Sensitive detection of abnormal aortic architecture in Marfan syndrome with high-frequency ultrasonic tissue characterization

A mechanistic model on the role of "radially-running" collagen fibers on dissection properties of human ascending thoracic aorta

Aortic dissection (AoD) is a common condition that often leads to life-threatening cardiovascular emergency. From a biomechanics viewpoint, AoD involves failure of load-bearing microstructural components of the aortic wall, mainly elastin and collagen fibers. Delamination strength of the aortic wall depends on the load-bearing capacity and local micro-architecture of these fibers, which may vary with age, disease and aortic location. Therefore, quantifying the role of fiber micro-architecture on the delamination strength of the aortic wall may lead to improved understanding of AoD. We present an experimentally-driven modeling paradigm towards this goal. Specifically, we utilize collagen fiber micro-architecture, obtained in a parallel study from multi-photon microscopy, in a predictive mechanistic framework to characterize the delamination strength. We then validate our model against peel test experiments on human aortic strips and utilize the model to predict the delamination strength of separate aortic strips and compare with experimental findings. We observe that the number density and failure energy of the radially-running collagen fibers control the peel strength. Furthermore, our model suggests that the lower delamination strength previously found for the circumferential direction in human aorta is related to a lower number density of radially-running collagen fibers in that direction. Our model sets the stage for an expanded future study that could predict AoD propagation in patient-specific aortic geometries and better understand factors that may influence propensity for occurrence.

Elastin and collagen fibre microstructure of the human aorta in ageing and disease: a review

Aortic disease is a significant cause of death in developed countries. The most common forms of aortic disease are aneurysm, dissection, atherosclerotic occlusion and ageing-induced stiffening. The microstructure of the aortic tissue has been studied with great interest, because alteration of the quantity and/or architecture of the connective fibres (elastin and collagen) within the aortic wall, which directly imparts elasticity and strength, can lead to the mechanical and functional changes associated with these conditions. This review article summarizes the state of the art with respect to characterization of connective fibre microstructure in the wall of the human aorta in ageing and disease, with emphasis on the ascending thoracic aorta and abdominal aorta where the most common forms of aortic disease tend to occur.

Extracellular matrix and the mechanics of large artery development

The large, elastic arteries, as their name suggests, provide elastic distention and recoil during the cardiac cycle in vertebrate animals. The arteries are distended from the pressure of ejecting blood during the active contraction of the left ventricle (LV) during systole and recoil to their original dimensions during relaxation of the LV during diastole. The cyclic distension occurs with minimal energy loss, due to the elastic properties of one of the major structural extracellular matrix (ECM) components, elastin. The maximum distension is limited to prevent damage to the artery by another major ECM component, collagen. The mix of ECM components in the wall largely determines the passive mechanical behavior of the arteries and the subsequent load on the heart during systole. While much research has focused on initial artery formation, there has been less attention on the continuing development of the artery to produce the mature composite wall complete with endothelial cells (ECs), smooth muscle cells (SMCs), and the necessary mix of ECM components for proper cardiovascular function. This review focuses on the physiology of large artery development, including SMC differentiation and ECM production. The effects of hemodynamic forces and ECM deposition on the evolving arterial structure and function are discussed. Human diseases and mouse models with genetic mutations in ECM proteins that affect large artery development are summarized. A review of constitutive models and growth and remodeling theories is presented, along with future directions to improve understanding of ECM and the mechanics of large artery development.

Quantitative evaluation of coronary artery wall echogenicity by integrated backscatter analysis in Kawasaki disease

BACKGROUND

Coronary artery wall echogenicity increases on echocardiograms during the acute phase of Kawasaki disease (KD). According to this background, echogenicity of the coronary artery wall in patients with KD is quantified by using integrated backscatter (IB) analysis.

METHODS

IB analysis is a quantitative method for evaluating echogenicity. We examined the value of IB in the wall of the left anterior descending coronary artery and compared it with that in adjacent intracardiac blood as a measure of background. The difference between these values is represented as corrected IB for the coronary artery wall.

RESULTS

Corrected IB for the coronary artery wall was higher in patients with KD than in controls (KD with pre-immunoglobulin therapy vs. controls: 27.4 +/- 5.3 dB vs. 22.0 +/- 3.5 dB, P < .05) and in patients with coronary enlargement after intravenous immunoglobulin (with vs. without coronary enlargement, 29.2 +/- 5.2 dB vs. 24.1 +/- 5.5 dB, P < .05).

CONCLUSION

The magnitude of IB from the coronary artery wall reflects the effectiveness of immunoglobulin therapy. Furthermore, this method and its value might be useful to predict the occurrence of coronary enlargement in patients with KD.

Histopathologic findings in ascending aortas from individuals with Loeys-Dietz syndrome (LDS)

Loeys-Dietz syndrome (LDS) is an autosomal dominant connective tissue disorder resulting from genetic mutations in the transforming growth factor beta receptors 1 and 2 (TGFBR1 and TGFBR2). The syndrome is characterized phenotypically by hypertelorism, bifid uvula, and/or cleft palate, and arterial tortuosity with aneurysms and dissections. LDS has a much more rapid clinical course than Marfan syndrome (MFS) and thus those diagnosed with LDS are currently being recommended for prophylactic aortic root replacement at younger ages and with smaller aortic dimensions. Aortic root tissue obtained at surgery was compared between 15 patients carrying a diagnosis of LDS, 11 patients with MFS and 11 control aortas to evaluate the range of histopathologic changes in LDS. Standard hematoxylin and eosin and Movat pentachrome stains were performed. LDS samples had increased medial collagen and a subtle but diffuse form of elastic fiber fragmentation and extracellular matrix deposition, referred to as diffuse medial degeneration. LDS samples had significantly more diffuse medial degeneration compared with MFS and control samples (P<0.05), significantly less medial degeneration of the "cystic" variety compared with MFS (P<0.01) and significantly more collagen deposition than control samples (P<0.01). Additionally, an immunohistochemical stain for pSmad2, a marker of TGFbeta activity, was significantly increased in LDS patients compared with controls (P<0.001). Overall, the histologic findings of LDS are best appreciated with special stains to evaluate fibrosis and elastic fiber fragmentation. The changes described, although not entirely specific for LDS, help differentiate this entity from other vascular diseases in the appropriate clinicopathologic setting.

Histopathology and fibrillin-1 distribution in severe early onset Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant condition which may involve the cardiovascular, ocular, skeletal, and other systems. Mutations causing MFS are found in the FBN1 gene, encoding fibrillin-1, an extracellular matrix protein involved in microfibril formation. In the most severe cases, mutations are generally found in exons 24-32, and children with these mutations usually die in the first years of life, of cardiopulmonary failure. We present clinical, molecular and histopathological studies on a patient with severe early onset MFS. He has a mutation in exon 25 of FBN1, a G>A transition at nucleotide position 3131 that converts the codon TGC, coding for cysteine at position 1044, to TAC, coding for tyrosine (C1044Y). This has resulted in abnormalities of the extracellular matrix and a severe clinical phenotype, although he has survived to the age of 14 years.

Ultrasonic delineation of aortic microstructure: the relative contribution of elastin and collagen to aortic elasticity

Aortic elasticity is an important factor in hemodynamic health, and compromised aortic compliance affects not only arterial dynamics but also myocardial function. A variety of pathologic processes (e.g., diabetes, Marfan's syndrome, hypertension) can affect aortic elasticity by altering the microstructure and composition of the elastin and collagen fiber networks within the tunica media. Ultrasound tissue characterization techniques can be used to obtain direct measurements of the stiffness coefficients of aorta by measurement of the speed of sound in specific directions. In this study we sought to define the contributions of elastin and collagen to the mechanical properties of aortic media by measuring the magnitude and directional dependence of the speed of sound before and after selective isolation of either the collagen or elastin fiber matrix. Formalin-fixed porcine aortas were sectioned for insonification in the circumferential, longitudinal, or radial direction and examined using high-frequency (50 MHz) ultrasound microscopy. Isolation of the collagen or elastin fiber matrices was accomplished through treatment with NaOH or formic acid, respectively. The results suggest that elastin is the primary contributor to aortic medial stiffness in the unloaded state, and that there is relatively little anisotropy in the speed of sound or stiffness in the aortic wall.

On the ultrasonic properties of tendon

The strong dependence of tendon echogenicity on insonation angle is explored by analyzing echo spectra. Combining echo spectra with high-resolution images from several modalities reveals that fluid spaces surrounding fascicles and bundles are likely sources of ultrasonic scatter. Mathematical models of tendon structure are proposed to explain how the anisotropic microstructure of tendon gives rise to angle-dependent echogenicity. Echo spectra from spontaneously damaged equine tendon samples were compared with normal equine tendon and found to exhibit a dramatic decrease in anisotropic properties that appears to be related to the spatial organization and type of collagen generated during repair. Variation in echo spectra with insonation angle is a robust indicator of mechanical damage.

Characterization of aortic microstructure with ultrasound: implications for mechanisms of aortic function and dissection

Specific ultrasonic tissue characterization parameters were correlated with the three-dimensional architecture and material properties (density, compressibility, size, and orientation) of aortic elastic elements at the microscopic level. The medial layer of 10 samples of normal canine aorta were insonified in vitro utilizing acoustic microscopy from 30 to 44 MHz. The following quantitative indexes exhibited substantial anisotropic elastic behavior in radial (R), circumferential (C), and longitudinal (L) directions: backscatter coefficient (R:0.9 +/- 0.2; C:0.008 +/- 0.0008; LL:0.0077 +/- 0.0008 sr(-1) cm(-1)); frequency dependence of backscatter (R:3.3; C:1.4; L:1.5); attenuation coefficients 1(R:105 +/- 22; L:135 +/- 13; C:131 +/- 14 dB/cm). Thus, the ultrasonic indexes are anisotropic: equivalent in the C and L directions, but markedly different in the R direction. These data are indicative of an aortic microstructure that interacts with ultrasonic waves as thin sheet-like elastic layers instead of independent elastin fibers. This specific sheet-like organization of elastin microfibers may function to limit shear injury to concentric aortic lamellae and prevent aortic dissection. The marked anisotropic behavior of normal aortas suggests that ultrasound may be useful for nondestructive characterization of vascular integrity.

Different roles of arteriosclerosis in the rupture of intracranial dissecting aneurysms

AIMS

Although intracranial dissecting aneurysm (IDA) is a newly described variant of the brain aneurysms that affects mainly the vertebrobasilar arterial system, its pathogenesis remains obscure. We aimed to clarify the role of arteriosclerosis in the pathogenesis of IDA based on histopathological findings in seven autopsy cases of IDA.

METHODS AND RESULTS

All cases exhibited systemic hypertension or left ventricular hypertrophy. Macroscopically, all cases exhibited subarachnoid haemorrhage. Two types of dissection were recognized in the vertebral artery. Six of seven IDA cases showed a widespread disruption of the entire thickness of the arterial wall with the formation of a dilated pseudoaneurysm, which consisted of thin adventitia (arterial wall disruption type). Medial disruption of the arterial wall and subadventitial dissecting haemorrhage were also found, resulting in the formation of a false lumen and stenosis of the 'true' lumen of the artery. However, these lesions were connected to the site of rupture of the entire arterial wall. Within 1 day after onset of IDA, the autopsy cases showed formation of fibrin thrombus, marked leucocyte infiltration and necrosis of the arterial wall at the site of the lesion. Cases that survived more than 1 week showed smooth muscle cell proliferation, macrophage accumulation and lymphocytic infiltration in the lesions. These cases showed no atherosclerotic plaque, but non-atherosclerotic fibrocellular intima. The thickness of intima and media was significantly less in the vertebral artery of IDA patients than that of non-IDA patients with systemic hypertension. On the other hand, the remaining case showed severe atherosclerosis with haemorrhage into the lipid core without connection to the arterial lumen (intra-atheromatous plaque haemorrhage type). However, unusual arterioles and neovascularization of the intra-and peri-arterial walls were observed.

CONCLUSIONS

Our results suggest that disruption of the entire arterial wall may be a critical event in the development of IDA and result in the medial disruption and subadventitial haemorrhage. Non-atheromatous intima might function as a protective factor in arterial wall disruption. On the other hand, atherosclerosis may predispose to intra-atheromatous plaque haemorrhage type of IDA through intramural haemorrhage originating from the newly formed vessels.

Delineation of the extracellular determinants of ultrasonic scattering from elastic arteries

Elastic arteries consist of three primary components: elastin fibers, extracellular collagen matrix and smooth muscle cells. However, the relative contribution of elastin and collagen fibers to overall ultrasonic scattering from an intact arterial wall is poorly understood. To define the principal source of extracellular scattering from the medial layer of elastic arteries, canine ascending aortas (n = 10) were excised, fixed and sectioned for insonification. Subsequently, aortic specimens were restudied after treatment to dissolve all tissue components except extracellular collagen matrix (n = 5) and elastin fibers (n = 5). Histological staining revealed very few elastin fibers and sparse intact collagen in collagen-isolated and elastin-isolated tissues, respectively. Integrated backscatter, attenuation and backscatter coefficients differentiated these two treated tissues. The backscatter coefficient for elastin-isolated tissue demonstrated a fivefold increase over collagen-isolated tissue, suggesting that elastin fibers represent a primary scattering component within elastic arteries, and the collagen fibers may provide a secondary component of scattering.

In vitro characterization of a novel, tissue-targeted ultrasonic contrast system with acoustic microscopy

Targeted ultrasonic contrast systems are designed to enhance the reflectivity of selected tissues in vivo [Lanza et al., Circulation 94, 3334 (1996)]. In particular, these agents hold promise for the minimally invasive diagnosis and treatment of a wide array of pathologies, most notably tumors, thromboses, and inflamed tissues. In the present study, acoustic microscopy was used to assess the efficacy of a novel, perfluorocarbon based contrast agent to enhance the inherent acoustic reflectivity of biological and synthetic substrates. Data from these experiments were used to postulate a simple model describing the observed enhancements. Frequency averaged reflectivity (30-55 MHz) was shown to increase 7.0 +/- 1.1 dB for nitrocellulose membranes with targeted contrast. Enhancements of 36.0 +/- 2.3 dB and 8.5 +/- 0.9 dB for plasma and whole blood clots, respectively, were measured between 20 and 35 MHz. A proposed acoustic transmission line model predicted the targeted contrast system would increase the acoustic reflectivity of the nitrocellulose membrane, whole blood clot, and fibrin plasma clot by 2.6, 8.0, and 31.8 dB, respectively. These predictions were in reasonable agreement with the experimental results of this paper. In conclusion, acoustic microscopy provides a rapid and sensitive approach for in vitro chracterization, development, and testing of mathematical models of targeted contrast systems. Given the current demand for targeted contrast systems for medical diagnostic and therapeutic use, the use of acoustic microscopy may provide a useful tool in the development of these agents.