4,310 research outputs found
NOViSE: a virtual natural orifice transluminal endoscopic surgery simulator
Purpose: Natural Orifice Transluminal Endoscopic Surgery (NOTES) is a novel technique in minimally invasive surgery whereby a flexible endoscope is inserted via a natural orifice to gain access to the abdominal cavity, leaving no external scars. This innovative use of flexible endoscopy creates many new challenges and is associated with a steep learning curve for clinicians. Methods: We developed NOViSE - the first force-feedback enabled virtual reality simulator for NOTES training supporting a flexible endoscope. The haptic device is custom built and the behaviour of the virtual flexible endoscope is based on an established theoretical framework – the Cosserat Theory of Elastic Rods. Results: We present the application of NOViSE to the simulation of a hybrid trans-gastric cholecystectomy procedure. Preliminary results of face, content and construct validation have previously shown that NOViSE delivers the required level of realism for training of endoscopic manipulation skills specific to NOTES Conclusions: VR simulation of NOTES procedures can contribute to surgical training and improve the educational experience without putting patients at risk, raising ethical issues or requiring expensive animal or cadaver facilities. In the context of an experimental technique, NOViSE could potentially facilitate NOTES development and contribute to its wider use by keeping practitioners up to date with this novel surgical technique. NOViSE is a first prototype and the initial results indicate that it provides promising foundations for further development
Fully Anisotropic String Cosmologies, Maxwell Fields and Primordial Shear
We present a class of exact cosmological solutions of the low energy string
effective action in the presence of a homogeneous magnetic fields. We discuss
the physical properties of the obtained (fully anisotropic) cosmologies paying
particular attention to their vacuum limit and to the possible isotropization
mechanisms. We argue that quadratic curvature corrections are able to
isotropize fully anisotropic solutions whose scale factors describe accelerated
expansion. Moreover, the degree of isotropization grows with the duration of
the string phase. We follow the fate of the shear parameter in a decelerated
phase where, dilaton, magnetic fields and radiation fluid are simultaneously
present. In the absence of any magnetic field a long string phase immediately
followed by radiation is able to erase large anisotropies. Conversely, if a
short string phase is followed by a long dilaton dominated phase the
anisotropies can be present, in principle, also at later times. The presence of
magnetic seeds after the end of the string phase can induce further
anisotropies which can be studied within the formalism reported in this paper.Comment: 19 pages in Revtex style, 14 Encapsulated figure
Homogeneous magnetic fields in fully anisotropic string cosmological backgrounds
We present new solutions of the string cosmological effective action in the
presence of a homogeneous Maxwell field with pure magnetic component. Exact
solutions are derived in the case of space-independent dilaton and vanishing
torsion background. In our examples the four dimensional metric is either of
Bianchi-type III and VI or Kantowski-Sachs.Comment: 4 page
The Dominant Balance at Cosmological Singularities
We define the notion of a finite-time singularity of a vector field and then
discuss a technique suitable for the asymptotic analysis of vector fields and
their integral curves in the neighborhood of such a singularity. Having in mind
the application of this method to cosmology, we also provide an analysis of the
time singularities of an isotropic universe filled with a perfect fluid in
general relativity.Comment: 13 pages, to appear in the Proceedings of the Greek Relativity
Meeting NEB12, June 29-July 2, 2006, Nauplia, Greec
Diseases of winter linseed : occurrence, effects and importance
In 1998, a survey of the incidence and severity of diseases was carried out on 30 crops of winter linseed at early flowering and again at crop maturity. Five crops each were selected in south west, east, east Midlands, west Midlands and north of England and from Scotland. Crops were predominantly cv. Oliver (90% crops), grown from certified seed (83%) and sown in September (97%). Pasmo (Mycosphaerella) was the most important disease, affecting leaves of 73% crops at early flowering and 90% crops at maturity. Powdery mildew (70% crops), Alternaria (30% crops) on leaves and Botrytis on capsules (70% crops) were also common. Regional differences were apparent for powdery mildew, which was present in all regions except the southwest, whilst Alternaria predominated in the Midlands. Half of the crops surveyed had received fungicide sprays, but this appeared to have made limited impact on disease severity. Pasmo is a new threat to UK linseed crops and this raises concerns about the threat it poses to spring linsee
Quantum fields and "Big Rip" expansion singularities
The effects of quantized conformally invariant massless fields on the
evolution of cosmological models containing a ``Big Rip'' future expansion
singularity are examined. Quantized scalar, spinor, and vector fields are found
to strengthen the accelerating expansion of such models as they approach the
expansion singularity.Comment: 7 pages; REVTeX
Plane-symmetric inhomogeneous Brans-Dicke cosmology with an equation of state
We present a new exact solution in Brans-Dicke theory. The solution describes
inhomogeneous plane-symmetric perfect fluid cosmological model with an equation
of state . Some main properties of the solution are discussed.Comment: 6 pages, Late
The Power of General Relativity
We study the cosmological and weak-field properties of theories of gravity
derived by extending general relativity by means of a Lagrangian proportional
to . This scale-free extension reduces to general relativity when
. In order to constrain generalisations of general relativity of
this power class we analyse the behaviour of the perfect-fluid Friedmann
universes and isolate the physically relevant models of zero curvature. A
stable matter-dominated period of evolution requires or . The stable attractors of the evolution are found. By considering the
synthesis of light elements (helium-4, deuterium and lithium-7) we obtain the
bound We evaluate the effect on the power spectrum of
clustering via the shift in the epoch of matter-radiation equality. The horizon
size at matter--radiation equality will be shifted by for a value of
We study the stable extensions of the Schwarzschild
solution in these theories and calculate the timelike and null geodesics. No
significant bounds arise from null geodesic effects but the perihelion
precession observations lead to the strong bound assuming that Mercury follows a timelike geodesic. The combination of
these observational constraints leads to the overall bound on theories of this type.Comment: 26 pages and 5 figures. Published versio
Cosmological dynamics of exponential gravity
We present a detailed investigation of the cosmological dynamics based on
gravity. We apply the dynamical system approach to both
the vacuum and matter cases and obtain exact solutions and their stability in
the finite and asymptotic regimes. The results show that cosmic histories exist
which admit a double de-Sitter phase which could be useful for describing the
early and the late-time accelerating universe.Comment: 17 pages LaTeX, 3 figure
Covariant and locally Lorentz-invariant varying speed of light theories
We propose definitions for covariance and local Lorentz invariance applicable
when the speed of light is allowed to vary. They have the merit of
retaining only those aspects of the usual definitions which are invariant under
unit transformations, and which can therefore legitimately represent the
outcome of an experiment. We then discuss some possibilities for invariant
actions governing the dynamics of such theories. We consider first the
classical action for matter fields and the effects of a changing upon
quantization. We discover a peculiar form of quantum particle creation due to a
varying . We then study actions governing the dynamics of gravitation and
the speed of light. We find the free, empty-space, no-gravity solution, to be
interpreted as the counterpart of Minkowksi space-time, and highlight its
similarities with Fock-Lorentz space-time. We also find flat-space string-type
solutions, in which near the string core is much higher. We label them
fast-tracks and compare them with gravitational wormholes. We finally discuss
general features of cosmological and black hole solutions, and digress on the
meaning of singularities in these theories.Comment: To be published in Physical Review
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