8,277 research outputs found
Anatomical landmark based registration of contrast enhanced T1-weighted MR images
In many problems involving multiple image analysis, an im- age registration step is required. One such problem appears in brain tumor imaging, where baseline and follow-up image volumes from a tu- mor patient are often to-be compared. Nature of the registration for a change detection problem in brain tumor growth analysis is usually rigid or affine. Contrast enhanced T1-weighted MR images are widely used in clinical practice for monitoring brain tumors. Over this modality, con- tours of the active tumor cells and whole tumor borders and margins are visually enhanced. In this study, a new technique to register serial contrast enhanced T1 weighted MR images is presented. The proposed fully-automated method is based on five anatomical landmarks: eye balls, nose, confluence of sagittal sinus, and apex of superior sagittal sinus. Af- ter extraction of anatomical landmarks from fixed and moving volumes, an affine transformation is estimated by minimizing the sum of squared distances between the landmark coordinates. Final result is refined with a surface registration, which is based on head masks confined to the sur- face of the scalp, as well as to a plane constructed from three of the extracted features. The overall registration is not intensity based, and it depends only on the invariant structures. Validation studies using both synthetically transformed MRI data, and real MRI scans, which included several markers over the head of the patient were performed. In addition, comparison studies against manual landmarks marked by a radiologist, as well as against the results obtained from a typical mutual information based method were carried out to demonstrate the effectiveness of the proposed method
Rheological Model for Wood
Wood as the most important natural and renewable building material plays an
important role in the construction sector. Nevertheless, its hygroscopic
character basically affects all related mechanical properties leading to
degradation of material stiffness and strength over the service life.
Accordingly, to attain reliable design of the timber structures, the influence
of moisture evolution and the role of time- and moisture-dependent behaviors
have to be taken into account. For this purpose, in the current study a 3D
orthotropic elasto-plastic, visco-elastic, mechano-sorptive constitutive model
for wood, with all material constants being defined as a function of moisture
content, is presented. The corresponding numerical integration approach, with
additive decomposition of the total strain is developed and implemented within
the framework of the finite element method (FEM). Moreover to preserve a
quadratic rate of asymptotic convergence the consistent tangent operator for
the whole model is derived.
Functionality and capability of the presented material model are evaluated by
performing several numerical verification simulations of wood components under
different combinations of mechanical loading and moisture variation.
Additionally, the flexibility and universality of the introduced model to
predict the mechanical behavior of different species are demonstrated by the
analysis of a hybrid wood element. Furthermore, the proposed numerical approach
is validated by comparisons of computational evaluations with experimental
results.Comment: 37 pages, 13 figures, 10 table
Assessing the outcome of orthognathic surgery by three-dimensional soft tissue analysis
Studies of orthognathic surgery often focus on pre-surgical versus post-surgical changes in facial shape. In contrast, this study provides an innovative comparison between post-surgical and control shape. Forty orthognathic surgery patients were included, who underwent three different types of surgical correction: Le Fort I maxillary advancement, bilateral sagittal split mandibular advancement, and bimaxillary advancement surgery. Control facial images were captured from volunteers from local communities in Glasgow, with patterns of age, sex, and ethnic background that matched those of the surgical patients. Facial models were fitted and Procrustes registration and principal components analysis used to allow quantitative analysis, including the comparison of group mean shape and mean asymmetry. The primary characteristic of the difference in shape was found to be residual mandibular prognathism in the group of female patients who underwent Le Fort I maxillary advancement. Individual cases were assessed against this type of shape difference, using a quantitative scale to aid clinical audit. Analysis of the combined surgical groups provided strong evidence that surgery reduces asymmetry in some parts of the face such as the upper lip region. No evidence was found that mean asymmetry in post-surgical patients is greater than that in controls
Geometry Processing of Conventionally Produced Mouse Brain Slice Images
Brain mapping research in most neuroanatomical laboratories relies on
conventional processing techniques, which often introduce histological
artifacts such as tissue tears and tissue loss. In this paper we present
techniques and algorithms for automatic registration and 3D reconstruction of
conventionally produced mouse brain slices in a standardized atlas space. This
is achieved first by constructing a virtual 3D mouse brain model from annotated
slices of Allen Reference Atlas (ARA). Virtual re-slicing of the reconstructed
model generates ARA-based slice images corresponding to the microscopic images
of histological brain sections. These image pairs are aligned using a geometric
approach through contour images. Histological artifacts in the microscopic
images are detected and removed using Constrained Delaunay Triangulation before
performing global alignment. Finally, non-linear registration is performed by
solving Laplace's equation with Dirichlet boundary conditions. Our methods
provide significant improvements over previously reported registration
techniques for the tested slices in 3D space, especially on slices with
significant histological artifacts. Further, as an application we count the
number of neurons in various anatomical regions using a dataset of 51
microscopic slices from a single mouse brain. This work represents a
significant contribution to this subfield of neuroscience as it provides tools
to neuroanatomist for analyzing and processing histological data.Comment: 14 pages, 11 figure
Skull Flexure from Blast Waves: A Mechanism for Brain Injury with Implications for Helmet Design
Traumatic brain injury [TBI] has become a signature injury of current
military conflicts, with debilitating, costly, and long-lasting effects.
Although mechanisms by which head impacts cause TBI have been well-researched,
the mechanisms by which blasts cause TBI are not understood. From numerical
hydrodynamic simulations, we have discovered that non-lethal blasts can induce
sufficient skull flexure to generate potentially damaging loads in the brain,
even without a head impact. The possibility that this mechanism may contribute
to TBI has implications for injury diagnosis and armor design.Comment: version in press, Physical Review Letters; 17 pages, 5 figures
(includes supplementary material
Mathematical modeling of local perfusion in large distensible microvascular networks
Microvessels -blood vessels with diameter less than 200 microns- form large,
intricate networks organized into arterioles, capillaries and venules. In these
networks, the distribution of flow and pressure drop is a highly interlaced
function of single vessel resistances and mutual vessel interactions. In this
paper we propose a mathematical and computational model to study the behavior
of microcirculatory networks subjected to different conditions. The network
geometry is composed of a graph of connected straight cylinders, each one
representing a vessel. The blood flow and pressure drop across the single
vessel, further split into smaller elements, are related through a generalized
Ohm's law featuring a conductivity parameter, function of the vessel cross
section area and geometry, which undergo deformations under pressure loads. The
membrane theory is used to describe the deformation of vessel lumina, tailored
to the structure of thick-walled arterioles and thin-walled venules. In
addition, since venules can possibly experience negative transmural pressures,
a buckling model is also included to represent vessel collapse. The complete
model including arterioles, capillaries and venules represents a nonlinear
system of PDEs, which is approached numerically by finite element
discretization and linearization techniques. We use the model to simulate flow
in the microcirculation of the human eye retina, a terminal system with a
single inlet and outlet. After a phase of validation against experimental
measurements, we simulate the network response to different interstitial
pressure values. Such a study is carried out both for global and localized
variations of the interstitial pressure. In both cases, significant
redistributions of the blood flow in the network arise, highlighting the
importance of considering the single vessel behavior along with its position
and connectivity in the network
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