Biomechanical Imaging Research Articles

MRI-based biomechanical imaging: initial study on early plaque progression and vessel remodeling

The goal of the study is to develop a noninvasive magnetic resonance imaging (MRI)-based biomechanical imaging technique to address biomechanical pathways of atherosclerotic progression and regression in vivo using a 3D fluid-structure interaction (FSI) model. Initial in vivo study was carried out in an early plaque model in pigs that underwent balloon-overstretch injury to the left carotid arteries. Consecutive MRI scans were performed while the pigs were maintained on high cholesterol (progression) or normal chow (regression), with an injection of a plaque-targeted contrast agent, Gadofluorine M. At the end of study, the specimens of carotid arterial segments were dissected and underwent dedicated mechanical testing to determine their material properties. 3D FSI computational model was applied to calculate structure stress and strain distribution. The plaque structure resembles early plaque with thickened intima. Lower maximal flow shear stress correlates with the growth of plaque volume during progression, but not during regression. In contrast, maximal principle structure stress/stain (stress-P1 and strain-P1) were shown to correlate strongly with the change in the plaque dimension during regression, but moderately during progression. This MRI-based biomechanical imaging method may allow for noninvasive dynamic assessment of local hemodynamic forces on the development of atherosclerotic plaques in vivo.

Neurophysiology of swallowing: Effects of age and bolus type

This study examined age-related changes in swallowing from an integrated biomechanical and functional imaging perspective in order to more comprehensively characterize changes in swallowing associated with age. We examined swallowing-related fMRI brain activity and videoflouroscopic biomechanics of three bolus types (saliva, water and barium) in 12 young and 11 older adults. We found that age-related neurophysiological changes in swallowing are evident. The group of older adults recruited more cortical regions than young adults, including the pericentral gyri and inferior frontal gyrus pars opercularis and pars triangularis (primarily right-sided). Saliva swallows elicited significantly higher BOLD responses in regions important for swallowing compared to water and barium. In separate videofluoroscopy sessions, we obtained durational measures of supine swallowing. The older cohort had significantly longer delays before the onset of the pharyngeal swallow response and increased residue of ingested material in the pharynx. These findings suggest that older adults without neurological insult elicit more cortical involvement to complete the same swallowing tasks as younger adults.

Effect of specimen conditioning on the microarchitectural parameters of trabecular bone assessed by micro-computed tomography

The best way to preserve the mechanical properties of bone specimens is hydration in NaCl, whereas the reference process in µCT analysis is defatting. However, for finite element modelling (FEM) it is necessary to use the same bone specimens for biomechanical testing and 3D imaging. This study aimed to evaluate the effect of sample conditioning on trabecular bone microarchitectural parameters. Trabecular bones were analysed by µCT under three successive conditions: first, the fatted samples were analysed immersed in NaCl (process N); second, they were hydrated for 24 h then imaged without immersion (process H); third, the samples were defatted before analysis (process D). The microarchitectural parameters bone volume/tissue volume (BV/TV), trabecular spacing (Tb.Sp), number (Tb.N) and thickness (Tb.Th) were calculated. Except for BV/TV, there was no significant difference between the processes N and D. In process H, BV/TV, Tb.Th and Tb.N were higher and BS/BV and Tb.Sp were lower than in process D. Results showed that the process D may be replaced by the process N. The process H induced significant differences in microarchitectural parameters when compared to process D. Nevertheless, this sample conditioning should be used to develop FEM when µCT images are to be acquired during compressive testing.

Application of Histomorphometric Methods to the Study of Bone Repair

Standardized methods for the histomorphometric assessment of bone are essential features of most studies of metabolic bone diseases and their treatments. These methodologies were developed to assess coupled remodeling, focusing primarily on osteoblasts and osteoclasts, the anabolic and catabolic rates of these cells, and structural features of mature bone. Research studies on bone healing and the development of new therapeutic approaches for the enhancement of bone repair also require a comprehensive understanding of the basic cellular and tissue level mechanisms that underlie these processes. However, the histological methods developed for metabolic bone disease studies are not completely suitable for studies of bone repair because they are based on assumptions that there is little variation in tissue composition within a sample of bone and not generally designed to quantify other types of tissues, such as cartilage, that contribute to bone healing. These techniques also do not provide tissue-based structural measurements that are relatable to the specific types of biomechanical and radiographic structural assessments that are used to determine rates of bone healing. These deficiencies in current histological approaches therefore point to the need to establish standardized criteria for the histomorphometric assessments that are specifically adapted for the study of bone repair in models of fracture healing and bone regeneration. In this Perspective, we outline what we believe to be the specific structural, tissue. and cellular aspects that need to be addressed to establish these standardized criteria for the histomorphometric assessment of bone repair. We present the specific technical considerations that need to be addressed to appropriately sample repair tissues to obtain statistically meaningful results and suggest specific procedures and definitions of nomenclatures for the application of this technology to bone repair. Finally, we present how aspects of histomorphometric measurements of bone repair can be related to biomechanical and radiographic imaging properties that functionally define rates of bone healing, and thus, how these tools can be used to provide corroborating data.

Basic science and translational research in female pelvic floor disorders: proceedings of an NIH-sponsored meeting.

To report the findings of a multidisciplinary group of scientists focusing on issues in basic science and translational research related to female pelvic floor disorders, and to produce recommendations for a research agenda for investigators studying female pelvic floor disorders. A National Institutes of Health (NIH)-sponsored meeting was held on November 14-15, 2002, bringing together scientists in diverse fields including obstetrics, gynecology, urogynecology, urology, gastroenterology, biomechanical engineering, neuroscience, endocrinology, and molecular biology. Recent and ongoing studies were presented and discussed, key gaps in knowledge were identified, and recommendations were made for research that would have the highest impact in making advances in the field of female pelvic floor disorders. The meeting included presentations and discussion on the use of animal models to better understand physiology and pathophysiology; neuromuscular injury (such as at childbirth) as a possible pathogenetic factor and mechanisms for recovery of function after injury; the use of biomechanical concepts and imaging to better understand the relationship between structure and function; and molecular and biochemical mechanisms that may underlie the development of female pelvic floor disorders. While the findings of current research will help elucidate the pathophysiologic pathways leading to the development of female pelvic floor disorders, much more research is needed for full understanding that will result in better care for patients through specific rather than empiric therapy, and lead to the potential for prevention on primary and secondary levels.

MR imaging of normal and matrix-depleted cartilage: correlation with biomechanical function and biochemical composition.

To correlate articular cartilage function, as reflected in biomechanical properties and biochemical composition, with magnetic resonance (MR) imaging parameters of normal articular cartilage and cartilage partially depleted of matrix components. Normal articular cartilage from 12 porcine patellae was evaluated biomechanically, biochemically, and with MR imaging (with and without gadolinium enhancement). The patellae were then enzymatically treated to deplete the matrix of either collagen or proteoglycan and then reevaluated biomechanically, biochemically, and with MR imaging. Correlations between cartilaginous tissue function and MR imaging parameters were made. Analysis of variance was performed to assess the effect of enzymatic treatment on measured parameters. Linear correlations among the MR imaging, biochemical, and biomechanical parameters were performed to determine the strengths of the relationships. P <.05 indicated statistically significant differences. Biochemical, biomechanical, and MR analyses enabled detection of changes caused by matrix depletion (P <.05). T2 was the most useful MR imaging parameter for distinguishing proteoglycan loss from collagen loss. T2 correlated significantly with both biomechanical modulus (indicative of cartilage stiffness; P <.001, R2 = 0.51) and biochemical proteoglycan content (P <.001, R2 = 0.44). Differentiation between proteoglycan loss and collagen loss in terms of T1 improved with gadolinium enhancement. With gadolinium enhancement, proteoglycan depletion was associated with a greater decrease in T1 than collagen depletion (P <.05). An association between biochemical and biomechanical functional status and MR imaging parameters of articular cartilage was demonstrated. Linear correlations existed between modulus and proteoglycan content in terms of T2. Additionally, proteoglycan loss and collagen loss had differing effects on gadolinium-enhanced T1 when it was expressed as the ratio of T1 after gadolinium enhancement/T1 before gadolinium enhancement.

Augmentation of Tendon Healing in an Intraarticular Bone Tunnel with Use of a Bone Growth Factor

We hypothesized that an exogenous bone growth factor could augment healing of a tendon graft in a bone tunnel in a rabbit anterior cruciate ligament-reconstruction model. Seventy rabbits underwent bilateral anterior cruciate ligament reconstructions with a semitendinosus tendon graft. One limb received a collagen sponge carrier vehicle containing a mixture of bone-derived proteins while the contralateral limb was treated with either no sponge or a sponge without bone-derived proteins. The reconstruction was evaluated at 2, 4, or 8 weeks with histologic, biomechanical, and magnetic resonance imaging analysis. Histologic analysis demonstrated that specimens treated with bone-derived proteins had a more consistent, dense interface tissue and closer apposition of new bone to the graft, with occasional formation of a fibrocartilaginous interface, when compared with control specimens. The treated specimens had significantly higher load-to-failure rates than did control specimens. Treatment with bone-derived proteins resulted in an average increase in tensile strength of 65%. The treated specimens were stronger than control specimens at each time point, but the difference was greatest at 8 weeks. On the basis of signal characteristics and new bone formation, magnetic resonance imaging was useful for predicting which limb was treated, the site of failure, and the limbs with higher load-to-failure values. This study demonstrates the potential for augmenting tendon healing in an intraarticular bone tunnel using an osteoinductive growth factor.

Evaluation and management of the posterior cruciate injured knee

Diagnosis and treatment of the posterior cruciate ligament (PCL) injured knee have evolved significantly over the past 15 years. With improved understanding from biomechanical data, anatomy, and imaging, diagnosis of PCL injuries is easier. Diagnoses can often be made on physical examination alone. However, imaging studies, such as radiographs and magnetic resonance imaging, are invaluable in assessing the PCL and associated injuries that may affect treatment and outcome. In general, grade I and II and some grade III isolated PCL injuries do well with nonsurgical treatment. Indications for surgery are becoming standard, although some controversy still exists. Experts still do not agree on the preferred method of reconstruction. Debate continues regarding single- and double-bundle techniques, as well as “inlay” techniques. No publication to date has definitively determined which reconstructive technique provides the best results. This article reviews the current diagnostic techniques, discusses nonsurgical and surgical indications, and presents current available techniques for treating the PCL injured knee.