STATEMENT OF SIGNIFICANCE: Transarterial chemoembolization (TACE), which employs drug-eluting embolic agents to obstruct the tumor-feeding vessels while locally releasing chemotherapeutic drugs into the tumor, has become the first-line treatment of unresectable liver cancer over past two decades. Nevertheless, the advancement of effective drug-eluting embolic agents has been retarded due to the lack of appropriate in vitro models for assessing the local embolization and chemotherapy performances in TACE. Here we developed a cirrhotic hepatocellular carcinoma-based decellularized liver cancer model, which preserves the aberrant vasculatures and tumor-specific extracellular matrix of liver cancer, for TACE evaluation. This model incorporates a blood flow simulation component to assess the dynamics of drug release behaviors of chemoembolic agents within tumor-mimicking conditions, more accurately replicating the in vivo environment for the locoregional assessments as compared to conventional in vitro models.

Hepatic portal vein branching patterns according to different liver assessment methods and classifications of branching type

BACKGROUND

It is known that there are varying frequencies of hepatic portal vein branching patterns found in the literature. Studies use different methods and classifications to evaluate the anatomy of the portal vein, which limits accurate comparison between studies and the determination of true frequency of branching patterns in different populations. The aim of the present study was to investigate the intrahepatic branching of the portal vein in corrosive samples using different methods - somatoscopic and computed tomography (CT) and compare with similar studies as well as compare the reclassified data according to the most popular classifications used in the literature.

METHODS

A total of 105 liver corrosion specimens from the 1960-1980 period (51 male and 54 female individuals; min-max age variation - 21-90 y., M=59,46 y.) were investigated. The branching patterns of the hepatic portal vein (HPV), left (HPV-LB) and right branch of hepatic portal vein (HPV-RB), and their segmental branches were examined and scanned by CT. Standard HPV ramification was considered, when HPV divided into HPV-LB and HPV-RB, HPV-RB bifurcated to the anterior and posterior branches, and further segmental ramification into the superior and inferior branches was considered standard. We compared the HPV main branch length and diameter measurements between manual and CT method. A review of the literature was performed on portal vein branching variations.

RESULTS

The standard HPV ramification pattern was detected in 85.7% of the cases in both somatoscopic and CT evaluation. Variations related to the main branches were HPV trifurcation - 7.6%, posterior branch of right branch of hepatic portal vein from HPV - 4.8% and 5.7%, HPV quadrifurcation 1.9% and 1% respectively, in somatoscopic and CT evaluation. There was a significant difference between HPV-LB length and diameter in CT and manual measurements. According to the literature, more variations are seen using the CT method versus somatoscopic corrosion cast evaluation. The varying frequency in studies may be explained by a lack of one unanimous classification of branching patterns (some authors do not consider segmental variations as standard HPV ramification) and different evaluation methods.

CONCLUSION

Somatoscopic evaluation of the branching patterns of the hepatic portal vein in corroded specimens and their CT reconstructions did not differ significantly (which allows relatively accurate comparison of old specimens with newer data). However, the ability to evaluate the reconstructed 3D images of the specimens allowed a more accurate assessment of segmental branching and measurements of lengths and diameters. Standard HPV branching (according to a self-developed classification) in this study was 85.7%. Depending on the classification, the rate of standard branching in the same corrosive samples varied from 63.8% to 84.8% of all cases, indicating that the lack of a unified and stable classification makes it difficult to compare the results of different studies. Deviations from standard branching are very important in surgical procedures and liver transplantation.

Magnetic particle image scanner based on asymmetric core-filled electromagnetic actuator

Monitoring the distribution of magnetic nanoparticles (MNPs) in the vascular system is an important task for the advancement of precision therapeutics and drug delivery. Despite active targeting using active motilities, it is required to visualize the position and concentration of carriers that reach the target, to promote the development of this technology. In this work, a feasibility study is presented on a tomographic scanner that allows monitoring of the injected carriers quantitatively in a relatively short interval. The device is based on a small-animal-scale asymmetric magnetic platform integrated with magnetic particle imaging technology. An optimized isotropic field-free region (FFR) generation method using a magnetic manipulation system (MMS) is derived and numerically investigated. The in-vitro and in-vivo tracking performances are demonstrated with a high position accuracy of approximately 1 mm. A newly proposed tracking method was developed, specialized in vascular system, with quick scanning time (about 1s). In this paper, the primary function of the proposed system is to track magnetic particles using a magnetic manipulation system. Through this, proposed method enables the conventional magnetic actuation systems to upgrade the functionalities of both manipulation and localization of magnetic objects.

From Corrosion Casting to Virtual Dissection: Contrast-Enhanced Vascular Imaging using Hafnium Oxide Nanocrystals

Vascular corrosion casting is a method used to visualize the three dimensional (3D) anatomy and branching pattern of blood vessels. A polymer resin is injected in the vascular system and, after curing, the surrounding tissue is removed. The latter often deforms or even fractures the fragile cast. Here, a method is proposed that does not require corrosion, and is based on in situ micro computed tomography (micro-CT) scans. To overcome the lack of CT contrast between the polymer cast and the animals' surrounding soft tissue, hafnium oxide nanocrystals (HfO2 NCs) are introduced as CT contrast agents into the resin. The NCs dramatically improve the overall CT contrast of the cast and allow for straightforward segmentation in the CT scans. Careful design of the NC surface chemistry ensures the colloidal stability of the NCs in the casting resin. Using only 5 m% of HfO2 NCs, high-quality cardiovascular casts of both zebrafish and mice can be automatically segmented using CT imaging software. This allows to differentiate even μ $\umu$ m-scale details without having to alter the current resin injection methods. This new method of virtual dissection by visualizing casts in situ using contrast-enhanced CT imaging greatly expands the application potential of the technique.

Lumped parameter liver simulation to predict acute haemodynamic alterations following partial resections

Partial liver resections are routinely performed in living donor liver transplantation and to debulk tumours in liver malignancies, but surgical decisions on vessel reconstruction for adequate inflow and outflow are challenging. Pre-operative evaluation is often limited to radiological imaging, which fails to account for post-resection haemodynamic alterations. Substantial evidence suggests post-surgical increase in local volume flow rate enhances shear stress, signalling hepatic regeneration, but excessive shear stress has been postulated to result in small for size syndrome and liver failure. Predicting haemodynamic alterations throughout the liver is particularly challenging due to the dendritic architecture of the vasculature, spanning several orders of magnitude in diameter. Therefore, we developed a mathematical lumped parameter model with realistic heterogeneities capturing inflow/outflow of the human liver to simulate acute perfusion alterations following surgical resection. Our model is parametrized using clinical measurements, relies on a single free parameter and accurately captures established perfusion characteristics. We quantify acute changes in volume flow rate, flow speed and wall shear stress following variable, realistic liver resections and make comparisons with the intact liver. Our numerical model runs in minutes and can be adapted to patient-specific anatomy, providing a novel computational tool aimed at assisting pre- and intra-operative surgical decisions for liver resections.

Volumetric Tissue Imaging of Surgical Tissue Specimens Using Micro-Computed Tomography: An Emerging Digital Pathology Modality for Nondestructive, Slide-Free Microscopy-Clinical Applications of Digital Pathology in 3 Dimensions

OBJECTIVES

Micro-computed tomography (micro-CT) is a novel, nondestructive, slide-free digital imaging modality that enables the acquisition of high-resolution, volumetric images of intact surgical tissue specimens. The aim of this systematic mapping review is to provide a comprehensive overview of the available literature on clinical applications of micro-CT tissue imaging and to assess its relevance and readiness for pathology practice.

METHODS

A computerized literature search was performed in the PubMed, Scopus, Web of Science, and CENTRAL databases. To gain insight into regulatory and financial considerations for performing and examining micro-CT imaging procedures in a clinical setting, additional searches were performed in medical device databases.

RESULTS

Our search identified 141 scientific articles published between 2000 and 2021 that described clinical applications of micro-CT tissue imaging. The number of relevant publications is progressively increasing, with the specialties of pulmonology, cardiology, otolaryngology, and oncology being most commonly concerned. The included studies were mostly performed in pathology departments. Current micro-CT devices have already been cleared for clinical use, and a Current Procedural Terminology (CPT) code exists for reimbursement of micro-CT imaging procedures.

CONCLUSIONS

Micro-CT tissue imaging enables accurate volumetric measurements and evaluations of entire surgical specimens at microscopic resolution across a wide range of clinical applications.

Fluid Mechanics Approach to Perfusion Quantification: Vasculature Computational Fluid Dynamics Simulation, Quantitative Transport Mapping (QTM) Analysis of Dynamics Contrast Enhanced MRI, and Application in Nonalcoholic Fatty Liver Disease Classification

OBJECTIVE

We quantify liver perfusion using quantitative transport mapping (QTM) method that is free of arterial input function (AIF). QTM method is validated in a vasculature computational fluid dynamics (CFD) simulation and is applied for processing dynamic contrast enhanced (DCE) MRI images in differentiating liver with nonalcoholic fatty liver disease (NAFLD) from healthy controls using pathology reference in a preclinical rabbit model.

METHODS

QTM method was validated on a liver perfusion simulation based on fluid dynamics using a rat liver vasculature model and the mass transport equation. In the NAFLD grading task, DCE MRI images of 7 adult rabbits with methionine choline-deficient diet-induced nonalcoholic steatohepatitis (NASH), 8 adult rabbits with simple steatosis (SS) were acquired and processed using QTM method and dual-input two compartment Kety's method respectively. Statistical analysis was performed on six perfusion parameters: velocity magnitude | u | derived from QTM, liver arterial blood flow LBFa, liver venous blood flow LBFv, permeability Ktrans, blood volume Vp and extravascular space volume Ve averaged in liver ROI.

RESULTS

In the simulation, QTM method successfully reconstructed blood flow, reduced error by 48% compared to Kety's method. In the preclinical study, only QTM |u| showed significant difference between high grade NAFLD group and low grade NAFLD group.

CONCLUSION

QTM postprocesses DCE-MRI automatically through deconvolution in space and time to solve the inverse problem of the transport equation. Comparing with Kety's method, QTM method showed higher accuracy and better differentiation in NAFLD classification task.

SIGNIFICANCE

We propose to apply QTM method in liver DCE MRI perfusion quantification.

Mixed reality navigation training system for liver surgery based on a high-definition human cross-sectional anatomy data set

OBJECTIVES

This study aims to use the three-dimensional (3D) mixed-reality model of liver, entailing complex intrahepatic systems and to deeply study the anatomical structures and to promote the training, diagnosis and treatment of liver diseases.

METHODS

Vascular perfusion human specimens were used for thin-layer frozen milling to obtain liver cross-sections. The 104-megapixel-high-definition cross sectional data set was established and registered to achieve structure identification and manual segmentation. The digital model was reconstructed and data was used to print a 3D hepatic model. The model was combined with HoloLens mixed reality technology to reflect the complex relationships of intrahepatic systems. We simulated 3D patient specific anatomy for identification and preoperative planning, conducted a questionnaire survey, and evaluated the results.

RESULTS

The 3D digital model and 1:1 transparent and colored model of liver established truly reflected intrahepatic vessels and their complex relationships. The reconstructed model imported into HoloLens could be accurately matched with the 3D model. Only 7.7% participants could identify accessory hepatic veins. The depth and spatial-relationship of intrahepatic structures were better understandable for 92%. The 100%, 84.6%, 69% and 84% believed the 3D models were useful in planning, safer surgical paths, reducing intraoperative complications and training of young surgeons respectively.

CONCLUSIONS

A detailed 3D model can be reconstructed using the higher quality cross-sectional anatomical data set. When combined with 3D printing and HoloLens technology, a novel hybrid-reality navigation-training system for liver surgery is created. Mixed Reality training is a worthy alternative to provide 3D information to clinicians and its possible application in surgery. This conclusion was obtained based on a questionnaire and evaluation. Surgeons with extensive experience in surgical operations perceived in the questionnaire that this technology might be useful in liver surgery, would help in precise preoperative planning, accurate intraoperative identification, and reduction of hepatic injury.

The Influence of Interdisciplinary Work towards Advancing Knowledge on Human Liver Physiology

The knowledge accumulated throughout the years about liver regeneration has allowed a better understanding of normal liver physiology, by reconstructing the sequence of steps that this organ follows when it must rebuild itself after being injured. The scientific community has used several interdisciplinary approaches searching to improve liver regeneration and, therefore, human health. Here, we provide a brief history of the milestones that have advanced liver surgery, and review some of the new insights offered by the interdisciplinary work using animals, in vitro models, tissue engineering, or mathematical models to help advance the knowledge on liver regeneration. We also present several of the main approaches currently available aiming at providing liver support and overcoming organ shortage and we conclude with some of the challenges found in clinical practice and the ethical issues that have concomitantly emerged with the use of those approaches.

Multifunctional microrobot with real-time visualization and magnetic resonance imaging for chemoembolization therapy of liver cancer

Microrobots that can be precisely guided to target lesions have been studied for in vivo medical applications. However, existing microrobots have challenges in vivo such as biocompatibility, biodegradability, actuation module, and intra- and postoperative imaging. This study reports microrobots visualized with real-time x-ray and magnetic resonance imaging (MRI) that can be magnetically guided to tumor feeding vessels for transcatheter liver chemoembolization in vivo. The microrobots, composed of a hydrogel-enveloped porous structure and magnetic nanoparticles, enable targeted delivery of therapeutic and imaging agents via magnetic guidance from the actuation module under real-time x-ray imaging. In addition, the microrobots can be tracked using MRI as postoperative imaging and then slowly degrade over time. The in vivo validation of microrobot system-mediated chemoembolization was demonstrated in a rat liver with a tumor model. The proposed microrobot provides an advanced medical robotic platform that can overcome the limitations of existing microrobots and current liver chemoembolization.

Preliminary Studies on the Intrahepatic Anatomy of the Venous Vasculature in Cats

Hepatic surgeries are often performed in cats to obtain a disease diagnosis, for the removal of masses, or for the treatment of shunts. Whereas the vascular anatomy of the liver has been studied in dogs, such evidence is lacking in cats. The current study used corrosion casts of portal and hepatic veins and computed tomography (CT) analysis of the casts to identify and describe the intrahepatic anatomy in healthy cat livers (n = 7). The results showed that feline livers had a consistent intrahepatic portal and venous anatomy, with only minor disparities in the numbers of secondary and tertiary branches. The feline portal vein consistently divided into two major branches and not three, as previously described in the literature for cats. The finding of a portal vein originating from the right medial lobe branch leading to the quadrate lobe in 4/7 specimens is a novelty of the feline anatomy that was not previously described in dogs. Partial to complete fusion of the caudate process of the caudate and the right lateral lobe, with a lack of clear venous separation between the lobes, was present in two specimens. These findings allowed a detailed description of the most common intrahepatic venous patterns in cats. Further anatomical studies should be encouraged to confirm the present findings and to investigate the utility of this information in surgical settings.

From Seeing to Simulating: A Survey of Imaging Techniques and Spatially-Resolved Data for Developing Multiscale Computational Models of Liver Regeneration

Liver regeneration, which leads to the re-establishment of organ mass, follows a specifically organized set of biological processes acting on various time and length scales. Computational models of liver regeneration largely focused on incorporating molecular and signaling detail have been developed by multiple research groups in the recent years. These modeling efforts have supported a synthesis of disparate experimental results at the molecular scale. Incorporation of tissue and organ scale data using noninvasive imaging methods can extend these computational models towards a comprehensive accounting of multiscale dynamics of liver regeneration. For instance, microscopy-based imaging methods provide detailed histological information at the tissue and cellular scales. Noninvasive imaging methods such as ultrasound, computed tomography and magnetic resonance imaging provide morphological and physiological features including volumetric measures over time. In this review, we discuss multiple imaging modalities capable of informing computational models of liver regeneration at the organ-, tissue- and cellular level. Additionally, we discuss available software and algorithms, which aid in the analysis and integration of imaging data into computational models. Such models can be generated or tuned for an individual patient with liver disease. Progress towards integrated multiscale models of liver regeneration can aid in prognostic tool development for treating liver disease.

Rigorous mathematical optimization of synthetic hepatic vascular trees

In this paper, we introduce a new framework for generating synthetic vascular trees, based on rigorous model-based mathematical optimization. Our main contribution is the reformulation of finding the optimal global tree geometry into a nonlinear optimization problem (NLP). This rigorous mathematical formulation accommodates efficient solution algorithms such as the interior point method and allows us to easily change boundary conditions and constraints applied to the tree. Moreover, it creates trifurcations in addition to bifurcations. A second contribution is the addition of an optimization stage for the tree topology. Here, we combine constrained constructive optimization (CCO) with a heuristic approach to search among possible tree topologies. We combine the NLP formulation and the topology optimization into a single algorithmic approach. Finally, we attempt the validation of our new model-based optimization framework using a detailed corrosion cast of a human liver, which allows a quantitative comparison of the synthetic tree structure with the tree structure determined experimentally down to the fifth generation. The results show that our new framework is capable of generating asymmetric synthetic trees that match the available physiological corrosion cast data better than trees generated by the standard CCO approach.

Anatomically- and physiologically-informed computational model of hepatic contrast perfusion for virtual imaging trials

PURPOSE

Virtual (in silico) imaging trials (VITs), involving computerized phantoms and models of the imaging process, provide a modern alternative to clinical imaging trials. VITs are faster, safer, and enable otherwise-impossible investigations. Current phantoms used in VITs are limited in their ability to model functional behavior such as contrast perfusion which is an important determinant of dose and image quality in CT imaging. In our prior work with the XCAT computational phantoms, we determined and modeled inter-organ (organ to organ) intravenous contrast concentration as a function of time from injection. However, intra-organ concentration, heterogeneous distribution within a given organ, was not pursued. We extend our methods in this work to model intra-organ concentration within the XCAT phantom with a specific focus on the liver.

METHODS

Intra-organ contrast perfusion depends on the organ's vessel network. We modeled the intricate vascular structures of the liver, informed by empirical and theoretical observations of anatomy and physiology. The developed vessel generation algorithm modeled a dual-input-single-output vascular network as a series of bifurcating surfaces to optimally deliver flow within the bounding surface of a given XCAT liver. Using this network, contrast perfusion was simulated within voxelized versions of the phantom by using knowledge of the blood velocities in each vascular structure, vessel diameters and length, and the time since the contrast entered the hepatic artery. The utility of the enhanced phantom was demonstrated through a simulation study with the phantom voxelized prior to CT simulation with the relevant liver vasculature prepared to represent blood and iodinated contrast media. The spatial extent of the blood-contrast mixture was compared to clinical data.

RESULTS

The vascular structures of the liver were generated with size and orientation which resulted in minimal energy expenditure required to maintain blood flow. Intravenous contrast was simulated as having known concentration and known total volume in the liver as calibrated from time-concentration curves (TCC). Measurements of simulated CT ROIs were found to agree with clinically-observed values of early arterial phase contrast enhancement of the parenchyma (∼5 HU). Similarly, early enhancement in the hepatic artery was found to agree with average clinical enhancement (180 HU).

CONCLUSIONS

The computational methods presented here furthered the development of the XCAT phantoms allowing for multi-timepoint contrast perfusion simulations, enabling more anthropomorphic virtual clinical trials intended for optimization of current clinical imaging technologies and applications. This article is protected by copyright. All rights reserved.

A dynamic blood flow model to compute absorbed dose to circulating blood and lymphocytes in liver external beam radiotherapy

We have developed a novel 4D dynamic liver blood flow model, capable of accurate dose estimation to circulating blood cells during liver-directed external beam radiotherapy, accounting for blood recirculation and radiation delivery time structure. Adult male and adult female liver computational phantoms with detailed vascular trees were developed to include the hepatic arterial, hepatic portal venous, and hepatic venous trees. A discrete time Markov Chain approach was applied to determine the spatiotemporal distribution of 10⁵blood particles (BP) in the human body based on reference values for cardiac output and organ blood volumes. For BPs entering the liver, an explicit Monte Carlo simulation was implemented to track their propagation along ∼2000 distinct vascular pathways through the liver. The model tracks accumulated absorbed dose from time-dependent radiation fields with a 0.1 s time resolution. The computational model was then evaluated for 3 male and 3 female patients receiving photon (VMAT and IMRT) and proton (passive SOBP and active PBS) treatments. The dosimetric impact of treatment modality, delivery time, and fractionation on circulating blood cells was investigated and quantified using the mean dose (μdose,b),V_>0Gy,V_>0.125Gy,andD_2%. Average reductions inμdose,b,V_>0Gy,V_>0.125GyandD_2%of 45%, 6%, 53%, 19% respectively, were observed for proton treatments as compared to photon treatments. Our simulation also showed thatV_>0Gy,V_>0.125Gy, andD_2%were highly sensitive to the beam-on time. BothV_>0GyandV_>0.125Gyincreased with beam-on time, whereasD_2%decreased with increasing beam-on time, demonstrating the tradeoff between low dose to a large fraction of blood cells and high dose to a small fraction of blood cells. Consequently, proton treatments are not necessarily advantageous in terms of dose to the blood simply based on integral dose considerations. Instead, both integral dose and beam-on time can substantially impact relevant dosimetric indices.

Organ-Specific Endothelial Cell Differentiation and Impact of Microenvironmental Cues on Endothelial Heterogeneity

Endothelial cells throughout the body are heterogeneous, and this is tightly linked to the specific functions of organs and tissues. Heterogeneity is already determined from development onwards and ranges from arterial/venous specification to microvascular fate determination in organ-specific differentiation. Acknowledging the different phenotypes of endothelial cells and the implications of this diversity is key for the development of more specialized tissue engineering and vascular repair approaches. However, although novel technologies in transcriptomics and proteomics are facilitating the unraveling of vascular bed-specific endothelial cell signatures, still much research is based on the use of insufficiently specialized endothelial cells. Endothelial cells are not only heterogeneous, but their specialized phenotypes are also dynamic and adapt to changes in their microenvironment. During the last decades, strong collaborations between molecular biology, mechanobiology, and computational disciplines have led to a better understanding of how endothelial cells are modulated by their mechanical and biochemical contexts. Yet, because of the use of insufficiently specialized endothelial cells, there is still a huge lack of knowledge in how tissue-specific biomechanical factors determine organ-specific phenotypes. With this review, we want to put the focus on how organ-specific endothelial cell signatures are determined from development onwards and conditioned by their microenvironments during adulthood. We discuss the latest research performed on endothelial cells, pointing out the important implications of mimicking tissue-specific biomechanical cues in culture.

Fibrosis and hepatic regeneration mechanism

Liver cirrhosis is the final stage of continuous hepatic inflammatory activity derived by viral, metabolic or autoimmune origin. In the last years, cirrhosis was considered a unique and static condition; recently was accepted some patients subgroups with different liver injury degrees that coexist under the same diagnosis, with implications about the natural disease history. The liver growth factor (LGF) is a potent in vivo and in vitro mitogenic agent and an inducer of hepatic regeneration (HR) through the hepatocytes DNA synthesis. The clinical implications of the LGF levels in cirrhosis, are not clear and even with having a fundamental role in the liver regeneration processes, the studies suggest that it could be a cirrhosis severity marker, in acute liver failure and in chronic hepatitis. Its role as predictor of mortality in fulminant hepatic insufficiency patients has been suggested. HR is one of the most enigmatic and fascinating biological phenomena. The rapid volume and liver function restoration after a major hepatectomy (>70%) or severe hepatocellular damage and its strict regulation of tissue damage response after the cessation, is an exclusive property of the liver. HR is the clinical applications fundament, such as extensive hepatic resections (>70% of the liver parenchyma), segmental transplantation or living donor transplantation, sequential hepatectomies, isolated portal embolization or associated with in situ hepatic transection, temporary artificial support in acute liver failure and the possible cell therapy clinical applications.

Hierarchical Modeling of the Liver Vascular System

The liver plays a key role in the metabolic homeostasis of the whole organism. To carry out its functions, it is endowed with a peculiar circulatory system, made of three main dendritic flow structures and lobules. Understanding the vascular anatomy of the liver is clinically relevant since various liver pathologies are related to vascular disorders. Here, we develop a novel liver circulation model with a deterministic architecture based on the constructal law of design over the entire scale range (from macrocirculation to microcirculation). In this framework, the liver vascular structure is a combination of superimposed tree-shaped networks and porous system, where the main geometrical features of the dendritic fluid networks and the permeability of the porous medium, are defined from the constructal viewpoint. With this model, we are able to emulate physiological scenarios and to predict changes in blood pressure and flow rates throughout the hepatic vasculature due to resection or thrombosis in certain portions of the organ, simulated as deliberate blockages in the blood supply to these sections. This work sheds light on the critical impact of the vascular network on mechanics-related processes occurring in hepatic diseases, healing and regeneration that involve blood flow redistribution and are at the core of liver resilience.

Geometrical model of lobular structure and its importance for the liver perfusion analysis

A convenient geometrical description of the microvascular network is necessary for computationally efficient mathematical modelling of liver perfusion, metabolic and other physiological processes. The tissue models currently used are based on the generally accepted schematic structure of the parenchyma at the lobular level, assuming its perfect regular structure and geometrical symmetries. Hepatic lobule, portal lobule, or liver acinus are considered usually as autonomous functional units on which particular physiological problems are studied. We propose a new periodic unit-the liver representative periodic cell (LRPC) and establish its geometrical parametrization. The LRPC is constituted by two portal lobulae, such that it contains the liver acinus as a substructure. As a remarkable advantage over the classical phenomenological modelling approaches, the LRPC enables for multiscale modelling based on the periodic homogenization method. Derived macroscopic equations involve so called effective medium parameters, such as the tissue permeability, which reflect the LRPC geometry. In this way, mutual influences between the macroscopic phenomena, such as inhomogeneous perfusion, and the local processes relevant to the lobular (mesoscopic) level are respected. The LRPC based model is intended for its use within a complete hierarchical model of the whole liver. Using the Double-permeability Darcy model obtained by the homogenization, we illustrate the usefulness of the LRPC based modelling to describe the blood perfusion in the parenchyma.

Investigation of vessel occlusion during cell seeding process

The seeding of cells into an organ is an important step in cell therapy because the final functional properties of the organ are related to the initial cell distribution throughout the organ. However, vessel occlusion is a serious problem that prevents uniform distribution of the cells in the entire organ. Understanding the mechanism of vessel occlusion can help optimize the seeding process. In this study, the vessel occlusion phenomenon under perfusion conditions during cell seeding was investigated. First, we applied a microfluidic system that enabled the observation of the occlusion events during injection. Second, we applied a multiphase numerical model that can describe the cell-cell interactions and cell-fluid interactions to investigate the vessel occlusion phenomenon during the seeding process. In particular, the effects of cell concentration and flow rate were investigated. The results indicate the importance of cell-cell interactions and cell-vessel interactions for the occurrence of vessel occlusion. In addition, it is found that the probability of occurrence of vessel occlusion increases with the increase in cell concentration and decrease in flow rate. The simulation model can help determine the optimum parameters to enhance cell seeding efficiency.

HEDOS-a computational tool to assess radiation dose to circulating blood cells during external beam radiotherapy based on whole-body blood flow simulations

We have developed a time-dependent computational framework, hematological dose (HEDOS), to estimate dose to circulating blood cells from radiation therapy treatment fields for any treatment site. Two independent dynamic models were implemented in HEDOS: one describing the spatiotemporal distribution of blood particles (BPs) in organs and the second describing the time-dependent radiation field delivery. A whole-body blood flow network based on blood volumes and flow rates from ICRP Publication 89 was simulated to produce the spatiotemporal distribution of BPs in organs across the entire body using a discrete-time Markov process. Constant or time-varying transition probabilities were applied and their impact on transition time was investigated. The impact of treatment time and anatomical site were investigated using imaging data and dose distributions from a liver cancer and a brain cancer patient. The simulations revealed different dose levels to the circulating blood for brain irradiation compared to liver irradiation even for similar field sizes due to the different blood flow properties of the two organs. The volume of blood receiving any dose (V>0 Gy) after a single radiation fraction increases from 1.2% for a 1 s delivery time to 20.9% for 120 s delivery time for the brain cancer treatment, and from 10% (1 s) to 48.7% (120 s) for a liver cancer treatment. Other measures of the low-dose bath to the circulating blood such as the dose to small volumes of blood (D2%) decreases with longer delivery time. Furthermore, we demonstrate that the blood dose-volume histogram is highly sensitive to changes in the treatment time, indicating that dynamic modeling of blood flow and radiation fields is necessary to evaluate dose to circulating blood cells for the assessment of radiation-induced lymphopenia. HEDOS is publicly available and allows for the estimation of patient-specific dose to circulating blood cells based on organ DVHs, thus enabling the study of the impact of different treatment plans, dose rates, and fractionation schemes.

Synapomorphic features of hepatic and pulmonary vasculatures include comparable purinergic signaling responses in host defense and modulation of inflammation

Hepatosplanchnic and pulmonary vasculatures constitute synapomorphic, highly comparable networks integrated with the external environment. Given functionality related to obligatory requirements of "feeding and breathing," these organs are subject to constant environmental challenges entailing infectious risk, antigenic and xenobiotic exposures. Host responses to these stimuli need to be both protective and tightly regulated. These functions are facilitated by dualistic, high-low pressure blood supply of the liver and lungs, as well as tolerogenic characteristics of resident immune cells and signaling pathways. Dysregulation in hepatosplanchnic and pulmonary blood flow, immune responses, and microbiome implicate common pathogenic mechanisms across these vascular networks. Hepatosplanchnic diseases, such as cirrhosis and portal hypertension, often impact lungs and perturb pulmonary circulation and oxygenation. The reverse situation is also noted with lung disease resulting in hepatic dysfunction. Others, and we, have described common features of dysregulated cell signaling during liver and lung inflammation involving extracellular purines (e.g., ATP, ADP), either generated exogenously or endogenously. These metabokines serve as danger signals, when released by bacteria or during cellular stress and cause proinflammatory and prothrombotic signals in the gut/liver-lung vasculature. Dampening of these danger signals and organ protection largely depends upon activities of vascular and immune cell-expressed ectonucleotidases (CD39 and CD73), which convert ATP and ADP into anti-inflammatory adenosine. However, in many inflammatory disorders involving gut, liver, and lung, these protective mechanisms are compromised, causing perpetuation of tissue injury. We propose that interventions that specifically target aberrant purinergic signaling might prevent and/or ameliorate inflammatory disorders of the gut/liver and lung axis.

Study of flow effects on temperature-controlled radiofrequency ablation using phantom experiments and forward simulations

PURPOSE

Blood flow is known to add variability to hepatic radiofrequency ablation (RFA) treatment outcomes. However, few studies exist on its impact on temperature-controlled RFA. Hence, we investigate large-scale blood flow effects on temperature-controlled RFA in flow channel experiments and numerical simulations.

METHODS

Ablation zones were induced in tissue-mimicking, thermochromic phantoms with a single flow channel, using an RF generator with temperature-controlled power delivery and a monopolar needle electrode. Channels were generated by molding the phantom around a removable rod. Channel radius and saline flow rate were varied to study the impact of flow on (i) the ablated cross-sectional area, (ii) the delivered generator power, and (iii) the occurrence of directional effects on the thermal lesion. Finite volume simulations reproducing the experimental geometry, flow conditions, and generator power input were conducted and compared to the experimental ablation outcomes.

RESULTS

Vessels of different channel radii r affected the ablation outcome in different ways. For r = 0.275  mm, the ablated area decreased with increasing flow rate while the energy input was hardly affected. For r = 0.9  mm and r = 2.3  mm, the energy input increased toward larger flow rates; for these radii, the ablated area decreased and increased toward larger flow rates, respectively, while still being reduced overall as compared to the reference experiment without flow. Directional effects, that is, local shrinking of the lesion upstream of the needle and an extension thereof downstream, were observed only for the smallest radius. The simulations qualitatively confirmed these observations. As compared to performing the simulations without flow, including flow effects in the simulations reduced the mean absolute error between experimental and simulated ablated areas from 0.23 to 0.12.

CONCLUSION

While the temperature control mechanism did not detect the heat sink effect in the case of the smallest channel radius, it counteracted the heat sink effect in the case of the larger channel radii with an increased energy input; this explains the increase in ablated area toward high flow rates (for r = 2.3  mm). The experiments in a simple phantom setup, thus, contribute to a good understanding of the phenomenon and are suitable for model validation.

Microvascular anatomy of the non-lobulated liver of adult Xenopus laevis: A scanning electron microscopic study of vascular casts

The microvascular anatomy of the non-lobulated liver of adult Xenopus laevis was studied by scanning electron microscopy of vascular corrosion casts. Hepatic portal veins and hepatic arteries entered hepatic lobes at the hiluses, hepatic veins left at these sites. Intraparenchymal, hepatic portal veins branched up to 10 times before terminal portal venules supplied liver sinusoids. Hepatic arteries closely followed portal vessels. Arteriolar side branches formed anastomoses with close by portal venules (arteriolar-portal anastomoses; APAs), liver sinusoids (arteriolar-sinusoidal anastomoses; ASAs), and peribiliary plexus vessels. Distally, hepatic arteries anastomosed with terminal portal venules having >100 μm in diameter. Liver sinusoids formed a dense three-dimensional network displaying signs of non-sprouting and sprouting angiogenesis evidenced by "holes" and blind ending tapering cast vascular structures (sprouts), respectively. Sinusoids drained via efferent hepatic veins. Right and left hepatic veins drained into the posterior caval vein. Locally, a dense honeycomb-like 3D-meshwork of resin structures was found around terminal portal venules and hepatic arteries. These networks were fed by hepatic arterioles and drained into adjacent terminal portal venules. As their morphologies differed significantly from sinusoids and they were found at sites where diffuse lymphoid tissue is described, we are convinced that they represent the vasculature of diffuse lymphoid tissue areas. Frequencies and diameter ratios of hepatic portal venules versus hepatic arterioles anastomosing with the former (APAs) implicate that the arterial supply contributes to the oxygenation of parenchymal and stromal cells rather than to a significant increase in blood flow towards hepatic sinusoids.

Super-resolution ultrasound localization microscopy based on a high frame-rate clinical ultrasound scanner: an in-human feasibility study

Non-invasive detection of microvascular alterations in deep tissuesin vivoprovides critical information for clinical diagnosis and evaluation of a broad-spectrum of pathologies. Recently, the emergence of super-resolution ultrasound localization microscopy (ULM) offers new possibilities for clinical imaging of microvasculature at capillary level. Currently, the clinical utility of ULM on clinical ultrasound scanners is hindered by the technical limitations, such as long data acquisition time, high microbubble (MB) concentration, and compromised tracking performance associated with low imaging frame-rate. Here we present a robust in-human ULM on a high frame-rate (HFR) clinical ultrasound scanner to achieve super-resolution microvessel imaging using a short acquisition time (<10 s). Ultrasound MB data were acquired from different human tissues, including a healthy liver and a diseased liver with acute-on-chronic liver failure, a kidney, a pancreatic tumor, and a breast mass using an HFR clinical scanner. By leveraging the HFR and advanced processing techniques including sub-pixel motion registration, MB signal separation, and Kalman filter-based tracking, MBs can be robustly localized and tracked for ULM under the circumstances of relatively high MB concentration associated with standard clinical MB administration and limited data acquisition time in humans. Subtle morphological and hemodynamic information in microvasculature were shown based on data acquired with single breath-hold and free-hand scanning. Compared with contrast-enhanced power Doppler generated based on the same MB dataset, ULM showed a 5.7-fold resolution improvement in a vessel based on a linear transducer, and provided a wide-range blood flow speed measurement that is Doppler angle-independent. Microvasculatures with complex hemodynamics can be well-differentiated at super-resolution in both normal and pathological tissues. This preliminary study implemented the ultrafast in-human ULM in various human tissues based on a clinical scanner that supports HFR imaging, indicating the potentials of the technique for various clinical applications. However, rigorous validation of the technique in imaging human microvasculature (especially for those tiny vessel structure), preferably with a gold standard, is still required.

Fiber Optic Sensors-Based Thermal Analysis of Perfusion-Mediated Tissue Cooling in Liver Undergoing Laser Ablation

The current challenge in the field of thermo-ablative treatments of tumors is to achieve a balance between complete destruction of malignant cells and safeguarding of the surrounding healthy tissue. Blood perfusion plays a key role for thermal ablation success, especially in the case of highly vascularized organs like liver. This work aims at monitoring the temperature within perfused swine liver undergoing laser ablation (LA). Temperature was measured through seven arrays of Fiber Bragg Grating sensors (FBGs) around the laser applicator. To mimic reality, blood perfusion within the ex-vivo liver was simulated using artificial vessels. The influence of blood perfusion on LA was carried out by comparing the temperature profiles in two different spatial configurations of vessels and fibers. The proposed setup permitted to accurately measure the heat propagation in real-time with a temperature resolution of 0.1 °C and to observe a relevant tissue cooling near to the vessel up to 65%.

Transarterial drug delivery for liver cancer: numerical simulations and experimental validation of particle distribution in patient-specific livers

Background: Transarterial therapies are routinely used for the locoregional treatment of unresectable hepatocellular carcinoma (HCC). However, the impact of clinical parameters (i.e. injection location, particle size, particle density etc.) and patient-specific conditions (i.e. hepatic geometry, cancer burden) on the intrahepatic particle distribution (PD) after transarterial injection of embolizing microparticles is still unclear. Computational fluid dynamics (CFD) may help to better understand this impact.Methods: Using CFD, both the blood flow and microparticle mass transport were modeled throughout the 3D-reconstructed arterial vasculature of a patient-specific healthy and cirrhotic liver. An experimental feasibility study was performed to simulate the PD in a 3D-printed phantom of the cirrhotic arterial network.Results: Axial and in-plane injection locations were shown to be effective parameters to steer particles toward tumor tissue in both geometries. Increasing particle size or density made it more difficult for particles to exit the domain. As cancer burden increased, the catheter tip location mattered less. The in vitro study and numerical results confirmed that PD largely mimics flow distribution, but that significant differences are still possible.Conclusions: Our findings highlight that optimal parameter choice can lead to selective targeting of tumor tissue, but that targeting potential highly depends on patient-specific conditions.

The liver, a functionalized vascular structure

The liver is not only the largest organ in the body but also the one playing one of the most important role in the human metabolism as it is in charge of transforming toxic substances in the body. Understanding the way its blood vasculature works is key. In this work we show that the challenge of predicting the hepatic multi-scale vascular network can be met thanks to the constructal law of design evolution. The work unveils the structure of the liver blood flow architecture as a combination of superimposed tree-shaped networks and porous system. We demonstrate that the dendritic nature of the hepatic artery, portal vein and hepatic vein can be predicted, together with their geometrical features (diameter ratio, duct length ratio) as the entire blood flow architectures follow the principle of equipartition of imperfections. At the smallest scale, the shape of the liver elemental systems-the lobules-is discovered, while their permeability is also predicted. The theory is compared with good agreement to anatomical data from the literature.

Utility of modified vascular corrosion casting technique in the diagnosis of fetal ductus arteriosus abnormalities

The anatomy of ductus arteriosus (DA) can be varied in different congenital heart defects (CHDs), and it is difficult to fully discover the DA and other associated cardiac anomalies by prenatal ultrasound. This study was aimed to use the modified vascular corrosion casting technique to prepare fetal cardiovascular casts with DA anomalies, assess the casting effectiveness in evaluating the great vessels of the fetal heart and investigate the utility of cardiovascular casting for the demonstration of fetal DA abnormalities. This retrospective study enrolled fourteen fetuses (23 to 28⁺² gestational weeks) with severe CHDs diagnosed by prenatal echocardiography and casting technique from January 2013 to July 2019. The sonographic features of DAs were carefully observed and other associated cardiovascular anomalies were also evaluated during the screening. The architectures of DAs and the whole cardiovascular system were observed and analyzed, and then the cast findings were compared with prenatal ultrasonic results. In fourteen cases, 18 ductal abnormities were indicated by prenatal echocardiography in fourteen cases, while 25 were revealed by casting. Cast findings included 4 cases of ductal stenosis, 1 case of ductal dilation, 6 cases of ductal circuity, 3 cases of right-sided ductus, 5 cases of anomalous ductal connection, 1 case of bilateral ductus and 5 cases of absent ductus. Cast findings consisted with ultrasound in 10 ductal abnormalities, revealed additional 15 ductal abnormalities miss-diagnosed by sonography, and corrected 6 abnormalities misdiagnosed prenatally. Meanwhile, 3 ductal abnormalities (reversed flow) could not be demonstrated by casts but only by ultrasound. Cast models can visually display the anatomical characteristics of ductus arteriosus, and could be successfully used in the demonstration of ductus abnormalities in fetuses with severe CHDs. Comparing with ultrasound, casting technique has its own superiority in exhibiting ductus abnormalities, especially in certain types such as course, origin and absence abnormalities of ductus.

Application of three-dimensional reconstruction and virtual reality technology in liver surgery

After three-dimensional (3D) reconstruction of the two-dimensional information obtained from routine computed tomography or magnetic resonance imaging examinations of the liver using software, surgeons can examine the volume of the liver, anatomical variation, the course of intrahepatic vessels, the location of the tumor, and its relationship with the surrounding vessels more intuitively, vividly, and from multiple angles. Preoperative 3D reconstruction and virtual reality technology can realize the measurement of liver volume and the implementation of simulated hepatectomy, which can further clarify the scope of surgical resection and ensure the residual liver volume and function to meet the needs of patients after operation. The virtual operation and image navigation before and during the operation can also prevent the injury to important blood vessels and bile ducts in the liver during the operation, significantly shorten the operation time, reduce the bleeding during the operation, and reduce the occurrence of complications such as liver dysfunction, bile leakage, and bleeding after the operation.

Corrosion casting of the cardiovascular structure in adult zebrafish for analysis by scanning electron microscopy and X-ray microtomography

Zebrafish have come to the forefront as a flexible, relevant animal model to study human disease, including cardiovascular disorders. Zebrafish are optically transparent during early developmental stages, enabling unparalleled imaging modalities to examine cardiovascular structure and function in vivo and ex vivo. At later stages, however, the options for systematic cardiovascular phenotyping are more limited. To visualise the complete vascular tree of adult zebrafish, we have optimised a vascular corrosion casting method. We present several improvements to the technique leading to increased reproducibility and accuracy. We designed a customised support system and used a combination of the commercially available Mercox II methyl methacrylate with the Batson's catalyst for optimal vascular corrosion casting of zebrafish. We also highlight different imaging approaches, with a focus on scanning electron microscopy (SEM) and X-ray microtomography (micro-CT) to obtain highly detailed, faithful three-dimensional reconstructed images of the zebrafish cardiovascular structure. This procedure can be of great value to a wide range of research lines related to cardiovascular biology in small specimens.

Corrosion casting in anatomy: Visualizing the architecture of hollow structures and surface details

Corrosion casting is the technique by which a solid, negative replica is created from a hollow anatomical structure and liberated from its surrounding tissues. For centuries, different types of hardening substances have been developed to create such casts, but nowadays, thermosetting polymers are mostly used as casting medium. Although the principle and initial set-up are relatively easy, producing high-quality casts that serve their intended purpose can be quite challenging. This paper evaluates some of the more popular casting resins that are currently available and provides a step-by-step overview of the corrosion casting procedure, including surface casts of anatomical structures. Hurdles and pitfalls are discussed, along with possible solutions to circumvent them, based on personal experience by the authors.

Ex vivo microangioCT: Advances in microvascular imaging

Therapeutic modulation of angiogenesis is believed to be a prospective powerful treatment strategy to modulate the microcirculation and therefore help millions of patients with cardiovascular and cancer diseases. The often-frustrating results from late-stage clinical studies indicate an urgent need for improved assessment of the pro- and anti-angiogenic compounds in preclinical stage of investigation. For such a proper assessment, detailed vascular visualization and adequate quantification are essential. Nowadays, there are few imaging modalities available, but none of them provides non-destructive 3D-visualization of the vasculature down to the capillary level. In many instances, the approaches cannot be combined with the subsequent histological or ultrastructural analysis. In this review, we address the latest developments in the microvascular imaging, namely, the microangioCT approach with a polymer-based contrast agent (μAngiofil). This approach allows time-efficient non-destructive 3D-imaging of the organ and its vasculature including the finest capillaries. Besides the superior visualization, the obtained detailed 3D information on the organ vasculature enables its 3D-skeletonization and further quantitative analysis. Probably the only significant limitation of the described approach is that it can be used only ex vivo, i.e., no longitudinal studies. In spite of this drawback, microangioCT with μAngiofil is a relatively simple and straightforward tool with a broad application range for studying physiological and pathological alterations in the microvasculature of any organ. It provides microvascular imaging at unprecedented level and enables correlative microscopy.

Effect of flow-encoding strength on intravoxel incoherent motion in the liver

PURPOSE

To study the impact of variable flow-encoding strength on intravoxel incoherent motion (IVIM) liver imaging of diffusion and perfusion.

THEORY

Signal attenuation in DWI arises from (1) intravoxel microvascular blood flow, which depends on the flow-encoding strength α (first gradient moment) of the diffusion-encoding waveform, and (2) intravoxel spin diffusion, which depends on the b-value of the diffusion-encoding gradient waveforms α and b-value. Both are linked to the diffusion-encoding gradient waveform and conventionally are not independently controlled.

METHODS

In this work a convex optimization framework was used to generate gradient waveforms with independent α and b-value. Thirty-six unique α and b-value sample points from 5 different gradient waveforms were used to reconstruct perfusion fraction (f), coefficient of diffusion (D), and blood velocity standard deviation (V_b ) maps using a recently proposed IVIM model. Faster acquisition strategies were evaluated with 1000 random subsampling strategies of 16, 8, and 4 α and b-value. Among the subsampled reconstructions, the sampling schemes that minimized the difference with the fully sampled reconstruction were reported.

RESULTS

Healthy volunteers (N = 9) were imaged on a 3T scanner. Liver perfusion and diffusion estimates using the fully sampled IVIM method were f = 0.19 ± 0.06, D = 1.15 ± 0.15 × 10^-3 mm² /s, and V_b = 5.22 ± 3.86 mm/s. No statistical differences were found between the fully sampled and 2-times undersampled reconstruction (f = 0.2 ± 0.07, D = 1.19 ± 0.15 × 10^-3 mm² /s, V_b = 5.79 ± 3.43 mm/s); 4-times undersampled (f = 0.2 ± 0.06, D = 1.15 ± 0.17 × 10^-3 mm² /s, V_b = 4.66 ± 3.61 mm/s), or 8-times undersampled ( f = 0.2 ± 0.06, D = 1.23 ± 0.22 × 10^-3 mm² /s, V_b = 4.99 ± 3.82 mm/s) approaches.

CONCLUSION

We demonstrate the IVIM signal's dependence on the b-value, the diffusion-encoding time and the flow-encoding strength and observe in vivo the ballistic regime signature of microperfusion in the liver. This work also demonstrates that using an IVIM model and sampling scheme matched to the ballistic regime, pixel-wise IVIM parameter maps are possible when sampling as few as 4 IVIM signals.

Quantitative analysis of hepatic macro- and microvascular alterations during cirrhogenesis in the rat

Cirrhosis represents the end-stage of any persistent chronically active liver disease. It is characterized by the complete replacement of normal liver tissue by fibrosis, regenerative nodules, and complete fibrotic vascularized septa. The resulting angioarchitectural distortion contributes to an increasing intrahepatic vascular resistance, impeding liver perfusion and leading to portal hypertension. To date, knowledge on the dynamically evolving pathological changes of the hepatic vasculature during cirrhogenesis remains limited. More specifically, detailed anatomical data on the vascular adaptations during disease development is lacking. To address this need, we studied the 3D architecture of the hepatic vasculature during induction of cirrhogenesis in a rat model. Cirrhosis was chemically induced with thioacetamide (TAA). At predefined time points, the hepatic vasculature was fixed and visualized using a combination of vascular corrosion casting and deep tissue microscopy. Three-dimensional reconstruction and data-fitting enabled cirrhogenic features to extracted at multiple scales, portraying the impact of cirrhosis on the hepatic vasculature. At the macrolevel, we noticed that regenerative nodules severely compressed pliant venous vessels from 12 weeks of TAA intoxication onwards. Especially hepatic veins were highly affected by this compression, with collapsed vessel segments severely reducing perfusion capabilities. At the microlevel, we discovered zone-specific sinusoidal degeneration, with sinusoids located near the surface being more affected than those in the middle of a liver lobe. Our data shed light on and quantify the evolving angioarchitecture during cirrhogenesis. These findings may prove helpful for future targeted invasive interventions.

Modeling of the contrast-enhanced perfusion test in liver based on the multi-compartment flow in porous media

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.

Review of quantitative multiscale imaging of breast cancer

Breast cancer is the most common cancer among women worldwide and ranks second in terms of overall cancer deaths. One of the difficulties associated with treating breast cancer is that it is a heterogeneous disease with variations in benign and pathologic tissue composition, which contributes to disease development, progression, and treatment response. Many of these phenotypes are uncharacterized and their presence is difficult to detect, in part due to the sparsity of methods to correlate information between the cellular microscale and the whole-breast macroscale. Quantitative multiscale imaging of the breast is an emerging field concerned with the development of imaging technology that can characterize anatomic, functional, and molecular information across different resolutions and fields of view. It involves a diverse collection of imaging modalities, which touch large sections of the breast imaging research community. Prospective studies have shown promising results, but there are several challenges, ranging from basic physics and engineering to data processing and quantification, that must be met to bring the field to maturity. This paper presents some of the challenges that investigators face, reviews currently used multiscale imaging methods for preclinical imaging, and discusses the potential of these methods for clinical breast imaging.

Evaluation and Minimization of the Pseudohepatic Anisotropy Artifact in Liver Intravoxel Incoherent Motion

PURPOSE

The aim of this study was to evaluate the effect of the pseudohepatic anisotropy artifact on liver intravoxel incoherent motion (IVIM) metrics and whether the use of multiple gradient directions in the IVIM acquisition minimizes the artifact.

MATERIALS AND METHODS

Multiple breath-holding and forced shallow free-breathing IVIM scans were performed on 8 healthy volunteers using 1 and 6 gradient directions. Cluster analysis was carried out to separate motion-contaminated parenchyma from liver parenchyma and vessels. Nonlinear motion analysis was also performed to look for a possible link between IVIM metrics and nonlinear liver motion.

RESULTS

On the basis of the resulted clusters, motion-contaminated parenchyma is often noted in the left liver lobe, where the prominent pseudohepatic artifact has previously been identified. A significant reduction in outliers was obtained with the acquisition of 6 noncoplanar gradient directions and when using forced shallow free-breathing.

CONCLUSION

The pseudohepatic anisotropy artifact can be minimized when using multiple diffusion-encoding gradient directions and forced free-breathing during IVIM acquisition.

Liver vessel segmentation and identification based on oriented flux symmetry and graph cuts

BACKGROUND AND OBJECTIVE

Accurate segmentation of liver vessels from abdominal computer tomography angiography (CTA) volume is very important for liver-vessel analysis and living-related liver transplants. This paper presents a novel liver-vessel segmentation and identification method.

METHODS

Firstly, an anisotropic diffusion filter is used to smooth noise while preserving vessel boundaries. Then, based on the gradient symmetry and antisymmetry pattern of vessel structures, optimal oriented flux (OOF) and oriented flux antisymmetry (OFA) measures are respectively applied to detect liver vessels and their boundaries, and further to slenderize vessels. Next, according to vessel geometrical structure, a centerline extraction measure based on height ridge traversal and leaf node line-growing (LNLG) is proposed for the extraction of liver-vessel centerlines, and an intensity model based on fast marching is integrated into graph cuts (GCs) for effective segmentation of liver vessels. Finally, a distance voting mechanism is applied to separate the hepatic vein and portal vein.

RESULTS

The experiment results on abdominal CTA images show that the proposed method can effectively segment liver vessels, achieving an average accuracy, sensitivity, and specificity of 97.7%, 79.8%, and 98.6%, respectively, and has a good performance on thin-vessel extraction.

CONCLUSIONS

The proposed method does not require manual selection of the centerlines and vessel seeds, and can effectively segment liver vessels and identify hepatic vein and portal vein.

Liver Preservation by Aortic Perfusion Alone Compared With Preservation by Aortic Perfusion and Additional Arterial Ex Situ Back-Table Perfusion With Histidine-Tryptophan-Ketoglutarate Solution: A Prospective, Randomized, Controlled, Multicenter Study

Background

Arterial ex situ back-table perfusion (BP) reportedly reduces ischemic-type biliary lesion after liver transplantation. We aimed to verify these findings in a prospective investigation.

Methods

Our prospective, randomized, controlled, multicenter study involved livers retrieved from patients in 2 German regions, and compared the outcomes of standard aortic perfusion to those of aortic perfusion combined with arterial ex situ BP. The primary endpoint was the incidence of ischemic-type biliary lesions over a follow-up of 2 years after liver transplantation, whereas secondary endpoints included 2-year graft survival, initial graft damage as reflected by transaminase levels, and functional biliary parameters at 6 months after transplantation.

Results

A total of 75 livers preserved via standard aortic perfusion and 75 preserved via standard aortic perfusion plus arterial BP were treated using a standardized protocol. The incidence of clinically apparent biliary lesions after liver transplantation (n = 9 for both groups; P = 0.947), the 2-year graft survival rate (standard aortic perfusion, 74%; standard aortic perfusion plus arterial BP, 68%; P = 0.34), and incidence of initial graft injury did not differ between the 2 perfusion modes. Although 33 of the 77 patients with cholangiography workups exhibited injured bile ducts, only 10 had clinical symptoms.

Conclusions

Contrary to previous findings, the present study indicated that additional ex situ BP did not prevent ischemic-type biliary lesions or ischemia-reperfusion injury after liver transplantation. Moreover, there was considerable discrepancy between cholangiography findings regarding bile duct changes and clinically apparent cholangiopathy after transplantation, which should be considered when assessing ischemic-type biliary lesions.

The use of porcine corrosion casts for teaching human anatomy

In teaching and learning human anatomy, anatomical autopsy and prosected specimens have always been indispensable. However, alternative methods must often be used to demonstrate particularly delicate structures. Corrosion casting of porcine organs with Biodur E20^® Plus is valuable for teaching and learning both gross anatomy and, uniquely, the micromorphology of cardiovascular, respiratory, digestive, and urogenital systems. Assessments of casts with a stereomicroscope and/or scanning electron microscope as well as highlighting cast structures using color coding help students to better understand how the structures that they have observed as two-dimensional images actually exist in three dimensions, and students found using the casts to be highly effective in their learning. Reconstructions of cast hollow structures from (micro-)computed tomography scans and videos facilitate detailed analyses of branching patterns and spatial arrangements in cast structures, aid in the understanding of clinically relevant structures and provide innovative visual aids. The casting protocol and teaching manual we offer can be adjusted to different technical capabilities and might also be found useful for veterinary or other biological science classes.