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Search Results: 1 - 10 of 81 matches for " Yasha Kadkhodayan "
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Endovascular and Surgical Treatment of Unruptured MCA Aneurysms: Meta-Analysis and Review of the Literature
Spiros L. Blackburn,Abdelrahman M. Abdelazim,Andrew B. Cutler,Kevin T. Brookins,Kyle M. Fargen,Brian L. Hoh,Yasha Kadkhodayan
Stroke Research and Treatment , 2014, DOI: 10.1155/2014/348147
Abstract: Introduction. The best treatment for unruptured middle cerebral artery (MCA) aneurysms is unclear. We perform a meta-analysis of recent publications to evaluate the results of unruptured MCA aneurysms treated with surgical clipping and endovascular coiling. Methods. A PubMed search for articles published between January 2004 and November 2013 was performed. The R statistical software package was used to create a random effects model for each desired incidence rate. Cochran’s Q test was used to evaluate possible heterogeneity among the rates observed in each study. Results. A total of 1891 unruptured MCA aneurysms, 1052 clipped and 839 coiled, were included for analysis. The complete occlusion rate at 6–9 months mean follow-up was 95.5% in the clipped group and 67.8% in the coiled group ( ). The periprocedural thromboembolism rate in the clipping group was 1.8% compared with 10.7% in the aneurysms treated by coiling ( ). The recanalization rate was 0% for clipping and 14.3% for coiling ( ). Modified Rankin scores of 0–2 were obtained in 98.9% of clipped patients compared to 95.5% of coiled (NS). Conclusions. This review weakly supports clipping as the preferred treatment of unruptured MCA aneurysms. Clinical outcomes did not differ significantly between the two groups. 1. Introduction Endovascular coiling has emerged as an option in the management of intracranial aneurysms that traditionally have been treated through open surgical clipping [1]. In the United States, the endovascular management of intracranial aneurysms continues to increase [2, 3]. To support this trend, growing literature is demonstrating low complication rates, durable treatment, and outcomes competitive with surgical results [1, 2, 4–7]. Unlike aneurysms in other locations, the unruptured middle cerebral artery (MCA) aneurysm has several characteristics favoring surgical treatment. This includes superficial location, a familiar surgical approach, easy proximal control at the supraclinoid carotid, and minimal perforator vessels. In contrast, endovascular therapy can be somewhat more difficult in this location due to the small parent vessels, difficulty with obtaining adequate working projection views, and incorporation of branch vessels in the aneurysm. However, the endovascular management of aneurysms has evolved, and coiling of unruptured MCA aneurysms is considered an appropriate alternative to clipping for some aneurysms [8]. As the trend for endovascular management of aneurysms has grown to incorporate MCA aneurysms, recent literature has emerged to promote the surgical option as
An Investigation into the Different Hardening Models in Reverse Forming of Thin Sheets
M. Kadkhodayan,J. Mosayebi
Advances in Mechanical Engineering , 2009, DOI: 10.1155/2009/874202
Abstract: This paper discusses a finite element analysis of the Bauschinger effect in the reverse cup drawing process, taken from the NUMISHEET'99 (Gelin and Picart, 1999) benchmark. In order to study the Bauschinger effect, several hardening models are considered such as isotropic, kinematic, and combined forms in the linear and nonlinear cases, including the well-known Yoshida and Chaboche's model. The obtained results have been compared with some experimental results reported in literature. The various factors, namely, normalized axial stress, von Mises stress, and the punch forces, for both first and second stages have been calculated for different materials and thicknesses. Results show that the combined model had acceptable agreement with the empirical data through both stages, while the bilinear models did not show this effectiveness. Generally, the nonlinear kinematic and combined models lead to more accurate results.
Regular Elements and Right Units of Semigroup Bx(D) Defined Semilattice D for Which V(D,а)=Q ∈ ∑3(X,8)  [PDF]
Giuli Tavdgiridze, Yasha Diasamidze
Applied Mathematics (AM) , 2015, DOI: 10.4236/am.2015.62035
Abstract: In this paper we take \"\" subsemilattice of X-semilattice of unions D which satisfies the following conditions:
\"\" We will investigate the properties of regular elements of the complete semigroup of binary relations Bx(D) satisfying V(D,а)=Q. For the case where X is a finite set we derive formulas by means of which we can calculate the numbers of regular elements and right units of the respective semigroup.
Monopole-Monopole solutions of Einstein-Yang-Mills-Higgs Theory
Yasha Shnir
Physics , 2005,
Abstract: New static regular axially symmetric solutions of SU(2) Yang-Mills-Higgs theory are constructed. They are asymptotically flat and represent gravitating monopole-monopole pairs. The solutions form two branches linked to the second Bartnik-McKinnon solution on upper mass branch and to the unstable monopole-monopole configuration in flat space on the lower branch, respectively.
Parity and reality properties of the EPRL spinfoam
Yasha Neiman
Physics , 2011, DOI: 10.1088/0264-9381/29/6/065008
Abstract: We study the parity behavior of the Lorentzian EPRL spinfoam model. We demonstrate that the vertex amplitude does not depend on the sign of the Immirzi parameter. We present numerical results for the transition amplitude and the graviton propagator in the large-spin 4-simplex approximation. The results suggest a simple relation between the contributions of the two parity-related critical points. Finally, we observe that the graviton propagator is not invariant under parity-odd permutations of equivalent nodes. Thus, the Lorentzian model has the same chirality problem as the Euclidean.
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman
Physics , 2013, DOI: 10.1103/PhysRevD.88.024037
Abstract: We study the imaginary part of the Lorentzian gravitational action for bounded regions, as described in arXiv:1301.7041. By comparing to a Euclidean calculation, we explain the agreement between the formula for this imaginary part and the formula for black hole entropy. We also clarify the topological structure of the imaginary part in Lovelock gravity. We then evaluate the action's imaginary part for some special regions. These include cylindrical slabs spanning the exterior of a stationary black hole spacetime, 'maximal diamonds' in various symmetric spacetimes, as well as local near-horizon regions. In the first setup, the black hole's entropy and conserved charges contribute to the action's imaginary and real parts, respectively. In the other two setups, the imaginary part coincides with the relevant entropy.
The Color--Flavor Transformation of induced QCD
Yasha Shnir
Physics , 2005, DOI: 10.1142/S0217751X05025401
Abstract: The Zirnbauer's color-flavor transformation is applied to the $U(N_c)$ lattice gauge model, in which the gauge theory is induced by a heavy chiral scalar field sitting on lattice sites. The flavor degrees of freedom can encompass several `generations' of the auxiliary field, and for each generation, remaining indices are associated with the elementary plaquettes touching the lattice site. The effective, color-flavor transformed theory is expressed in terms of gauge singlet matrix fields carried by lattice links. The effective action is analyzed for a hypercubic lattice in arbitrary dimension. We investigate the corresponding d=2 and d=3 dual lattices. The saddle points equations of the model in the large-$N_c$ limit are discussed.
Higher-spin gravity as a theory on a fixed (anti) de Sitter background
Yasha Neiman
Physics , 2015, DOI: 10.1007/JHEP04(2015)144
Abstract: We study Vasiliev's higher-spin gravity in 3+1d. We formulate the theory in the so-called compensator formalism, where the local isometry group SO(4,1) is reduced to the Lorentz group SO(3,1) by a choice of spacelike direction in an internal 4+1d space. We present a consistent extension of Vasiliev's equations that allows this internal direction to become spacetime-dependent. This allows a new point of view on the theory, where spacetime is identified with the de Sitter space of possible internal directions. We thus obtain an interacting theory of higher-spin massless gauge fields on a fixed, maximally symmetric background spacetime. We expect implications for the physical interpretation of higher-spin gravity, for the search for a Lagrangian formulation and/or quantization, as well as for higher-spin holography.
On-shell actions with lightlike boundary data
Yasha Neiman
Physics , 2012,
Abstract: We argue that finite-region observables in quantum gravity are best approached in terms of boundary data on null hypersurfaces. This has far-reaching effects on the basic notions of classical and quantum mechanics, such as Hamiltonians and canonical conjugates. Such radical properties are not unexpected in finite-region quantum gravity. We are thus motivated to reformulate field theory in terms of null boundary data. As a starting point, we consider the on-shell action functional for classical field theory in finite null-bounded regions. Closed-form results are obtained for free scalars and for Maxwell fields. The action of classical gravity is also discussed, to the extent possible without solving the field equations. These action functionals exhibit non-locality and, in special cases, a "holographic" reduction of the degrees of freedom. Also, they cannot be used to define global charges. Whereas for ordinary field theory these are just artifacts of a restrictive formalism, in quantum gravity they are expected to be genuine features. This further supports a connection between quantum gravity and null-boundary observables. In our treatment of the GR action, we identify a universal imaginary term that reproduces the Bekenstein entropy formula.
The imaginary part of the gravity action and black hole entropy
Yasha Neiman
Physics , 2013, DOI: 10.1007/JHEP04(2013)071
Abstract: As observed recently in arXiv:1212.2922, the action of General Relativity (GR) in finite spacetime regions has an imaginary part that resembles the Bekenstein entropy. In this paper, we expand on that argument, with attention to different causal types of boundaries. This property of the GR action may open a new approach to the puzzles of gravitational entropy. In particular, the imaginary action can be evaluated for non-stationary finite regions, where the notion of entropy is not fully understood. As a first step in constructing the precise relationship between the imaginary action and entropy, we focus on stationary black hole spacetimes. There, we identify a class of bounded regions for which the action's imaginary part precisely equals the black hole entropy. As a powerful test on the validity of the approach, we also calculate the imaginary action for Lovelock gravity. The result is related to the corresponding entropy formula in the same way as in GR.
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