10,625 research outputs found
In vivo measurement of human brain elasticity using a light aspiration device
The brain deformation that occurs during neurosurgery is a serious issue
impacting the patient "safety" as well as the invasiveness of the brain
surgery. Model-driven compensation is a realistic and efficient solution to
solve this problem. However, a vital issue is the lack of reliable and easily
obtainable patient-specific mechanical characteristics of the brain which,
according to clinicians' experience, can vary considerably. We designed an
aspiration device that is able to meet the very rigorous sterilization and
handling process imposed during surgery, and especially neurosurgery. The
device, which has no electronic component, is simple, light and can be
considered as an ancillary instrument. The deformation of the aspirated tissue
is imaged via a mirror using an external camera. This paper describes the
experimental setup as well as its use during a specific neurosurgery. The
experimental data was used to calibrate a continuous model. We show that we
were able to extract an in vivo constitutive law of the brain elasticity: thus
for the first time, measurements are carried out per-operatively on the
patient, just before the resection of the brain parenchyma. This paper
discloses the results of a difficult experiment and provide for the first time
in-vivo data on human brain elasticity. The results point out the softness as
well as the highly non-linear behavior of the brain tissue.Comment: Medical Image Analysis (2009) accept\'
The Invalidity of the Laplace Law for Biological Vessels and of Estimating Elastic Modulus from Total Stress vs. Strain: a New Practical Method
The quantification of the stiffness of tubular biological structures is often
obtained, both in vivo and in vitro, as the slope of total transmural hoop
stress plotted against hoop strain. Total hoop stress is typically estimated
using the "Laplace law." We show that this procedure is fundamentally flawed
for two reasons: Firstly, the Laplace law predicts total stress incorrectly for
biological vessels. Furthermore, because muscle and other biological tissue are
closely volume-preserving, quantifications of elastic modulus require the
removal of the contribution to total stress from incompressibility. We show
that this hydrostatic contribution to total stress has a strong
material-dependent nonlinear response to deformation that is difficult to
predict or measure. To address this difficulty, we propose a new practical
method to estimate a mechanically viable modulus of elasticity that can be
applied both in vivo and in vitro using the same measurements as current
methods, with care taken to record the reference state. To be insensitive to
incompressibility, our method is based on shear stress rather than hoop stress,
and provides a true measure of the elastic response without application of the
Laplace law. We demonstrate the accuracy of our method using a mathematical
model of tube inflation with multiple constitutive models. We also re-analyze
an in vivo study from the gastro-intestinal literature that applied the
standard approach and concluded that a drug-induced change in elastic modulus
depended on the protocol used to distend the esophageal lumen. Our new method
removes this protocol-dependent inconsistency in the previous result.Comment: 34 pages, 13 figure
Anisotropic behaviour of human gallbladder walls
Inverse estimation of biomechanical parameters of soft tissues from non-invasive measurements has clinical significance in patient-specific modelling and disease diagnosis. In this paper, we propose a fully nonlinear approach to estimate the mechanical properties of the human gallbladder wall muscles from in vivo ultrasound images. The iteration method consists of a forward approach, in which the constitutive equation is based on a modified Hozapfel–Gasser–Ogden law initially developed for arteries. Five constitutive parameters describing the two orthogonal families of fibres and the matrix material are determined by comparing the computed displacements with medical images. The optimisation process is carried out using the MATLAB toolbox, a Python code, and the ABAQUS solver. The proposed method is validated with published artery data and subsequently applied to ten human gallbladder samples. Results show that the human gallbladder wall is anisotropic during the passive refilling phase, and that the peak stress is 1.6 times greater than that calculated using linear mechanics. This discrepancy arises because the wall thickness reduces by 1.6 times during the deformation, which is not predicted by conventional linear elasticity. If the change of wall thickness is accounted for, then the linear model can used to predict the gallbladder stress and its correlation with pain. This work provides further understanding of the nonlinear characteristics of human gallbladder
A coupled mitral valve -- left ventricle model with fluid-structure interaction
Understanding the interaction between the valves and walls of the heart is
important in assessing and subsequently treating heart dysfunction. With
advancements in cardiac imaging, nonlinear mechanics and computational
techniques, it is now possible to explore the mechanics of valve-heart
interactions using anatomically and physiologically realistic models. This
study presents an integrated model of the mitral valve (MV) coupled to the left
ventricle (LV), with the geometry derived from in vivo clinical magnetic
resonance images. Numerical simulations using this coupled MV-LV model are
developed using an immersed boundary/finite element method. The model
incorporates detailed valvular features, left ventricular contraction,
nonlinear soft tissue mechanics, and fluid-mediated interactions between the MV
and LV wall. We use the model to simulate the cardiac function from diastole to
systole, and investigate how myocardial active relaxation function affects the
LV pump function. The results of the new model agree with in vivo measurements,
and demonstrate that the diastolic filling pressure increases significantly
with impaired myocardial active relaxation to maintain the normal cardiac
output. The coupled model has the potential to advance fundamental knowledge of
mechanisms underlying MV-LV interaction, and help in risk stratification and
optimization of therapies for heart diseases.Comment: 25 pages, 6 figure
A 3D discrete model of the diaphragm and human trunk
In this paper, a 3D discrete model is presented to model the movements of the
trunk during breathing. In this model, objects are represented by physical
particles on their contours. A simple notion of force generated by a linear
actuator allows the model to create forces on each particle by way of a
geometrical attractor. Tissue elasticity and contractility are modeled by local
shape memory and muscular fibers attractors. A specific dynamic MRI study was
used to build a simple trunk model comprised of by three compartments: lungs,
diaphragm and abdomen. This model was registered on the real geometry.
Simulation results were compared qualitatively as well as quantitatively to the
experimental data, in terms of volume and geometry. A good correlation was
obtained between the model and the real data. Thanks to this model, pathology
such as hemidiaphragm paralysis can also be simulated.Comment: published in: "Lung Modelling", France (2006
Determining the Biomechanical Behavior of the Liver Using Medical Image Analysis and Evolutionary Computation
Modeling the liver deformation forms the basis for the development of
new clinical applications that improve the diagnosis, planning and guidance
in liver surgery. However, the patient-specific modeling of this organ and its
validation are still a challenge in Biomechanics. The reason is the difficulty
to measure the mechanical response of the in vivo liver tissue. The current
approach consist of performing minimally invasive or open surgery aimed at
estimating the elastic constant of the proposed biomechanical models.
This dissertation presents how the use of medical image analysis and evolutionary
computation allows the characterization of the biomechanical behavior
of the liver, avoiding the use of these minimally invasive techniques. In particular,
the use of similarity coefficients commonly used in medical image analysis
has permitted, on one hand, to estimate the patient-specific biomechanical
model of the liver avoiding the invasive measurement of its mechanical response.
On the other hand, these coefficients have also permitted to validate
the proposed biomechanical models.
Jaccard coefficient and Hausdorff distance have been used to validate the
models proposed to simulate the behavior of ex vivo lamb livers, calculating
the error between the volume of the experimentally deformed samples of the
livers and the volume from biomechanical simulations of these deformations.
These coefficients has provided information, such as the shape of the samples
and the error distribution along their volume. For this reason, both coefficients
have also been used to formulate a novel function, the Geometric Similarity
Function (GSF). This function has permitted to establish a methodology to
estimate the elastic constants of the models proposed for the human liver using
evolutionary computation. Several optimization strategies, using GSF as cost
function, have been developed aimed at estimating the patient-specific elastic
constants of the biomechanical models proposed for the human liver.
Finally, this methodology has been used to define and validate a biomechanical
model proposed for an in vitro human liver.MartĂnez MartĂnez, F. (2014). Determining the Biomechanical Behavior of the Liver Using Medical Image Analysis and Evolutionary Computation [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/39337TESI
Minihepcidins are rationally designed small peptides that mimic hepcidin activity in mice and may be useful for the treatment of iron overload
Iron overload is the hallmark of hereditary hemochromatosis and a complication of iron-loading anemias such as β-thalassemia. Treatment can be burdensome and have significant side effects, and new therapeutic options are needed. Iron overload in hereditary hemochromatosis and β-thalassemia intermedia is caused by hepcidin deficiency. Although transgenic hepcidin replacement in mouse models of these diseases prevents iron overload or decreases its potential toxicity, natural hepcidin is prohibitively expensive for human application and has unfavorable pharmacologic properties. Here, we report the rational design of hepcidin agonists based on the mutagenesis of hepcidin and the hepcidin-binding region of ferroportin and computer modeling of their docking. We identified specific hydrophobic/aromatic residues required for hepcidin-ferroportin binding and obtained evidence in vitro that a thiol-disulfide interaction between ferroportin C326 and the hepcidin disulfide cage may stabilize binding. Guided by this model, we showed that 7–9 N-terminal amino acids of hepcidin, including a single thiol cysteine, comprised the minimal structure that retained hepcidin activity, as shown by the induction of ferroportin degradation in reporter cells. Further modifications to increase resistance to proteolysis and oral bioavailability yielded minihepcidins that, after parenteral or oral administration to mice, lowered serum iron levels comparably to those after parenteral native hepcidin. Moreover, liver iron concentrations were lower in mice chronically treated with minihepcidins than those in mice treated with solvent alone. Minihepcidins may be useful for the treatment of iron overload disorders
In vivo imaging enables high resolution preclinical trials on patients' leukemia cells growing in mice.
Xenograft mouse models represent helpful tools for preclinical studies on human tumors. For modeling the complexity of the human disease, primary tumor cells are by far superior to established cell lines. As qualified exemplary model, patients' acute lymphoblastic leukemia cells reliably engraft in mice inducing orthotopic disseminated leukemia closely resembling the disease in men. Unfortunately, disease monitoring of acute lymphoblastic leukemia in mice is hampered by lack of a suitable readout parameter
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Cavitation in soft matter
Cavitation is the sudden, unstable expansion of a void or bubble within a liquid or solid subjected to a negative hydrostatic stress. Cavitation rheology is a field emerging from the development of a suite of materials characterization, damage quantification, and therapeutic techniques that exploit the physical principles of cavitation. Cavitation rheology is inherently complex and broad in scope with wide-ranging applications in the biology, chemistry, materials, and mechanics communities. This perspective aims to drive collaboration among these communities and guide discussion by defining a common core of high-priority goals while highlighting emerging opportunities in the field of cavitation rheology. A brief overview of the mechanics and dynamics of cavitation in soft matter is presented. This overview is followed by a discussion of the overarching goals of cavitation rheology and an overview of common experimental techniques. The larger unmet needs and challenges of cavitation in soft matter are then presented alongside specific opportunities for researchers from different disciplines to contribute to the field
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