The authors would like to acknowledge the support provided by The University of Adelaide, Royal Adelaide Hospital (RAH) and the South Australian Health and Medical Research Institute (SAHMRI). The COCOMO-ACS trial is an investigator-initiated study funded by project grants from the National Health and Medical Research Council (NHMRC) of Australia (ID1127159) and National Heart Foundation of Australia (ID101370). H.J.C. is supported by a scholarship from the Westpac Scholars Trust (Future Leaders Scholarship) and acknowledges support from The University of Adelaide, School of Mechanical Engineering and the Department of Education, Skills and Employment Research Training Program (RTP) scholarship. S.J.N. receives a Principal Research Fellowship from the NHMRC (ID1111630). P.J.P. receives a Level 2 Future Leader Fellowship from the National Heart Foundation of Australia (FLF102056) and Level 2 Career Development Fellowship from the NHMRC (CDF1161506).

The authors have no conflicts to declare regarding the preparation of this article. S.J.N. has received research support from AstraZeneca, Amgen, Anthera, Eli Lilly, Esperion, Novartis, Cerenis, The Medicines Company, Resverlogix, InfraReDx, Roche, Sanofi-Regeneron, and Liposcience and is a consultant for AstraZeneca, Akcea, Eli Lilly, Anthera, Kowa, Omthera, Merck, Takeda, Resverlogix, Sanofi-Regeneron, CSL Behring, Esperion, and Boehringer Ingelheim. P.J.P. has received research support from Abbott Vascular, consulting fees from Amgen and Esperion and speaker honoraria from AstraZeneca, Bayer, Boehringer Ingelheim, Merck Schering-Plough, and Pfizer.

The use of FSI methods to analyze coronary biomechanics is still a developing field from both numerical modelling and clinical result aspects. Here we have described the outline of setting up a patient specific FSI analysis, based on the finite element/finite volume methods, utilizing OCT and angiographic imaging. While the method we describe here utilizes a commercial finite element solver, the procedure can be applied to any FSI capable software. There are still several limitations to be improved upon in the methodology. Firstly, we acknowledge the limitation of only presenting representative results for a single patient; however, we present the current protocol to better outline the process of biomechanical FSI simulation of arteries in the hope the presented methodology can be further developed and applied to larger cohorts in the future. Furthermore, during the image analysis stage, much of the process described is manual, introducing potential interobserver variability when segmenting the lumen, lipids, and outer wall. Further developments can be made to automate these processes55 or include multiple expert analyses which will be particularly useful for providing a more robust comparison between baseline and follow-up images. The OCT imaging used in this procedure was also not ECG-gated, resulting in slight misalignment of cross-sections due to cardiac motion/contraction. Taking this into account in the future could also increase comparison accuracy.
As the focus of the study was on the relationship between biomechanical forces and plaque composition, an OCT centered approach was taken to make use of OCT's high accuracy (10-20 &#181;m). This, however, presents challenges due to the significant attenuation that results from OCT's limited tissue penetration depth8. Unlike purely CFD techniques, which only require detailing the lumen shape from OCT images for fluid analyses, FSI also requires detailed information on lipids and the artery wall. To overcome this limitation an estimation technique was presented in this methodology, as during normal clinical practice only one intravascular imaging modality is used. Future research should be undertaken to validate the accuracy of this technique and its impact on biomechanical results through co-registration of IVUS and OCT. For this procedure, we have assumed that this impact is negligible, as lumen shape and fibrous cap thickness56,57 are generally more dominant drivers of stress results in the wall. Combining OCT and Intravascular Ultrasound (IVUS) could overcome this difficulty58; the superior tissue penetration of IVUS allowing analysis of plaque characteristics and a near 10-fold increase in spatial resolution of OCT leading to highly accurate lumen representation59. Developments in multimodal catheters present a significant opportunity to address this in the future58. Similarly, for the angiographic centerlines, further automation to more accurately account for table panning, artery compression/relaxation, and foreshortening can be made by building on this methodology60.
The numerical simulation can be further improved by including the anisotropic nature of coronary tissue. The isotropic hyperelastic tissue described in the methodology does not consider the impact of artery microstructure. This has been shown to be important for stress magnitude and distribution in the artery wall31. Tissue anisotropy could be included in the future by creating user-defined material models. Alternate finite element solvers also have built-in anisotropic hyperelastic models developed specifically for biological tissue behavior, such as the Holzapfel-Gasser-Odgen model61. Current simulations are also limited by primarily using data provided from the literature42 excised from cadavers and mechanically tested outside of the physiological environment. As intravascular imaging techniques improve, we also see the possibility of defining tissue properties tailored to each specific patient. Similarly, the intima, media and adventitia were simplified to a single layered structure as separating these layers from OCT presents challenges, especially when ensuring no overlap occurs in the reconstruction process. The added numerical cost is also significant, requiring further investigation to find the balance between cost/time and accuracy as the multiple layers do play a role in the overall stress response62,63.
Furthermore, from an intravascular flow perspective, while we only present representative results of LNH, several helical flow indices have been developed in the literature, with results such as the h2 index, which quantifies helicity intensity, suggested as being atheroprotective. These indices can be added to the protocol by defining them in custom field functions (step 4.16) in the same manner as LNH and we direct interested readers to the cited literature24,25,51 for further information.
Finally, our methodology is limited to applying velocity and pressure boundary conditions measured in the literature, rather than measured patient-specific ones. This could be improved by extracting blood velocity and pressure boundary conditions specific to each patient using doppler echocardiography64 and quantitative/fractional flow reserve measurements65 in the future. Such improvements would be particularly useful in ensuring that artery displacement could be exactly matched to blood velocity and pressure phases (start/end systole in particular), something we provide an estimate for in this analysis. Furthermore, while it has been suggested that fluid boundary conditions only impact the very near region of the boundary, the region of interest for assessment of plaque growth/regression of remodeling should be set around the mid-section of the simulation geometry to minimize any impact of these inlet and outlet conditions. The impact of simulating a sub-optimal artery section that is too short has been highlighted in Figure 12, where the inlet and outlet conditions greatly influence the presented results, rather than the artery geometry and dynamics. This will have direct consequences for predictive capability if not adequately accounted for. Furthermore, while we present a simulation using the Power Law non-Newtonian viscosity model based on suggestions in the literature46,48, more recent investigations47 have suggested the Carreau model as a more appropriate viscosity model. Different viscosity models can be chosen in step 4.7 of the protocol. By building directly from this methodology, incorporation of these advances could greatly increase result specificity and accuracy in the future.
During the development of the solid model, it is important to take steps to improve the likelihood of a high-quality mesh and reduce error prone regions. Sharing topology between the lipid and artery wall is an important first step, enabling mesh nodes to be shared between the solid surfaces when the meshing step is carried out. By sharing nodes, the possibility of errors associated with imperfect mesh alignment and mesh penetration is reduced, a not insignificant risk due to the complex geometry of the model. Following the setup of both the solid and fluid analyses components, it is also critical to check for mesh independence. This ensures that all results are not influenced by the size of the generated mesh. For mesh independence, results should remain stable despite changes in mesh size. For efficient calculations, choose the largest possible mesh (least number of elements) that ensures this independence. Furthermore, ensuring that both the Fluent based residuals converge and the system coupling data transfers converge is a critical step. Generally, it is good practice for the structural and fluid data transfer residuals to converge to within 10% of each other at the end of each time step. As the simulation finds an equilibrium state, the first few iterations of any FSI simulation will sometimes not completely converge, as previously described in Figure 7. Generally, after the first 5%-10% of simulation time is complete, convergence should be reached for every step. While we also demonstrate the methodology here over one cardiac cycle, a simplification frequently made in coronary biomechanics, multiple cardiac cycles are often required for numerical convergence. Due to the computational cost associated with coronary biomechanics simulations, however, multiple cardiac cycles are often not feasible. This is a limitation that warrants further development.
Several possible errors may also arise during simulation. Most common of these are element distortion/deformation from the structural side and negative cell volume from the fluid side. Element distortion can result from stress intensifiers such as pinch points/poor element quality areas or from the applied force exceeding material strength. To find the locations where errors are occurring, insert Newton-Raphson residuals (from under the Solution tab in transient structural). Newton-Raphson residuals will show the regions of the geometry with the largest residual error. If mesh quality is low in this region, refining the mesh may help. Viewing the force/displacement convergence from the drop-down list in the Solution tab may also help troubleshoot this error. From the fluid perspective, negative cell volume is generally associated with the dynamic mesh settings. Review these settings carefully and try increasing the margin between the mesh zone maximum/minimum sizes and the manually specified maxima/minima. Shallow/poor quality elements in the boundary region may also impact this error due to the deformation occurring during system coupling. Reviewing mesh quality in the boundary region may also assist in troubleshooting. When troubleshooting, solving the structural and fluid components separately can reduce the time spent addressing errors.
Moving forward, FSI-based simulation shows significant potential for coronary lesion analyses as it overcomes the limitations of purely structural or fluid (CFD) based approaches. The presented methodology also has further applications in coronary stenting66, ventricle function67, and spontaneous coronary artery dissection68 analysis. However, there remains the challenge of correlating biomechanical results to clinical outcomes. This remains difficult due to the computation cost associated with FSI techniques, limiting the number of analyses able to be performed within realistic timeframes. This means that currently results cannot be used in near-real time (such as in the catheterization laboratory) to directly assist percutaneous coronary intervention (PCI), but rather are retrospective analyses leading to delays in data access. This timeframe may still be helpful in guiding staged PCI procedures or individualizing more aggressive pharmacotherapy or surveillance for non-culprit coronary lesions that are identified as being at high risk of progression or destabilizing due to their biomechanical profile. With multiple biomechanical markers emerging with potential links to CAD, a multifactorial approach capable of simulating and comparing results will give a clearer picture of the relationship between artery biomechanics and atheroprone sites, a possibility that FSI simulations are uniquely placed to complete. The possibility for this FSI simulation to further integrate and inform machine learning algorithms could also overcome the limitations associated with each separate approach69. Overall, this methodology can assist predicting coronary atherosclerosis progression, and with further development could become an integral part of high-risk patient care.

Coronary artery disease (CAD) is the most common type of heart disease and one of the leading causes of death and economic burden globally1,2. In the United States, roughly one in every eight deaths is attributed to CAD3,4, while most global deaths from CAD are now seen in low- and middle-income countries5. Atherosclerosis is the predominant driver of these deaths, with plaque rupture or erosion leading to coronary artery occlusion and acute myocardial infarction (AMI)6. Even after revascularization of culprit coronary lesions, patients have substantial risk of recurrent major adverse cardiovascular events (MACE) after AMI, largely due to the concomitant presence of other non-culprit plaques that are also vulnerable to rupture7. Intracoronary imaging provides an opportunity to detect these high-risk plaques8. Although intravascular ultrasound (IVUS) is the gold standard for evaluating plaque volume, it has limited resolution to identify microstructural features of vulnerable plaque in contrast to the high resolution (10-20 &#181;m) of optical coherence tomography (OCT). A thin and inflamed fibrous cap overlying a large lipid pool has been demonstrated to be the most important signature of a vulnerable plaque9 and is best identified and measured by OCT among currently available intracoronary imaging modalities10. Importantly, OCT is also able to assess other high-risk plaque features, including: lipid arc; macrophage infiltration; presence of thin cap fibroatheroma (TCFA), which is defined as lipid-rich core with overlying thin fibrous cap (&#60;65 &#181;m); spotty calcification; and plaque microchannels. OCT detection of these high-risk features in non-culprit plaques post-AMI has been associated with up to a 6-fold increased risk of future MACE11. However, despite this, the ability of angiography and OCT imaging to predict which coronary plaques will progress and ultimately rupture or erode is limited, with positive predictive values of only 20%-30%8. This limited predictive ability hinders clinical decision-making around which non-culprit plaques to treat (e.g., by stenting)7,12.
In addition to patient factors and the biological characteristics of plaque, biomechanical forces in the coronary arteries are also important determinants of plaque progression and instability13. One technique that shows promise for helping to comprehensively evaluate these forces is fluid-structure interaction (FSI)14 simulation. Wall shear stress (WSS), also called endothelial shear stress, has been a traditional focal point for coronary biomechanics research15, with a general understanding that WSS plays an etiological role in atherosclerosis formation16. Predominantly simulated using computational fluid dynamics (CFD) techniques, low WSS regions have been associated with intimal thickening17, vascular remodeling18 and the prediction of lesion progression19 and future MACE20. Recent advances in these analyses suggest the underlying WSS vector field topology21, and its multidirectional characteristics22, as a better predictor of atherosclerosis risk than WSS magnitude alone. However, WSS only captures a glimpse of the overall biomechanical system at the lumen wall, and much like imaging modalities, no one biomechanical metric can reliably discern high risk atherosclerotic features.
Further metrics are emerging as potentially important in atherosclerosis formation. Intraluminal flow characteristics23 are one such example, with helical flow, quantified through various indices24, suggested as playing an atheroprotective role by suppressing disturbed flow patterns25,26. While CFD techniques can analyze these flow characteristics and present a wide range of useful results, they do not consider the underlying interactions between the blood flow, artery structure and general heart motion. This simplification of the dynamic system to a rigid wall misses potentially critical results such as fibrous cap stress. While the debate both for and against the need for FSI over CFD continues27,28,29, many comparisons neglect to include the impact of ventricle function. This limitation can be overcome with FSI, which has shown that dynamic bending and compression exerted on the artery through the influence of the ventricle function can significantly impact plaque and artery structural stress as well as flow metrics such as WSS30,31,32. This is important as structural stresses are also a key metric for analyzing and predicting plaque rupture33,34 and have been suggested to co-locate with regions of plaque increase14,35. Capturing these interactions allows for a more realistic representation of the coronary environment and the potential mechanisms of disease progression.
Addressing this, here we outline the process of developing a patient-specific geometry from OCT imaging36 and the setting up and running of an artery FSI simulation using a commercial finite element solver. The process to manually extract the lumen, lipid and outer artery wall is detailed before the three-dimensional computational reconstruction of the patient&#39;s artery. We outline the simulation set-up, coupling and the process of comparing baseline, and follow-up OCT imaging parameters to determine lesion progression. Finally, we discuss the post-processing of numerical results and how these data may have clinical relevance by comparing the biomechanical results with lesion progression/regression. The overall method is demonstrated on non-culprit, mildly stenotic, lipid-rich plaques in the right coronary artery (RCA) of a 58-year-old Caucasian male patient who presented with an acute non-ST elevation myocardial infarction in the setting of hypertension, type 2 diabetes mellitus, obesity (BMI 32.6) and a family history of premature CAD. Coronary angiography and OCT imaging were performed during his initial admission, and then 12 months later as part of an ongoing clinical trial (COCOMO-ACS trial ACTRN12618000809235). We anticipate that this technique can be further refined and used for identifying coronary plaques that are at high risk of progressing.

<table><tbody><tr><td>ANSYS Workbench (version 19.0)</td><td>ANSYS</td><td></td><td>Commercial finite element solver</td></tr><tr><td>MATLAB (version 2019b)</td><td>Mathworks</td><td></td><td>Commercial programming platform</td></tr><tr><td>MicroDicom/ImageJ</td><td>MicroDicom/ImageJ</td><td></td><td>Open Source DICOM reader</td></tr><tr><td>Visual Studio (version 2019)</td><td>Microsoft</td><td></td><td>Commercial Integrated Development Environment</td></tr></tbody></table>

optical coherence tomography based biomechanical fluid-structure interaction analysis of coronary atherosclerosis progression

In this paper, we present a complete workflow for the biomechanical analysis of atherosclerotic plaque in the coronary vasculature. With atherosclerosis as one of the leading causes of global death, morbidity and economic burden, novel ways of analyzing and predicting its progression are needed. One such computational method is the use of fluid-structure interaction (FSI) to analyze the interaction between the blood flow and artery/plaque domains. Coupled with in vivo imaging, this approach could be tailored to each patient, assisting in differentiating between stable and unstable plaques. We outline the three-dimensional reconstruction process, making use of intravascular Optical Coherence Tomography (OCT) and invasive coronary angiography (ICA). The extraction of boundary conditions for the simulation, including replicating the three-dimensional motion of the artery, is discussed before the setup and analysis is conducted in a commercial finite element solver. The procedure for describing the highly nonlinear hyperelastic properties of the artery wall and the pulsatile blood velocity/pressure is outlined along with setting up the system coupling between the two domains. We demonstrate the procedure by analyzing a non-culprit, mildly stenotic, lipid-rich plaque in a patient following myocardial infarction. Established and emerging markers related to atherosclerotic plaque progression, such as wall shear stress and local normalized helicity, respectively, are discussed and related to the structural response in the artery wall and plaque. Finally, we translate the results to potential clinical relevance, discuss limitations, and outline areas for further development. The method described in this paper shows promise for aiding in the determination of sites at risk of atherosclerotic progression and, hence, could assist in managing the significant death, morbidity, and economic burden of atherosclerosis.

Watch this Scientific Journal Video about Optical Coherence Tomography Based Biomechanical Fluid-Structure Interaction Analysis of Coronary Atherosclerosis Progression at JoVE.com

Optical Coherence Tomography Based Biomechanical Fluid-Structure Interaction Analysis of Coronary Atherosclerosis Progression

There is a need to determine which atherosclerotic lesions will progress in the coronary vasculature to guide intervention before myocardial infarction occurs. This article outlines the biomechanical modeling of arteries from Optical Coherence Tomography using fluid-structure interaction techniques in a commercial finite element solver to help predict this progression.

In this paper, we present a complete workflow for the biomechanical analysis of atherosclerotic plaque in the coronary vasculature. With atherosclerosis ...

optical-coherence-tomography-based-biomechanical-fluid-structure

Nanyang Technological University

Research

Education

JoVE Journal

JoVE Core

Civil Engineering

Mechanical Engineering

Bioengineering

Unit 1

Finite Control Volume Analysis

Force Vectors

SCIENTISTS IN ACTION

3.8K Views.  University of Adelaide. Atherosclerose is een van de belangrijkste oorzaken van wereldwijde dood en morbiditeit, en de multifactoriële en complexe aard maakt een multidisciplinaire aanpak belangrijk. Nu is beeldvorming een krachtig hulpmiddel om zwarte morfologie te analyseren, maar het kan ons nog geen inzicht geven in de onderliggende mechanismen van werk. En dit is waar computationele simulatie om de hoek komt kijken.Vanuit het oogpunt van vloeistofdynamica weten we dat factoren zoals wandschuifstress in ...

Video: Optical Coherence Tomography Based Biomechanical Fluid-Structure Interaction Analysis of Coronary Atherosclerosis Progression

Directed Cellular Self-Assembly to Fabricate Cell-Derived Tissue Rings for Biomechanical Analysis and Tissue Engineering

Each year, hundreds of thousands of patients undergo coronary artery bypass surgery in the United States.1 Approximately one third of these patients do not have suitable autologous donor vessels due to disease progression or previous harvest. The aim of vascular tissue engineering is to develop a suitable alternative source for these bypass grafts. In addition, engineered vascular tissue may prove valuable as living vascular models to study cardiovascular diseases. Several promising approaches to engineering blood vessels have been explored, with many recent studies focusing on development and analysis of cell-based methods.2-5 Herein, we present a method to rapidly self-assemble cells into 3D tissue rings that can be used in vitro to model vascular tissues.
To do this, suspensions of smooth muscle cells are seeded into round-bottomed annular agarose wells. The non-adhesive properties of the agarose allow the cells to settle, aggregate and contract around a post at the center of the well to form a cohesive tissue ring.6,7 These rings can be cultured for several days prior to harvesting for mechanical, physiological, biochemical, or histological analysis. We have shown that these cell-derived tissue rings yield at 100-500 kPa ultimate tensile strength8 which exceeds the value reported for other tissue engineered vascular constructs cultured for similar durations (&lt;30 kPa).9,10 Our results demonstrate that robust cell-derived vascular tissue ring generation can be achieved within a short time period, and offers the opportunity for direct and quantitative assessment of the contributions of cells and cell-derived matrix (CDM) to vascular tissue structure and function.

This article outlines a versatile method to create cell-derived tissue rings by cellular self-assembly. Smooth muscle cells seeded into ring-shaped agarose wells aggregate and contract to form robust three-dimensional (3D) tissues within 7 days. Millimeter-scale tissue rings are conducive to mechanical testing and serve as building blocks for tissue assembly.

Each year, hundreds of thousands of patients undergo coronary artery bypass surgery in the United States.1 Approximately one third of these patients do ...

Integrated Photoacoustic Ophthalmoscopy and Spectral-domain Optical Coherence Tomography

Both the clinical diagnosis and fundamental investigation of major ocular diseases greatly benefit from various non-invasive ophthalmic imaging technologies. Existing retinal imaging modalities, such as fundus photography1, confocal scanning laser ophthalmoscopy (cSLO)2, and optical coherence tomography (OCT)3, have significant contributions in monitoring disease onsets and progressions, and developing new therapeutic strategies. However, they predominantly rely on the back-reflected photons from the retina. As a consequence, the optical absorption properties of the retina, which are usually strongly associated with retinal pathophysiology status, are inaccessible by the traditional imaging technologies.
Photoacoustic ophthalmoscopy (PAOM) is an emerging retinal imaging modality that permits the detection of the optical absorption contrasts in the eye with a high sensitivity4-7 . In PAOM nanosecond laser pulses are delivered through the pupil and scanned across the posterior eye to induce photoacoustic (PA) signals, which are detected by an unfocused ultrasonic transducer attached to the eyelid. Because of the strong optical absorption of hemoglobin and melanin, PAOM is capable of non-invasively imaging the retinal and choroidal vasculatures, and the retinal pigment epithelium (RPE) melanin at high contrasts 6,7. More importantly, based on the well-developed spectroscopic photoacoustic imaging5,8 , PAOM has the potential to map the hemoglobin oxygen saturation in retinal vessels, which can be critical in studying the physiology and pathology of several blinding diseases 9 such as diabetic retinopathy and neovascular age-related macular degeneration.
Moreover, being the only existing optical-absorption-based ophthalmic imaging modality, PAOM can be integrated with well-established clinical ophthalmic imaging techniques to achieve more comprehensive anatomic and functional evaluations of the eye based on multiple optical contrasts6,10 . In this work, we integrate PAOM and spectral-domain OCT (SD-OCT) for simultaneously in vivo retinal imaging of rat, where both optical absorption and scattering properties of the retina are revealed. The system configuration, system alignment and imaging acquisition are presented.

Photoacoustic ophthalmology (PAOM), an optical-absorption-based imaging modality, provides the complementary evaluation of the retina to the currently available ophthalmic imaging technologies. We report the using of PAOM integrated with spectral-domain optical coherence tomography (SD-OCT) for simultaneous multimodal retinal imaging in rats.

Both the clinical diagnosis and fundamental investigation of major ocular diseases greatly benefit from various non-invasive ophthalmic imaging ...

Automated 3D Optical Coherence Tomography to Elucidate Biofilm Morphogenesis Over Large Spatial Scales

Biofilms are a most successful microbial lifestyle and prevail in a multitude of environmental and engineered settings. Understanding biofilm morphogenesis, that is the structural diversification of biofilms during community assembly, represents a remarkable challenge across spatial and temporal scales. Here, we present an automated biofilm imaging system based on optical coherence tomography (OCT). OCT is an emerging imaging technique in biofilm research. However, the amount of data that currently can be acquired and processed hampers the statistical inference of large scale patterns in biofilm morphology. The automated OCT imaging system allows covering large spatial and extended temporal scales of biofilm growth. It combines a commercially available OCT system with a robotic positioning platform and a suite of software solutions to control the positioning of the OCT scanning probe, as well as the acquisition and processing of 3D biofilm imaging datasets. This setup allows the in situ and non-invasive automated&#160;monitoring of biofilm development and may be further developed to couple OCT imaging with macrophotography and microsensor profiling.

Microbial biofilms form complex architectures at interphases and develop into highly scale-dependent spatial patterns. Here, we introduce an experimental system (hard- and software) for the automated acquisition of 3D optical coherence tomography (OCT) datasets. This toolset allows the non-invasive and multi-scale characterization of biofilm morphogenesis in space and time.

Biofilms are a most successful microbial lifestyle and prevail in a multitude of environmental and engineered settings. Understanding biofilm ...

Environment

Computed Tomography and Optical Imaging of Osteogenesis-angiogenesis Coupling to Assess Integration of Cranial Bone Autografts and Allografts

A major parameter determining the success of a bone-grafting procedure is vascularization of the area surrounding the graft. We hypothesized that implantation of a bone autograft would induce greater bone regeneration by abundant blood vessel formation. To investigate the effect of the graft on neovascularization at the defect site, we developed a micro&#8211;computed tomography (&#181;CT) approach to characterize newly forming blood vessels, which involves systemic perfusion of the animal with a polymerizing contrast agent. This method enables detailed vascular analysis of an organ in its entirety. Additionally, blood perfusion was assessed using fluorescence imaging (FLI) of a blood-borne fluorescent agent. Bone formation was quantified by FLI using a hydroxyapatite-targeted probe and &#181;CT analysis. Stem cell recruitment was monitored by bioluminescence imaging (BLI) of transgenic mice that express luciferase under the control of the osteocalcin promoter. Here we describe and demonstrate preparation of the allograft, calvarial defect surgery, &#181;CT scanning protocols for the neovascularization study and bone formation analysis (including the in vivo perfusion of contrast agent), and the protocol for data analysis.
The 3D high-resolution analysis of vasculature demonstrated significantly greater angiogenesis in animals with implanted autografts, especially with respect to arteriole formation. Accordingly, blood perfusion was significantly higher in the autograft group by the 7th day after surgery. We observed superior bone mineralization and measured greater bone formation in animals that received autografts. Autograft implantation induced resident stem cell recruitment to the graft-host bone suture, where the cells differentiated into bone-forming cells between the 7th and 10th postoperative day. This finding means that enhanced bone formation may be attributed to the augmented vascular feeding that characterizes autograft implantation. The methods depicted may serve as an optimal tool to study bone regeneration in terms of tightly bounded bone formation and neovascularization.

Implantation of autologous and allogeneic bone grafts constitute accepted approaches to treat major craniofacial bone loss. Yet the effect of graft composition on the interplay among neovascularization, cell differentiation and bone formation is unclear. We present a multimodal imaging protocol aimed to elucidate the angiogenesis-osteogenesis interdependence at the graft proximity.

A major parameter determining the success of a bone-grafting procedure is vascularization of the area surrounding the graft. We hypothesized that ...

Biomechanical Characterization of Human Soft Tissues Using Indentation and Tensile Testing

Regenerative medicine aims to engineer materials to replace or restore damaged or diseased organs. The mechanical properties of such materials should mimic the human tissues they are aiming to replace; to provide the required anatomical shape, the materials must be able to sustain the mechanical forces they will experience when implanted at the defect site. Although the mechanical properties of tissue-engineered scaffolds are of great importance, many human tissues that undergo restoration with engineered materials have not been fully biomechanically characterized. Several compressive and tensile protocols are reported for evaluating materials, but with large variability it is difficult to compare results between studies. Further complicating the studies is the often destructive nature of mechanical testing. Whilst an understanding of tissue failure is important, it is also important to have knowledge of the elastic and viscoelastic properties under more physiological loading conditions.
This report aims to provide a minimally destructive protocol to evaluate the compressive and tensile properties of human soft tissues. As examples of this technique, the tensile testing of skin and the compressive testing of cartilage are described. These protocols can also be directly applied to synthetic materials to ensure that the mechanical properties are similar to the native tissue. Protocols to assess the mechanical properties of human native tissue will allow a benchmark by which to create suitable tissue-engineered substitutes.

Tissue biomechanics is important for maintaining cell shape and function and for determining phenotype. This report demonstrates non-destructive mechanical protocols for characterizing elastic and viscoelastic properties of human soft tissues, which can be directly applied to tissue-engineered substrates to allow a close matching of engineered materials to native tissue.

Regenerative medicine aims to engineer materials to replace or restore damaged or diseased organs. The mechanical properties of such materials should ...

Semiautomated Longitudinal Microcomputed Tomography-based Quantitative Structural Analysis of a Nude Rat Osteoporosis-related Vertebral Fracture Model

Osteoporosis-related vertebral compression fractures (OVCFs) are a common and clinically unmet need with increasing prevalence as the world population ages. Animal OVCF models are essential to the preclinical development of translational tissue engineering strategies. While a number of models currently exist, this protocol describes an optimized method for inducing multiple highly reproducible vertebral defects in a single nude rat. A novel longitudinal semiautomated microcomputed tomography (&#181;CT)-based quantitative structural analysis of the vertebral defects is also detailed. Briefly, rats were imaged at multiple time points post-op. The day 1 scan was reoriented to a standard position, and a standard volume of interest was defined. Subsequent &#181;CT scans of each rat were automatically registered to the day 1 scan so the same volume of interest was then analyzed to assess for new bone formation. This versatile approach can be adapted to a variety of other models where longitudinal imaging-based analysis could benefit from precise 3D semiautomated alignment. Taken together, this protocol describes a readily quantifiable and easily reproducible system for osteoporosis and bone research. The suggested protocol takes 4 months to induce osteoporosis in nude ovariectomized rats and between 2.7 and 4 h to generate, image, and analyze two vertebral defects, depending on tissue size and equipment.

The goal of this protocol is to generate a nude rat osteoporosis-related vertebral compression fracture model that can be longitudinally evaluated in vivo using a semiautomated microcomputed tomography-based quantitative structural analysis.

Osteoporosis-related vertebral compression fractures (OVCFs) are a common and clinically unmet need with increasing prevalence as the world population ...

Synthesis of Monocyte-targeting Peptide Amphiphile Micelles for Imaging of Atherosclerosis

Atherosclerosis is a major contributor to cardiovascular disease, the leading cause of death worldwide, which claims 17.3 million lives annually. Atherosclerosis is also the leading cause of sudden death and myocardial infarction, instigated by unstable plaques that rupture and occlude the blood vessel without warning. Current imaging modalities cannot differentiate between stable and unstable plaques that rupture. Peptide amphiphiles micelles (PAMs) can overcome this drawback as they can be modified with a variety of targeting moieties that bind specifically to diseased tissue. Monocytes have been shown to be early markers of atherosclerosis, while large accumulation of monocytes is associated with plaques prone to rupture. Hence, nanoparticles that can target monocytes can be used to discriminate different stages of atherosclerosis. To that end, here, we describe a protocol for the preparation of monocyte-targeting PAMs (monocyte chemoattractant protein-1 (MCP-1) PAMs). MCP-1 PAMs are self-assembled through synthesis under mild conditions to form nanoparticles of 15 nm in diameter with near neutral surface charge. In vitro, PAMs were found to be biocompatible and had a high binding affinity for monocytes. The methods described herein show promise for a wide range of applications in atherosclerosis as well as other inflammatory diseases.

This paper presents a method involving the synthesis and characterization of monocyte-targeting peptide amphiphile micelles and the corresponding assays to test for biocompatibility and ability of the micelle to bind to monocytes.

Atherosclerosis is a major contributor to cardiovascular disease, the leading cause of death worldwide, which claims 17.3 million lives annually. ...

 IVUS-Based Computational Method to Quantify Coronary Mechanical Properties

Intravascular Ultrasound Image-Based Finite Element Modeling Approach for Quantifying In Vivo Mechanical Properties of Human Coronary Artery

Quantifying the mechanical properties of coronary arterial walls could provide meaningful information for the diagnosis, management, and treatment of coronary artery diseases. Since patient-specific coronary samples are not available for patients requiring continuous monitoring, direct experimental testing of vessel material properties becomes impossible. Current coronary models typically use material parameters from available literature, leading to significant mechanical stress/strain calculation errors. Here, we would introduce a finite element model-based updating approach (FEMBUA) to quantify patient-specific in vivo material properties of coronary arteries based on medical images. In vivo cine intravascular ultrasound (IVUS) and virtual histology (VH)-IVUS images of coronary arteries were acquired from a patient with coronary artery disease. Cine IVUS images showing the vascular movement over one cardiac cycle were segmented, and two IVUS frames with maximum and minimum lumen circumferences were selected to represent the coronary geometry under systolic and diastolic pressure conditions, respectively. VH-IVUS image was also segmented to obtain the vessel contours, and a layer thickness of 0.05 cm was added to the VH-IVUS contours to reconstruct the coronary geometry. A computational finite element model was created with an anisotropic Mooney-Rivlin material model used to describe the vessel's mechanical properties and pulsatile blood pressure conditions prescribed to the coronary luminal surface to make it contract and expand. Then, an iterative updating approach was employed to determine the material parameters of the anisotropic Mooney-Rivlin model by matching minimum and maximum lumen circumferences from the computational finite element model with those from cine IVUS images. This image-based finite element model-based updating approach could be successfully extended to determine the material properties of arterial walls in various vascular beds and holds the potential for risk assessment of cardiovascular diseases.

In vivo cine intravascular ultrasound images show the coronary cross-sectional movement corresponding to different pressure loading conditions. Based on a finite element model, an iterative scheme was employed to determine the patient-specific mechanical properties of coronary arteries in vivo by matching coronary motion from the computational model and medical images.

Quantifying the mechanical properties of coronary arterial walls could provide meaningful information for the diagnosis, management, and treatment of ...

Engineering

Nondestructive Monitoring of Degradable Scaffold-Based Tissue-Engineered Blood Vessel Development Using Optical Coherence Tomography

Engineered vascular grafts with structural and mechanical properties similar to natural blood vessels are expected to meet the growing demand for arterial bypass. Characterization of the growth dynamics and remodeling process of degradable polymer scaffold-based tissue-engineered blood vessels (TEBVs) with pulsatile stimulation is crucial for vascular tissue engineering. Optical imaging techniques stand out as powerful tools for monitoring vascularization of engineered tissue enabling high-resolution imaging in real-time culture. This paper demonstrates a nondestructive and fast real-time imaging strategy to monitor the growth and remodeling of TEBVs in long-term culture by using optical coherence tomography (OCT). Geometric morphology is evaluated, including vascular remodeling process, wall thickness, and comparison of TEBV thickness in different culture time points and presence of pulsatile stimulation. Finally, OCT provides practical possibilities for real-time observation of the degradation of polymer in the reconstructing tissues under pulsatile stimulation or not and in each vessel segment, by compared with the assessment of polymer degradation using scanning electron microscopic(SEM) and polarized microscope.

A step by step protocol for nondestructive and long-period monitoring the process of vascular remodeling and scaffold degradation in real-time culture of biodegradable polymeric scaffold-based tissue-engineered blood vessels with pulsatile stimulation using optical coherence tomography is described here.

Engineered vascular grafts with structural and mechanical properties similar to natural blood vessels are expected to meet the growing demand for arterial ...

A Method to Study the Correlation Between Local Collagen Structure and Mechanical Properties of Atherosclerotic Plaque Fibrous Tissue

The rupture of atherosclerotic plaques in coronary and carotid arteries is the primary cause of fatal cardiovascular events. However, the rupture mechanics of the heterogeneous, highly collagenous plaque tissue, and how this is related to the tissue's fibrous structure, are not known yet. Existing pipelines to study plaque mechanics are limited to obtaining only gross mechanical characteristics of the plaque tissue, based on the assumption of structural homogeneity of the tissue. However, fibrous plaque tissue is structurally heterogeneous, arguably mainly due to local variation in the collagen fiber architecture.
The mechano-imaging pipeline described here has been developed to study the heterogeneous structural and mechanical plaque properties. In this pipeline, the tissue's local collagen architecture is characterized using multiphoton microscopy (MPM) with second-harmonic generation (SHG), and the tissue's failure behavior is characterized under uniaxial tensile testing conditions using digital image correlation (DIC) analysis. This experimental pipeline enables correlation of the local predominant angle and dispersion of collagen fiber orientation, the rupture behavior, and the strain fingerprints of the fibrous plaque tissue. The obtained knowledge is key to better understand, predict, and prevent atherosclerotic plaque rupture events.

We have developed a mechano-imaging pipeline to study the heterogeneous structural and mechanical atherosclerotic plaque properties. This pipeline enables correlation of the local predominant angle and dispersion of collagen fiber orientation, the rupture behavior, and the strain fingerprints of the fibrous plaque tissue.

Optical Coherence Tomography Based Biomechanical Fluid-Structure Interaction Analysis of Coronary Atherosclerosis Progression

Samenvatting

Meer video's verkennen