Mathematical model for isometric and isotonic muscle contractions.

A new mathematical model is presented to describe both the active and passive mechanics of muscles. In order to account for the active response, a two-layer kinematics that introduces both the visible and rest lengths of the muscle is presented within a rational mechanics framework. The formulation is based on an extended version of the principle of virtual power and the dissipation principle. By using an accurate constitutive description of muscle mobility under activation, details of microscopic processes that lead to muscle contraction are glossed over while macroscopic effects of chemical/electrical stimuli on muscle mechanics are retained. The model predictions are tested with isometric and isotonic experimental data collected from murine extensor digitorum muscle. It is shown that the proposed model captures experimental observations with only three scalar parameters.

Do the ornamented osteoderms influence the heat conduction through the skin? A finite element analysis in Crocodylomorpha.

In order to assess the implication of the crocodylomorph ornamented osteoderms on the skin conduction during basking, we have performed three dimensional modeling and finite element analyses on a sample which includes both extant dry bones and well-preserved fossils tracing back to the Early Jurassic. In purpose to reveal the possible implication of the superficial ornamentation on the osteoderm heat conduction, we repeated the simulation on an equivalent set of smoothed 3D-modeled osteoderms. The comparison of the results evidenced that the presence of the apical sculpture has no significant impact on the osteoderm global conduction. Furthermore, as we also aimed to assess the influence of the inner bone porosity on the osteoderm conduction, we modified the heat equation parameters so that the 3D-modeled osteoderms successively score the compact and the cancellous bone properties (i.e. mass density, heat capacity, thermal conductivity and thermal diffusivity). Finally, we repeated the analyses using the soft-dermis properties which lead to outline that neither the degree of porosity nor the presence of the osteoderms (in itself) significantly modifies the heat conduction through the crocodylomorph skin. Consequently, as hypothesized by previous authors, if the dermal shield happens to be involved into heat capture during basking for crocodylians, this process must mainly rely on a convective effect based on the osteoderm relative degree of vascularization. This last assumption could thus explain why the crocodylians which produce little metabolic heat would carry an entire vascularized osteoderm shield.

The conceptual framework of ontogenetic trajectories: parallel transport allows the recognition and visualization of pure deformation patterns.

Ontogeny is usually studied by analyzing a deformation series spanning over juvenile to adult shapes. In geometric morphometrics, this approach implies applying generalized Procrustes analysis coupled with principal component analysis on multiple individuals or multiple species datasets. The trouble with such a procedure is that it mixes intra- and inter-group variation. While MANCOVA models are relevant statistical/mathematical tools to draw inferences about the similarities of trajectories, if one wants to observe and interpret the morphological deformation alone by filtering inter-group variability, a particular tool, namely parallel transport, is necessary. In the context of ontogenetic trajectories, one should firstly perform separate multivariate regressions between shape and size, using regression predictions to estimate within-group deformations relative to the smallest individuals. These deformations are then applied to a common reference (the mean of per-group smallest individuals). The estimation of deformations can be performed on the Riemannian manifold by using sophisticated connection metrics. Nevertheless, parallel transport can be effectively achieved by estimating deformations in the Euclidean space via ordinary Procrustes analysis. This approach proved very useful in comparing ontogenetic trajectories of species presenting large morphological differences at early developmental stages.

An electromechanical model of cardiac tissue: constitutive issues and electrophysiological effects.

We present an electromechanical model of myocardium tissue coupling a modified FitzHugh-Nagumo type system, describing the electrical activity of the excitable media, with finite elasticity, endowed with the capability of describing muscle contractions. The high degree of deformability of the medium makes it mandatory to set the diffusion process in a moving domain, thereby producing a direct influence of the deformation on the electrical activity. Various mechano-electric effects concerning the propagation of cylindrical waves, the rotating spiral waves, and the spiral breakups are discussed.

Elastic energies for nematic elastomers.

We discuss several elastic energies for nematic elastomers and their small strain expansions both in the regime of large director rotations, and in the case that director changes are small. We propose two fully non-linear model anisotropic energies and compare the behavior they predict with the currently available experimental evidence.

Swelling effects on localized adhesion of an elastic ribbon.

We investigate the adhesion mechanism between an elastic strip of vinylpolysiloxane bent in a racquet-like shape, and a thick elastomeric substrate with the aim to understand how local swelling modifies adhesion. Using a modified loop-tack adhesion test, we place a droplet of silicone oil in between the two materials, vary the dwell time and measure the force required to separate the two interfaces. The experiments are then compared with an analytical model that describes how the critical peel force is modified as the interfacial surface energy changes over time. Our study reveals that in certain circumstances swelling can enhance adhesion. More specifically, strong adhesion is obtained when most of the droplet is absorbed by the solid. By contrast, when the droplet remains at the interface a small adhesive force is measured.

Non-invasive assessment of functional strain lines in the real human left ventricle via speckle tracking echocardiography.

A mechanics-based analysis of data from three-dimensional speckle tracking echocardiography is proposed, aimed at investigating deformations in myocardium and at assessing shape and function of distinct strain lines corresponding to the principal strain lines of the cardiac tissue. The analysis is based on the application of a protocol of measurement of the endocardial and epicardial principal strain lines, which was already tested on simulated left ventricles. In contrast with similar studies, it is established that endocardial principal strain lines cannot be identified with any structural fibers, not even along the systolic phase and is suggested that it is due to the capacity of the endocardial surface to contrast the dilation of the left ventricle.

Association of deep white matter infarction with chronic communicating hydrocephalus: implications regarding the possible origin of normal-pressure hydrocephalus.

The coexistence of cerebrovascular disease leading to deep white matter infarction and normal-pressure hydrocephalus has been noted previously in clinical studies, as both diseases can present with the triad of gait disturbance, dementia, and incontinence. The purpose of this MR study was to determine if the two diseases demonstrated a statistical association. Evidence of patchy periventricular hyperintensity representing presumed deep white matter infarction was sought in 20 patients shunted for normal-pressure hydrocephalus and in 35 additional consecutive patients with clinical symptoms and MR findings consistent with normal-pressure hydrocephalus. Deep white matter infarction was also sought in 62 consecutive age-matched control subjects. There was a statistically significant (p less than .001) higher association (58%) of marked infarction in the 55 patients with normal-pressure hydrocephalus than in the age-matched controls (24%). MR findings of communicating hydrocephalus (ventriculomegaly and increased aqueductal CSF flow void) were sought in 78 consecutive patients with presumed deep white matter infarction, and the degree of severity of the two diseases was also found to be statistically significant (p less than .05). In view of this association, the possibility that the two diseases are related was considered. A potential mechanism is discussed whereby deep white matter infarction leading to decreased periventricular tensile strength could result in communicating hydrocephalus. It is plausible that normal-pressure hydrocephalus may result from a number of different insults to the brain.

Magnetic resonance imaging of the larynx.

Rarely does any radiologic imaging modality play a significant role in reaching a diagnosis of malignancy in the larynx and hypopharynx. These regions are so readily accessible to clinical examination that the combination of cytology and visual inspection usually strongly indicates the diagnosis of cancer. Therefore, the primary role of MR is the same as that of CT in imaging the larynx and hypopharynx: to define the extent of the disease. Compared to CT, MR consistently shows superior soft tissue definition. The use of direct coronal and sagittal scan planes allows the visualization of intrinsic laryngeal musculature in addition to better defining cranial caudal tumor extension. Thus, MR is now the imaging study of choice for the evaluation of cancer of the larynx. The role of MR can also be extended to replace CT scanning in the evaluation of the laryngeal airway and benign lesions of the larynx.

Magnetic resonance imaging of the facial nerve.

Magnetic resonance imaging (MRI) of the facial nerve was evaluated by studying normal volunteers and patients with diseases of the facial nerve with a 0.3 Tesla permanent-magnet MRI system with special surface coils. The normal MR images were correlated with the anatomy of thin cryosection specimens of fresh cadavers. The seventh nerve was followed from its nucleus in the brainstem through the temporal bone to the parotid gland bed. The entire labyrinth and tympanic portions, as well as the geniculate ganglion, could be shown with appropriate scan planes. Examples of brainstem diseases affecting the facial nerve and nucleus, facial neuromas, parotid tumors involving the facial nerve, and other diseases were studied. MRI is a technique that allows unique evaluation of the entire course of the facial nerve. It produces superior images of the facial nerve with high-contrast resolution. Unlike computed tomography, there is no beam-hardening artifact from the temporal bone or exposure to ionizing radiation and contrast agents. MRI also allows visualization of the main trunks of the facial nerve in the parotid bed not possible with any other imaging technique.

Low cost MRI of the paranasal sinuses.

The use of automated tuning, automated MR protocol sequences, surface coils, and grouping of patients with similar examinations, patient throughput can be greatly accelerated for MRI scanning. These measures coupled with high volume scanning permit significant reductions in cost so that MRI can be performed at a price range equal to or less than plain films for some examinations. The ability of MRI to perform axial, sagittal and coronal scans with excellent tissue contrast raises the possibility of substituting MRI for sinus x-rays. Based on our preliminary experience we have the opinion that MR is equal to/or better than CT in this region. This preference is currently being investigated. Only the cost issue will be addressed in this report.

MR imaging of acute subarachnoid hemorrhage.

The MR appearance of acute subarachnoid hemorrhage experimentally produced in Macaca monkeys and observed in patients with clinically documented acute subarachnoid hemorrhage is presented. Subarachnoid hemorrhages were produced in two Macaca Nemestrema monkeys using the technique of Frazee. CT and MR imaging were performed immediately after the procedure and at frequent intervals up to two week post hemorrhage. MR including T1 and T2 weighted multiplanar spin echo images were obtained. The imaging studies were compared with clinical evaluations and pathological specimens of all animals. Findings in the experimental animals are correlated with those observed in patients with clinically documented subarachnoid hemorrhage. The results show that acute subarachnoid hemorrhage (SAH) can be detected with MRI as isointense signal replacing normally black CSF spaces on T1 weighted images. Signal changes most likely relate to protein water binding associated with the clotting mechanism rather than oxidative denaturation of hemoglobin. Imaging performed experimentally and clinically beyond four days, however, showed a marked increase in signal intensity on T1 weighted images which probably does result from methemoglobin formation within the clot matrix. Although CT remains the gold standard in detecting acute SAH, MR does provide some sensitivity to its presence.

MR of hemorrhagic acoustic neuromas.

The magnetic resonance (MR) appearance of hemorrhagic acoustic neuromas has not previously been reported. Four patients with surgically proven acoustic neuromas containing spontaneous hemorrhage were preoperatively evaluated with MR imaging. All patients presented with new onset of symptoms and three had subarachnoid hemorrhage. Signal changes on T1- and T2-weighted spin echo sequences were characteristic for acute, subacute and chronic intraparenchymal hemorrhage. Recognition of hemorrhagic changes on MR is important since more rapid surgical intervention is required in this patient subgroup.

A comparison of MRI and clinical examination of acute lateral ankle sprains.

Because of its excellent soft tissue contrast and ability to demonstrate soft tissue structures, magnetic resonance imaging is ideally suited to the evaluation of the soft tissues surrounding the ankle, including the lateral collateral ligaments. This study was undertaken to compare the clinical evaluation of 15 patients who suffered inversion injuries of the ankle with the results found on magnetic resonance imaging within 48 hours of the injury. Physical examination was found to be 100% accurate in the diagnosis of grade III ligament injuries but only 25% accurate in the diagnosis of grade II injuries. Clinicians most often underestimate the damage with a grade II ligament tear. Furthermore, other associated injuries, such as significant capsule ruptures and tendon damage, were often overlooked at physical examination.

Testing convergent and parallel adaptations in talpids humeral mechanical performance by means of geometric morphometrics and finite element analysis.

The shape and mechanical performance in Talpidae humeri were studied by means of Geometric Morphometrics and Finite Element Analysis, including both extinct and extant taxa. The aim of this study was to test whether the ability to dig, quantified by humerus mechanical performance, was characterized by convergent or parallel adaptations in different clades of complex tunnel digger within Talpidae, that is, Talpinae+Condylura (monophyletic) and some complex tunnel diggers not belonging to this clade. Our results suggest that the pattern underlying Talpidae humerus evolution is evolutionary parallelism. However, this insight changed to true convergence when we tested an alternative phylogeny based on molecular data, with Condylura moved to a more basal phylogenetic position. Shape and performance analyses, as well as specific comparative methods, provided strong evidence that the ability to dig complex tunnels reached a functional optimum in distantly related taxa. This was also confirmed by the lower phenotypic variance in complex tunnel digger taxa, compared to non-complex tunnel diggers. Evolutionary rates of phenotypic change showed a smooth deceleration in correspondence with the most recent common ancestor of the Talpinae+Condylura clade.

Torsion of the human left ventricle: experimental analysis and computational modeling.

We set a twofold investigation: we assess left ventricular (LV) rotation and twist in the human heart through 3D-echocardiographic speckle tracking, and use representative experimental data as benchmark with respect to numerical results obtained by solving our mechanical model of the LV. We aim at new insight into the relationships between myocardial contraction patterns and the overall behavior at the scale of the whole organ. It is concluded that torsional rotation is sensitive to transmural gradients of contractility which is assumed linearly related to action potential duration (APD). Pressure-volume loops and other basic strain measures are not affected by these gradients. Therefore, realistic torsional behavior of human LV may indeed correspond to the electrophysiological and functional differences between endocardial and epicardial cells recently observed in non-failing hearts. Future investigations need now to integrate the mechanical model proposed here with minimal models of human ventricular APD to drive excitation-contraction coupling transmurally.

Critical voltages and blocking stresses in nematic gels : dynamics of director rotation for nematic elastomers under electro-mechanical loads.

We present a model of the dynamics of director rotation in nematic gels under combined electro-mechanical loading. Focusing on a model specimen, we describe the critical voltages that must be exceeded to achieve director reorientation, and the blocking stresses that prevent alignment of the nematic director with the applied electric field. The corresponding phase diagram shows that the dynamic thresholds defined above are different from those predicted on the sole basis of energetics. Multistep loading programs are used to explore the energy landscape of our model specimen, showing the existence of multiple local minima under the same voltage and applied stress. This leads us to conclude that hysteresis should be expected in the electro-mechanical response of nematic gels.

Magnetic resonance angiography.

Magnetic resonance (MR) angiography is a broad and expanding field. The technology of MR angiograms is evolving to produce higher spatial resolution, faster acquisition times, and reduced artifact. Rather than a straight, linear evolution, this progress is going forward in a number of areas inherent to the MR imaging process. Considerable progress has been demonstrated in such diverse areas as flow-sensitized radiofrequency pulses, reduced background signal with off-resonance pulses, improved vessel depiction with reduced echo times via improved hardware and reconstruction techniques, and improved display with more powerful computer algorithms. This review is a brief survey and comparison of available techniques for the visualization of blood vessels within the human body.

MR of the paranasal sinuses.

The purpose of this project was to examine the anatomy and pathology of the paranasal sinuses as seen by MR imaging. This was accomplished through correlations of MR images of normal volunteers with matched cadaver whole organ cryosections. The information obtained by MRI was compared to that of plain films and CT in the detection of a variety of conditions affecting the paranasal sinuses. The majority of the pathological processes could be quite adequately imaged by T1 weighted pulsing sequences. When more tissue specific information was required in some infiltrating malignant lesions, T2 weighting pulsing sequences are quite helpful for tumors that crossed the subarachnoid space into the central nervous system or in characterizing tissues in airless sinuses. Other than the single case of osteoid osteoma where X-ray studies were superior, magnetic resonance provided equal or superior information to the X-ray examinations.