In vivo magnetic resonance elastography to estimate left ventricular stiffness in a myocardial infarction induced porcine model

The role of the dystrophin glycoprotein complex in muscle cell mechanotransduction

Dystrophin is the central protein of the dystrophin-glycoprotein complex (DGC) in skeletal and heart muscle cells. Dystrophin connects the actin cytoskeleton to the extracellular matrix (ECM). Severing the link between the ECM and the intracellular cytoskeleton has a devastating impact on the homeostasis of skeletal muscle cells, leading to a range of muscular dystrophies. In addition, the loss of a functional DGC leads to progressive dilated cardiomyopathy and premature death. Dystrophin functions as a molecular spring and the DGC plays a critical role in maintaining the integrity of the sarcolemma. Additionally, evidence is accumulating, linking the DGC to mechanosignalling, albeit this role is still less understood. This review article aims at providing an up-to-date perspective on the DGC and its role in mechanotransduction. We first discuss the intricate relationship between muscle cell mechanics and function, before examining the recent research for a role of the dystrophin glycoprotein complex in mechanotransduction and maintaining the biomechanical integrity of muscle cells. Finally, we review the current literature to map out how DGC signalling intersects with mechanical signalling pathways to highlight potential future points of intervention, especially with a focus on cardiomyopathies.

A review of the function of the Dystrophic Glycoprotein Complex (DGC) in mechanosignaling provides an overview of the various components of DGC and potential mechanopathogenic mechanisms, particularly as they relate to muscular dystrophy.

Passive myocardial mechanical properties: meaning, measurement, models

Passive mechanical tissue properties are major determinants of myocardial contraction and relaxation and, thus, shape cardiac function. Tightly regulated, dynamically adapting throughout life, and affecting a host of cellular functions, passive tissue mechanics also contribute to cardiac dysfunction. Development of treatments and early identification of diseases requires better spatio-temporal characterisation of tissue mechanical properties and their underlying mechanisms. With this understanding, key regulators may be identified, providing pathways with potential to control and limit pathological development. Methodologies and models used to assess and mimic tissue mechanical properties are diverse, and available data are in part mutually contradictory. In this review, we define important concepts useful for characterising passive mechanical tissue properties, and compare a variety of in vitro and in vivo techniques that allow one to assess tissue mechanics. We give definitions of key terms, and summarise insight into determinants of myocardial stiffness in situ. We then provide an overview of common experimental models utilised to assess the role of environmental stiffness and composition, and its effects on cardiac cell and tissue function. Finally, promising future directions are outlined.

Shear wave cardiovascular MR elastography using intrinsic cardiac motion for transducer-free non-invasive evaluation of myocardial shear wave velocity

Changes in myocardial stiffness may represent a valuable biomarker for early tissue injury or adverse remodeling. In this study, we developed and validated a novel transducer-free magnetic resonance elastography (MRE) approach for quantifying myocardial biomechanics using aortic valve closure-induced shear waves. Using motion-sensitized two-dimensional pencil beams, septal shear waves were imaged at high temporal resolution. Shear wave speed was measured using time-of-flight of waves travelling between two pencil beams and corrected for geometrical biases. After validation in phantoms, results from twelve healthy volunteers and five cardiac patients (two left ventricular hypertrophy, two myocardial infarcts, and one without confirmed pathology) were obtained. Torsional shear wave speed in the phantom was 3.0 ± 0.1 m/s, corresponding with reference speeds of 2.8 ± 0.1 m/s. Geometrically-biased flexural shear wave speed was 1.9 ± 0.1 m/s, corresponding with simulation values of 2.0 m/s. Corrected septal shear wave speeds were significantly higher in patients than healthy volunteers [14.1 (11.0-15.8) m/s versus 3.6 (2.7-4.3) m/s, p = 0.001]. The interobserver 95%-limits-of-agreement in healthy volunteers were ± 1.3 m/s and interstudy 95%-limits-of-agreement - 0.7 to 1.2 m/s. In conclusion, myocardial shear wave speed can be measured using aortic valve closure-induced shear waves, with cardiac patients showing significantly higher shear wave speeds than healthy volunteers. This non-invasive measure may provide valuable insights into the pathophysiology of heart failure.

Mix and (mis-)match - The mechanosensing machinery in the changing environment of the developing, healthy adult and diseased heart

The composition and the stiffness of cardiac microenvironment change during development and/or in heart disease. Cardiomyocytes (CMs) and their progenitors sense these changes, which decides over the cell fate and can trigger CM (progenitor) proliferation, differentiation, de-differentiation or death. The field of mechanobiology has seen a constant increase in output that also includes a wealth of new studies specific to cardiac or cardiomyocyte mechanosensing. As a result, mechanosensing and transduction in the heart is increasingly being recognised as a main driver of regulating the heart formation and function. Recent work has for instance focused on measuring the molecular, physical and mechanical changes of the cellular environment - as well as intracellular contributors to the passive stiffness of the heart. On the other hand, a variety of new studies shed light into the molecular machinery that allow the cardiomyocytes to sense these properties. Here we want to discuss the recent work on this topic, but also specifically focus on how the different components are regulated at various stages during development, in health or disease in order to highlight changes that might contribute to disease progression and heart failure.

Evaluating Novel Targets of Ischemia Reperfusion Injury in Pig Models

Coronary heart diseases are of high relevance for health care systems in developed countries regarding patient numbers and costs. Disappointingly, the enormous effort put into the development of innovative therapies and the high numbers of clinical studies conducted are counteracted by the low numbers of therapies that become clinically effective. Evidently, pre-clinical research in its present form does not appear informative of the performance of treatments in the clinic and, even more relevant, it appears that there is hardly any consent about how to improve the predictive capacity of pre-clinical experiments. According to the steadily increasing relevance that pig models have gained in biomedical research in the recent past, we anticipate that research in pigs can be highly predictive for ischemia-reperfusion injury (IRI) therapies as well. Thus, we here describe the significance of pig models in IRI, give an overview about recent developments in evaluating such models by clinically relevant methods and present the latest insight into therapies applied to pigs under IRI.

Non-invasive Measurement of Dynamic Myocardial Stiffness Using Acoustic Radiation Force Impulse Imaging

Myocardial stiffness exhibits cyclic variations over the course of the cardiac cycle. These trends are closely tied to the electromechanical and hemodynamic changes in the heart. Characterization of dynamic myocardialstiffness can provide insights into the functional state of the myocardium, as well as allow for differentiation between the underlying physiologic mechanisms that lead to congestive heart failure. Previous work has revealed the potential of acoustic radiation force impulse (ARFI) imaging to capture temporal trends in myocardial stiffness in experimental preparations such as the Langendorff heart, as well as on animals in open-chest and intracardiac settings. This study was aimed at investigating the potential of ARFI to measure dynamic myocardial stiffness in human subjects, in a non-invasive manner through transthoracic imaging windows. ARFI imaging was performed on 12 healthy volunteers to track stiffness changes within the interventricular septum in parasternal long-axis and short-axis views. Myocardial stiffness dynamics over the cardiac cycle was quantified using five indices: stiffness ratio, rates of relaxation and contraction and time constants of relaxation and contraction. The yield of ARFI acquisitions was evaluated based on metrics of signal strength and tracking fidelity such as displacement signal-to-noise ratio, signal-to-clutter level, temporal coherence of speckle and spatial similarity within the region of excitation. These were quantified using the mean ARF-induced displacements over the cardiac cycle, the contrast between the myocardium and the cardiac chambers, the minimum correlation coefficients of radiofrequency signals and the correlation between displacement traces across simultaneously acquired azimuthal beams, respectively. Forty-one percent of ARFI acquisitions were determined to be "successful" using a mean ARF-induced displacement threshold of 1.5 μm. "Successful" acquisitions were found to have higher (i) signal-to-clutter levels, (ii) temporal coherence and (iii) spatial similarity compared with "unsuccessful" acquisitions. Median values of these three metrics, between the two groups, were measured to be 13.42dB versus 5.42dB, 0.988 versus 0.976 and 0.984 versus 0.849, respectively. Signal-to-clutter level, temporal coherence and spatial similarity were also found to correlate with each other. Across the cohort of healthy volunteers, the stiffness ratio measured was 2.74 ± 0.86; the rate of relaxation, 7.82 ± 4.69/s; and the rate of contraction, -7.31±3.79 /s. The time constant of relaxation was 35.90 ± 20.04ms, and that of contraction was 37.24 ± 19.85ms. ARFI-derived indices of myocardial stiffness were found to be similar in both views. These results indicate the feasibility of using ARFI to measure dynamic myocardial stiffness trends in a non-invasive manner and also highlightthe technical challenges of implementing this method in the transthoracic imaging environment.

Cardiovascular magnetic resonance elastography: A review

Cardiovascular diseases are the leading cause of death worldwide. These cardiovascular diseases are associated with mechanical changes in the myocardium and aorta. It is known that stiffness is altered in many diseases, including the spectrum of ischemia, diastolic dysfunction, hypertension and hypertrophic cardiomyopathy. In addition, the stiffness of the aortic wall is altered in multiple diseases, including hypertension, coronary artery disease and aortic aneurysm formation. For example, in diastolic dysfunction in which the ejection fraction is preserved, stiffness can potentially be an important biomarker. Similarly, in aortic aneurysms, stiffness can provide valuable information with regard to rupture potential. A number of studies have addressed invasive and non-invasive approaches to test and measure the mechanical properties of the myocardium and aorta. One of the non-invasive approaches is magnetic resonance elastography (MRE). MRE is a phase-contrast magnetic resonance imaging technique that measures tissue stiffness non-invasively. This review article highlights the technical details and application of MRE in the quantification of myocardial and aortic stiffness in different disease states.

Advances and Future Direction of Magnetic Resonance Elastography

The mechanical properties of soft tissues are closely associated with a variety of diseases. This motivates the development of elastography techniques in which tissue mechanical properties are quantitatively estimated through imaging. Magnetic resonance elastography (MRE) is a noninvasive phase-contrast MR technique wherein shear modulus of soft tissue can be spatially and temporally estimated. MRE has recently received significant attention due to its capability in noninvasively estimating tissue mechanical properties, which can offer considerable diagnostic potential. In this work, recent technology advances of MRE, its future clinical applications, and the related limitations will be discussed.

Estimation of transversely isotropic material properties from magnetic resonance elastography using the optimised virtual fields method

Magnetic resonance elastography (MRE) has been used to estimate isotropic myocardial stiffness. However, anisotropic stiffness estimates may give insight into structural changes that occur in the myocardium as a result of pathologies such as diastolic heart failure. The virtual fields method (VFM) has been proposed for estimating material stiffness from image data. This study applied the optimised VFM to identify transversely isotropic material properties from both simulated harmonic displacements in a left ventricular (LV) model with a fibre field measured from histology as well as isotropic phantom MRE data. Two material model formulations were implemented, estimating either 3 or 5 material properties. The 3-parameter formulation writes the transversely isotropic constitutive relation in a way that dissociates the bulk modulus from other parameters. Accurate identification of transversely isotropic material properties in the LV model was shown to be dependent on the loading condition applied, amount of Gaussian noise in the signal, and frequency of excitation. Parameter sensitivity values showed that shear moduli are less sensitive to noise than the other parameters. This preliminary investigation showed the feasibility and limitations of using the VFM to identify transversely isotropic material properties from MRE images of a phantom as well as simulated harmonic displacements in an LV geometry.

Biomechanical assessment of myocardial infarction using optical coherence elastography

Myocardial infarction (MI) leads to cardiomyocyte loss, impaired cardiac function, and heart failure. Molecular genetic analyses of myocardium in mouse models of ischemic heart disease have provided great insight into the mechanisms of heart regeneration, which is promising for novel therapies after MI. Although biomechanical factors are considered an important aspect in cardiomyocyte proliferation, there are limited methods for mechanical assessment of the heart in the mouse MI model. This prevents further understanding the role of tissue biomechanics in cardiac regeneration. Here we report optical coherence elastography (OCE) of the mouse heart after MI. Surgical ligation of the left anterior descending coronary artery was performed to induce an infarction in the heart. Two OCE methods with assessment of the direction-dependent elastic wave propagation and the spatially resolved displacement damping provide complementary analyses of the left ventricle. In comparison with sham, the infarcted heart features a fibrotic scar region with reduced elastic wave velocity, decreased natural frequency, and less mechanical anisotropy at the tissue level at the sixth week post-MI, suggesting lower and more isotropic stiffness. Our results indicate that OCE can be utilized for nondestructive biomechanical characterization of MI in the mouse model, which could serve as a useful tool in the study of heart repair.

Quantification of breast stiffness using MR elastography at 3 Tesla with a soft sternal driver: A reproducibility study

PURPOSE

Previous studies of breast MR elastography (MRE) evaluated the technique at magnetic field strengths of 1.5 Tesla (T) with the breast in contact with the driver. The aim of this study is to evaluate breast stiffness measurements and their reproducibility using a soft sternal driver at 3T and compare the results with qualitative measures of breast density.

MATERIALS AND METHODS

Twenty-two healthy volunteers each underwent two separate breast MRE scans in a 3T MRI. MRE vibrations were introduced into the breasts at 60 Hz using a soft sternal driver and axial slices were collected using a gradient echo MRE sequence. Mean stiffness measurements were calculated for each volunteer as well as a measure of reproducibility using concordance correlation between scans. Mean stiffness values for each volunteer were assessed and related to amounts of fibroglandular tissue (i.e., breast lobules, ducts, and fibrous connective tissue).

RESULTS

The stiffness values were reproducible with a significant P-value < 0.0001 between two scans with concordance correlation of 0.87 and 0.91 for center slice and grouping all slices, respectively. Volunteers with dense breasts (i.e., higher grades of fibroglandular tissue) had mean stiffness values of 0.96 kPa (center slice) and 0.92 kPa (all slices) while those without dense breasts had mean stiffness values of 0.85 kPa (center slice) and 0.83 kPa (all slices) (P ≤ 0.05).

CONCLUSION

Breast MRE is a reproducible technique at 3T using a soft sternal driver. Dense breasts had significantly higher stiffness measurements compared with nondense breasts.

LEVEL OF EVIDENCE

2 J. MAGN. RESON. IMAGING 2017;45:1379-1384.

Regional assessment of in vivo myocardial stiffness using 3D magnetic resonance elastography in a porcine model of myocardial infarction

PURPOSE

The stiffness of a myocardial infarct affects the left ventricular pump function and remodeling. Magnetic resonance elastography (MRE) is a noninvasive imaging technique for measuring soft-tissue stiffness in vivo. The purpose of this study was to investigate the feasibility of assessing in vivo regional myocardial stiffness with high-frequency 3D cardiac MRE in a porcine model of myocardial infarction, and compare the results with ex vivo uniaxial tensile testing.

METHODS

Myocardial infarct was induced in a porcine model by embolizing the left circumflex artery. Fourteen days postinfarction, MRE imaging was performed in diastole using an echocardiogram-gated spin-echo echo-planar-imaging sequence with 140-Hz vibrations and 3D MRE processing. The MRE stiffness and tensile modulus from uniaxial testing were compared between the remote and infarcted myocardium.

RESULTS

Myocardial infarcts showed increased in vivo MRE stiffness compared with remote myocardium (4.6 ± 0.7 kPa versus 3.0 ± 0.6 kPa, P = 0.02) within the same pig. Ex vivo uniaxial mechanical testing confirmed the in vivo MRE results, showing that myocardial infarcts were stiffer than remote myocardium (650 ± 80 kPa versus 110 ± 20 kPa, P = 0.01).

CONCLUSIONS

These results demonstrate the feasibility of assessing in vivo regional myocardial stiffness with high-frequency 3D cardiac MRE. Magn Reson Med 79:361-369, 2018. © 2017 International Society for Magnetic Resonance in Medicine.