Novel cardiovascular magnetic resonance phenotyping of the myocardium

INTRODUCTION Left ventricular (LV) microstructure is unique, composed of a winding helical pattern of myocytes and rotating aggregations of myocytes called sheetlets. Hypertrophic cardiomyopathy (HCM) is a cardiovascular disease characterised by left ventricular hypertrophy (LVH), however the link between LVH and underlying microstructural aberration is poorly understood. In vivo cardiovascular diffusion tensor imaging (cDTI) is a novel cardiovascular MRI (CMR) technique, capable of characterising LV microstructural dynamics non-invasively. In vivo cDTI may therefore improve our understanding microstructural-functional relationships in health and disease. METHODS AND RESULTS The monopolar diffusion weighted stimulated echo acquisition mode (DW-STEAM) sequence was evaluated for in vivo cDTI acquisitions at 3Tesla, in healthy volunteers (HV), patients with hypertensive LVH, and HCM patients. Results were contextualised in relation to extensively explored technical limitations. cDTI parameters demonstrated good intra-centre reproducibility in HCM, and good inter-centre reproducibility in HV. In all subjects, cDTI was able to depict the winding helical pattern of myocyte orientation known from histology, and the transmural rate of change in myocyte orientation was dependent on LV size and thickness. In HV, comparison of cDTI parameters between systole and diastole revealed an increase in transmural gradient, combined with a significant re-orientation of sheetlet angle. In contrast, in HCM, myocyte gradient increased between phases, however sheetlet angulation retained a systolic-like orientation in both phases. Combined analysis with hypertensive patients revealed a proportional decrease in sheetlet mobility with increasing LVH. CONCLUSION In vivo DW-STEAM cDTI can characterise LV microstructural dynamics non-invasively. The transmural rate of change in myocyte angulation is dependent on LV size and wall thickness, however inter phase changes in myocyte orientation are unaffected by LVH. In contrast, sheetlet dynamics demonstrate increasing dysfunction, in proportion to the degree of LVH. Resolving technical limitations is key to advancing this technique, and improving the understanding of the role of microstructural abnormalities in cardiovascular disease expression.Open Acces

Micro-computed tomography for high resolution soft tissue imaging; applications in the normal and failing heart

The normal structure and function of the heart, the common pathological changes that cause abnormal function and the interventions proposed to improve or restore its function are fundamentally based on cardiac anatomy. Therefore in all these areas a detailed and accurate understanding of 3D structure is essential. However there is still disparity over some aspects of the form and function of the healthy heart. Furthermore, in heart failure (HF) the transition from compensated to decompensated HF is poorly understood, and details of ventricular, and particularly atrial, remodelling and their effects on cardiac function are yet to be fully elucidated. In addition little is known on how the 3D morphology of the cardiac conduction system is affected in disease, and further knowledge is required on the structural substrates for arrhythmogenesis associated with HF. Here we have developed contrast enhanced micro-CT for soft tissue imaging, allowing non-invasive high resolution (~5 µm attainable) differentiation of multiple soft tissue types including; muscle, connective tissue and fat. Micro-CT was optimised for imaging of whole intact mammalian hearts and from these data we reveal novel morphological and anatomical detail in healthy hearts and in hearts after experimental HF (volume and pressure overload). Remodelling of the myocardium in HF was dramatic with significant hypertrophy and dilatation observed in both atria and ventricles. The atria showed a 67% increase in myocardial volume, with the left atrium showing a 93% increase. The pectinate muscle: wall thickness ratio was significantly increased in both atria (p

Modeling and simulation of the electric activity of the heart using graphic processing units

Mathematical modelling and simulation of the electric activity of the heart (cardiac electrophysiology) offers and ideal framework to combine clinical and experimental data in order to help understanding the underlying mechanisms behind the observed respond under physiological and pathological conditions. In this regard, solving the electric activity of the heart possess a big challenge, not only because of the structural complexities inherent to the heart tissue, but also because of the complex electric behaviour of the cardiac cells. The multi- scale nature of the electrophysiology problem makes difficult its numerical solution, requiring temporal and spatial resolutions of 0.1 ms and 0.2 mm respectively for accurate simulations, leading to models with millions degrees of freedom that need to be solved for thousand time steps. Solution of this problem requires the use of algorithms with higher level of parallelism in multi-core platforms. In this regard the newer programmable graphic processing units (GPU) has become a valid alternative due to their tremendous computational horsepower. This thesis develops around the implementation of an electrophysiology simulation software entirely developed in Compute Unified Device Architecture (CUDA) for GPU computing. The software implements fully explicit and semi-implicit solvers for the monodomain model, using operator splitting and the finite element method for space discretization. Performance is compared with classical multi-core MPI based solvers operating on dedicated high-performance computer clusters. Results obtained with the GPU based solver show enormous potential for this technology with accelerations over 50× for three-dimensional problems when using an implicit scheme for the parabolic equation, whereas accelerations reach values up to 100× for the explicit implementation. The implemented solver has been applied to study pro-arrhythmic mechanisms during acute ischemia. In particular, we investigate on how hyperkalemia affects the vulnerability window to reentry and the reentry patterns in the heterogeneous substrate caused by acute regional ischemia using an anatomically and biophysically detailed human biventricular model. A three dimensional geometrically and anatomically accurate regionally ischemic human heart model was created. The ischemic region was located in the inferolateral and posterior side of the left ventricle mimicking the occlusion of the circumflex artery, and the presence of a washed-out zone not affected by ischemia at the endocardium has been incorporated. Realistic heterogeneity and fi er anisotropy has also been considered in the model. A highly electrophysiological detailed action potential model for human has been adapted to make it suitable for modeling ischemic conditions (hyperkalemia, hipoxia, and acidic conditions) by introducing a formulation of the ATP-sensitive K+ current. The model predicts the generation of sustained re-entrant activity in the form single and double circus around a blocked area within the ischemic zone for K+ concentrations bellow 9mM, with the reentrant activity associated with ventricular tachycardia in all cases. Results suggest the washed-out zone as a potential pro-arrhythmic substrate factor helping on establishing sustained ventricular tachycardia.Colli-Franzone P, Pavarino L. A parallel solver for reaction-diffusion systems in computational electrocardiology, Math. Models Methods Appl. Sci. 14 (06):883-911, 2004.Colli-Franzone P, Deu hard P, Erdmann B, Lang J, Pavarino L F. Adaptivity in space and time for reaction-diffusion systems in electrocardiology, SIAM J. Sci. Comput. 28 (3):942-962, 2006.Ferrero J M(Jr), Saiz J, Ferrero J M, Thakor N V. Simulation of action potentials from metabolically impaired cardiac myocytes: Role of atp-sensitive K+ current. Circ Res, 79(2):208-221, 1996.Ferrero J M (Jr), Trenor B. Rodriguez B, Saiz J. Electrical acticvity and reentry during acute regional myocardial ischemia: Insights from simulations.Int J Bif Chaos, 13:3703-3715, 2003.Heidenreich E, Ferrero J M, Doblare M, Rodriguez J F. Adaptive macro finite elements for the numerical solution of monodomain equations in cardiac electrophysiology, Ann. Biomed. Eng. 38 (7):2331-2345, 2010.Janse M J, Kleber A G. Electrophysiological changes and ventricular arrhythmias in the early phase of regional myocardial ischemia. Circ. Res. 49:1069-1081, 1981.ten Tusscher K HWJ, Panlov A V. Alternans and spiral breakup in a human ventricular tissue model. Am. J.Physiol. Heart Circ. Physiol. 291(3):1088-1100, 2006.<br /

The development of a platform to manipulate cardiomyocyte structure and function

Cardiac tissue engineering to replace damaged areas of the postmitotic heart is still presented with significant challenges, due to the complex and dynamic interplay of electrical, mechanical and biochemical signals involved in the myocardium. The advancement of regenerative approaches is focussed on understanding the underlying regulatory mechanisms involved throughout cardiac development. However, current knowledge of how biophysical cues in the stem cell niche can modulate cell behaviour is limited. Firstly, polyacrylamide-co-acrylic acid was used as an in vitro stiffness-tuneable platform to test the effect of substrate mechanics on human induced pluripotent stem cell (hiPSC) differentiation into cardiomyocytes (CM). The results showed that the optimum differentiation efficiency level peaked at the embryonic-like stiffness of 560 Pa, with increased upregulation of cardiac genes. Functionally, hiPSC-CMs showed a biphasic relationship with a faster calcium transient and higher force generation at cardiac physiological stiffness. Next, shape was incorporated into the experimental design via CardioArray, a custom-built platform which mimics both the stiffness and shape of an adult human CM. This system can accommodate individual hiPSC-CMs to adopt the 3D geometry of an adult CM, while at the same time providing the relevant stiffness cues from the underlying substrate. The results highlighted the specific contribution of stiffness and 3D shape to α-sarcomeric structure, cell membrane stiffness, single cell gene expression and intracellular calcium cycling. Finally, the electrical microenvironment was investigated as a third infleuncing factor on hiPSC-CM development. A hybrid conductive polyaniline-Scl2 scaffold was fabricated, showing long term electronic stability and no cell toxicity when interfaced with electrosensitive hiPSC-CMs. This could provide electromechanical stability in model studies. Improvement of conduction velocity was observed in an in vitro myocardial slice model. As a whole, this thesis demonstrates the differential effects of substrate mechanics on hiPSC cardiac differentiation, providing a novel crucial understanding of how biophysical cues modulate the stem cell niche during differentiation and in vitro culture.Open Acces

Investigating left ventricular infarct extension after myocardial infarction using cardiac imaging and patient-specific modelling

Acute myocardial infarction (MI) is one of the leading causes of death worldwide that commonly affects the left ventricle (LV). Following MI, the LV mechanical loading is altered and may undergo a maladaptive compensatory mechanism that progressively leads to adverse LV remodelling and then heart failure. One of the remodelling processes is the infarct extension which involves necrosis of healthy myocardium in the border zone (BZ), progressively enlarging the infarct zone (IZ) and recruiting the remote zone (RZ) into the BZ. The mechanisms underlying infarct extension remain unclear, but myocyte stretching has been suggested as the most likely cause. A recent personalized LV modelling work found that infarct extension was correlated to inadequate diastolic fibre stretch and higher infarct stiffness. However, other possible factors of infarct extension may not have been elucidated in this work due to the limited number of myocardial locations analysed at the subendocardium only. Using human patient-specific left- ventricular (LV) models established from cardiac magnetic resonance imaging (MRI) of 6 MI patients, the correlation between infarct extension and regional mechanics impairment was studied. Prior to the modelling, a 2D-4D registration-cum-segmentation framework for the delineation of LV in late gadolinium enhanced (LGE) MRI was first developed, which is a pre-requisite for infarct scar quantification and localization in patient-specific 3D LV models. This framework automatically corrects for motion artifacts in multimodal MRI scans, resolving the issue of inaccurate infarct mapping and geometry reconstruction which is typically done manually in most patient-specific modelling work. The registration framework was evaluated against cardiac MRI data from 27 MI patients and showed high accuracy and robustness in delineating LV in LGE MRI of various quality and different myocardial features. This framework allows the integration of LV data from both LGE and cine scans and to facilitate the reconstruction of accurate 3D LV and infarct geometries for subsequent computational study. In the patient-specific LV mechanical modelling, the LV mechanics were formulated using a quasi-static and nearly incompressible hyperelastic material law with transversely isotropic behaviour. The patient-specific models were incorporated with realistic fibre orientation and excitable contracting myocardium. Optimisation of passive and active material parameters were done by minimizing the myocardial wall distance between the reference and end-diastole/end-systole LV geometries. Full cardiac cycle of the LV models was then simulated and stress/strain data were extracted to determine the correlation between regional mechanics abnormality and infarct extension. The fibre stress-strain loops (FSSLs) were analysed and its abnormality was characterized using the directional regional external work (DREW) index, which measures FSSL area and loop direction. Sensitivity studies were also performed to investigate the effect of infarct stiffness on regional myocardial mechanics and potential for infarct extension. It was found that infarct extension was correlated to severely abnormal FSSL in the form of counter-clockwise loop, as indicated by negative DREW values. In regions demonstrating negative DREW values, substantial isovolumic relaxation (IVR) fibre stretching was observed. Further analysis revealed that the occurrence of severely abnormal FSSL near the RZ-BZ boundary was due to a large amount of surrounding infarcted tissue that worsen with excessively stiff IZ

Computational biomechanics of acute myocardial infarction and its treatment

The intramyocardial injection of biomaterials is an emerging therapy for myocardial infarction. Computational methods can help to study the mechanical effect s of biomaterial injectates on the infarcted heart s and can contribute to advance and optimise the concept of this therapy. The distribution of polyethylene glycol hydrogel injectate delivered immediately after the infarct induction was studied using rat infarct model. A micro-structural three-dimensional geometrical model of the entire injectate was reconstructed from histological micro graphs. The model provides a realistic representation of biomaterial injectates in computational models at macroscopic and microscopic level. Biaxial and compression mechanical testing was conducted for healing rat myocardial infarcted tissue at immediate (0 day), 7, 14 and 28 days after infarction onset. Infarcts were found to be mechanically anisotropic with the tissue being stiffer in circumferential direction than in longitudinal direction. The 0, 7, 14 and 28 days infarcts showed 443, 670, 857 and 1218 kPa circumferential tensile moduli. The 28 day infarct group showed a significantly higher compressive modulus compared to the other infarct groups (p= 0.0055, 0.028, and 0.018 for 0, 7 and 14 days groups). The biaxial mechanical data were utilized to establish material constitutive models of rat healing infarcts. Finite element model s and genetic algorithms were employed to identify the parameters of Fung orthotropic hyperelastic strain energy function for the healing infarcts. The provided infarct mechanical data and the identified constitutive parameters offer a platform for investigations of mechanical aspects of myocardial infarction and therapies in the rat, an experimental model extensively used in the development of infarct therapies. Micro-structurally detailed finite element model of a hydrogel injectate in an infarct was developed to provide an insight into the micromechanics of a hydrogel injectate and infarct during the diastolic filling. The injectate caused the end-diastolic fibre stresses in the infarct zone to decrease from 22.1 to 7.7 kPa in the 7 day infarct and from 35.7 to 9.7 kPa in the 28 day infarct. This stress reduction effect declined as the stiffness of the biomaterial increased. It is suggested that the gel works as a force attenuating system through micromechanical mechanisms reducing the force acting on tissue layers during the passive diastolic dilation of the left ventricle and thus reducing the stress induced in these tissue layers

Establishing the material parameters of the neonatal porcine ventricular myocardium

Neonatal heart disorders represent a major clinical challenge and congenital heart disease alone affects 36,000 new-borns annually within the European Union. Engineering-based computational modelling is increasingly used in adult cardiology to simulate normal and pathophysiological conditions, leading to the development of new clinical interventions. However, the application of such technologies to advance care in young patients is limited by the need to characterise the structural and biomechanical properties of neonatal tissue in estimating the material parameters. Consequently, this research aims to establish the first material parameters that describe neonatal cardiac tissue. A porcine model was used to perform the microstructural and biomechanical analysis. Established (histology) and emerging—two-photon excited fluorescence and second-harmonic generation (TPEF/SHG) and diffusion tensor magnetic resonance imaging (DT-MRI)—imaging methodologies enabled quantification of the in-plane and out-plane cardiomyocytes’ and collagen fibrils’ orientation and dispersion, in addition to the cardiomyocytes’ architecture and regional fractional anisotropy. The biomechanical analysis was performed via uniaxial tensile, biaxial and simple shear tests. The analyses identified the regional variations within the anterior and posterior aspects of both ventricles. The surface area analysis quantified the greater ratio of collagen to cardiomyocytes in the posterior wall in both ventricles. TPEF/SHG identified the greater cardiomyocytes rotation in the posterior wall of both ventricles. The anterior ventricular walls were stiffest. The one-day-old porcine tissue was identified as exhibiting one-half the stiffness of adult porcine tissue in uniaxial testing, one-third in biaxial testing, and one-fourth Thesis summary iv stiffness in simple shear testing. Finally, these data were used with the structurally-based Holzapfel–Ogden model of the myocardium to establish the material parameters. The estimated material parameters in this study will enable a simulation that specifically reflects neonatal ventricular tissue behaviour. This will ultimately create new opportunities for researchers, bioengineers and clinicians to identify novel treatments and interventions of neonatal heart diseases

Mathematical modelling of cardiac function: constitutive law, fibre dispersion, growth and remodelling

The heart is an immensely complex living organ. Myocardium has continually been undergoing adaptive or maladaptive response to surrounding environments, in which the significant importance of growth and remodelling (G&R) has been valued. This PhD project intends to study mechanics modelling of myocardium towards predictive stress/strain-driven growth. Constitutive laws and fibre structures in myocardium work together to determine the mechanical clues which trigger the growth mechanically. Therefore, this project includes two parts: (1) constitutive characterization of myocardium, and (2) myocardial G&R. Constitutive laws and myofibre architectures hold the key to accurately model the biomechanical behaviours of the heart. In the first part of this thesis, we firstly perform an analysis using combinations of uniaxial tension, biaxial tension and simple shear from three different sets of myocardial experimental tissue studies to investigate the descriptive and predictive capabilities of a general invariant-based model that is developed by Holzapfel and Ogden, denoted the HO model. We aim to reduce the constitutive law using the Akaike information criterion to maintain its mechanical integrity whilst achieve minimal computational cost. Our study shows that single-mode tests are insufficient to determine the myocardium responses. It is also essential to consider the transmural fibre rotation within the myocardial samples. We conclude that a competent myocardial material model can be obtained from the general HO model using Akaike information criterion analysis and a suitable combination of tissue tests. Secondly, we develop a neonatal porcine bi-ventricle model with three different myofibre architectures for the left side of the heart. The most realistic one is derived from ex vivo diffusion tensor magnetic resonance image, and the other two simplifications are based on the rule-based methods. We show that the most realistic myofibre architecture model can achieve better cardiac pump functions compared to those of the rule-based models under the same pre/after loads. Our results also reveal that when the cross-fibre contraction is included, the active stress seems to play a dual role: the sheet-normal component enhances the ventricular contraction while the sheet component does the opposite. This study highlights the importance of including myofibre dispersion in cardiac modelling if rule-based methods are used, especially in personalized model. To further describe the detailed fibre distribution, discrete fibre dispersion method is employed to compute passive response because of its advantages in excluding compressed fibres. An additive active stress method that includes cross-fibre active stress is proposed according to the generalised structure tensor method. We find that end-systolic volumes of simulated heart models are much more sensitive to dispersion parameter than end-diastolic volumes. G&R is the focus in the second part of this thesis. An updated reference approach is employed to track the evolution of the reference configuration during G&R, in which the nodal positions and the fibre structure are updated at the beginning of each new growth cycle. Moreover, the homogenised constrained mixture theory is used to describe the G&R process of each constituent within myocardium, which are the ground matrix, collagen network and myofibres. Our models can reproduce the eccentric growth driven by fibre stretch at the diastole, concentric growth driven by fibre stress at the systole, and heterogeneous growth after acute myocardium infarction. Ventricular wall G&R mainly occurs in endocardium, in which the myocyte is the primary responder for the G&R process. G&R laws of collagen fibre have significant impacts on G&R of heart. For example, purely remodeled collagen network without new deposition causes increasingly softer heart wall, leading to excessive heart dilation. Finally, the effects of fibre dispersion on G&R is investigated by including fibre dispersion model in the G&R of infarction model. Highly dispersed fibre structure in the infarcted zone significantly reduces the pump function. This thesis has been focusing on mathematical modelling of biomechanical behaviours of myocardium, firstly on the nonlinear cardiac mechanics including constitutive laws and fibre structures, and then on the G&R process of heart under different pathological conditions. These studies support to choose suitable constitutive laws and fibre architectures in G&R model and illustrate the underlying mechanism of mechanical triggers in G&R. It presents the potential for understanding the mechanics of heart failure and reveal hidden roles of different constituents in myocardium