11,282 research outputs found
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Cerebral aneurysm treatment: modern neurovascular techniques.
Endovascular treatment of cerebral aneurysm continues to evolve with the development of new technologies. This review provides an overview of the recent major innovations in the neurointerventional space in recent years
Broad boron sheets and boron nanotubes: An ab initio study of structural, electronic, and mechanical properties
Based on a numerical ab initio study, we discuss a structure model for a
broad boron sheet, which is the analog of a single graphite sheet, and the
precursor of boron nanotubes. The sheet has linear chains of sp hybridized
sigma bonds lying only along its armchair direction, a high stiffness, and
anisotropic bonds properties. The puckering of the sheet is explained as a
mechanism to stabilize the sp sigma bonds. The anisotropic bond properties of
the boron sheet lead to a two-dimensional reference lattice structure, which is
rectangular rather than triangular. As a consequence the chiral angles of
related boron nanotubes range from 0 to 90 degrees. Given the electronic
properties of the boron sheets, we demonstrate that all of the related boron
nanotubes are metallic, irrespective of their radius and chiral angle, and we
also postulate the existence of helical currents in ideal chiral nanotubes.
Furthermore, we show that the strain energy of boron nanotubes will depend on
their radii, as well as on their chiral angles. This is a rather unique
property among nanotubular systems, and it could be the basis of a different
type of structure control within nanotechnology.Comment: 16 pages, 17 figures, 2 tables, Versions: v1=preview, v2=first final,
v3=minor corrections, v4=document slightly reworke
Generalised Super Resolution for Quantitative MRI Using Self-supervised Mixture of Experts
Multi-modal and multi-contrast imaging datasets have diverse voxel-wise intensities. For example, quantitative MRI acquisition protocols are designed specifically to yield multiple images with widely-varying contrast that inform models relating MR signals to tissue characteristics. The large variance across images in such data prevents the use of standard normalisation techniques, making super resolution highly challenging. We propose a novel self-supervised mixture-of-experts (SS-MoE) paradigm for deep neural networks, and hence present a method enabling improved super resolution of data where image intensities are diverse and have large variance. Unlike the conventional MoE that automatically aggregates expert results for each input, we explicitly assign an input to the corresponding expert based on the predictive pseudo error labels in a self-supervised fashion. A new gater module is trained to discriminate the error levels of inputs estimated by Multiscale Quantile Segmentation. We show that our new paradigm reduces the error and improves the robustness when super resolving combined diffusion-relaxometry MRI data from the Super MUDI dataset. Our approach is suitable for a wide range of quantitative MRI techniques, and multi-contrast or multi-modal imaging techniques in general. It could be applied to super resolve images with inadequate resolution, or reduce the scanning time needed to acquire images of the required resolution. The source code and the trained models are available at https://github.com/hongxiangharry/SS-MoE
Precision spectral manipulation of optical pulses using a coherent photon echo memory
Photon echo schemes are excellent candidates for high efficiency coherent
optical memory. They are capable of high-bandwidth multi-pulse storage, pulse
resequencing and have been shown theoretically to be compatible with quantum
information applications. One particular photon echo scheme is the gradient
echo memory (GEM). In this system, an atomic frequency gradient is induced in
the direction of light propagation leading to a Fourier decomposition of the
optical spectrum along the length of the storage medium. This Fourier encoding
allows precision spectral manipulation of the stored light. In this letter, we
show frequency shifting, spectral compression, spectral splitting, and fine
dispersion control of optical pulses using GEM
Problems and Progress in Flare Fast Particle Diagnostics
Recent progress in the diagnosis of flare fast particles is critically
discussed with the main emphasis on high resolution Hard X-Ray (HXR) data from
RHESSI and coordinated data from other instruments. Spectacular new photon data
findings are highlighted as are advances in theoretical aspects of their use as
fast particle diagnostics, and some important comparisons made with
interplanetary particle data. More specifically the following topics are
addressed
(a) RHESSI data on HXR (electron) versus gamma-ray line (ion) source
locations.
(b) RHESSI hard X-ray source spatial structure in relation to theoretical
models and loop density structure.
(c) Energy budget of flare electrons and the Neupert effect.
(d) Spectral deconvolution methods including blind target testing and results
for RHESSI HXR spectra, including the reality and implications of dips inferred
in electron spectra
(e) The relation between flare in-situ and interplanetary particle data.Comment: 15 pages, 13 figures, submitted to Advances in Space Researc
Regularized energy-dependent solar flare hard x-ray spectral index
The deduction from solar flare X-ray photon spectroscopic data of the energy
dependent model-independent spectral index is considered as an inverse problem.
Using the well developed regularization approach we analyze the energy
dependency of spectral index for a high resolution energy spectrum provided by
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The regularization
technique produces much smoother derivatives while avoiding additional errors
typical of finite differences. It is shown that observations imply a spectral
index varying significantly with energy, in a way that also varies with time as
the flare progresses. The implications of these findings are discussed in the
solar flare context.Comment: 13 pages; 5 figures, Solar Physics in pres
Induction chemotherapy in the treatment of nasopharyngeal carcinoma: Clinical outcomes and patterns of care
Teachers' classroom feedback: still trying to get it right
This article examines feedback traditionally given by teachers in schools. Such feedback tends to focus on children's acquisition and retrieval of externally prescribed knowledge which is then assessed against mandated tests. It suggests that, from a sociocultural learning perspective, feedback directed towards such objectives may limit children's social development. In this article, I draw on observation and interview data gathered from a group of 27 9- to 10-year olds in a UK primary school. These data illustrate the children's perceived need to conform to, rather than negotiate, the teacher's feedback comments. They highlight the children's sense that the teacher's feedback relates to school learning but not to their own interests. The article also includes alternative examples of feedback which draw on children's own inquiries and which relate to the social contexts within which, and for whom, they act. It concludes by suggesting that instead of looking for the right answer to the question of what makes teachers' feedback effective in our current classrooms, a more productive question might be how a negotiation can be opened up among teachers and learners themselves, about how teachers' feedback could support children's learning most appropriately
Thermalisation of self-interacting solar flare fast electrons
Most theoretical descriptions of the production of solar flare bremsstrahlung
radiation assume the collision of dilute accelerated particles with a cold,
dense target plasma, neglecting interactions of the fast particles with each
other. This is inadequate for situations where collisions with this background
plasma are not completely dominant, as may be the case in, for example,
low-density coronal sources. We aim to formulate a model of a self-interacting,
entirely fast electron population in the absence of a dense background plasma,
to investigate its implications for observed bremsstrahlung spectra and the
flare energy budget. We derive approximate expressions for the time-dependent
distribution function of the fast electrons using a Fokker-Planck approach. We
use these expressions to generate synthetic bremsstrahlung X-ray spectra as
would be seen from a corresponding coronal source. We find that our model
qualitatively reproduces the observed behaviour of some flares. As the flare
progresses, the model's initial power-law spectrum is joined by a lower energy,
thermal component. The power-law component diminishes, and the growing thermal
component proceeds to dominate the total emission over timescales consistent
with flare observations. The power-law exhibits progressive spectral hardening,
as is seen in some flare coronal sources. We also find that our model requires
a factor of 7 - 10 fewer accelerated electrons than the cold, thick target
model to generate an equivalent hard X-ray flux. This model forms the basis of
a treatment of self-interactions among flare fast electrons, a process which
affords a more efficient means to produce bremsstrahlung photons and so may
reduce the efficiency requirements placed on the particle acceleration
mechanism. It also provides a useful description of the thermalisation of fast
electrons in coronal sources.Comment: 9 pages, 7 figures, accepted for Astronomy & Astrophysics; this
version clarifies arguments around Eqs. (11) and (20
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Overcoming myelosuppression due to synthetic lethal toxicity for FLT3-targeted acute myeloid leukemia therapy.
Activating mutations in FLT3 confer poor prognosis for individuals with acute myeloid leukemia (AML). Clinically active investigational FLT3 inhibitors can achieve complete remissions but their utility has been hampered by acquired resistance and myelosuppression attributed to a 'synthetic lethal toxicity' arising from simultaneous inhibition of FLT3 and KIT. We report a novel chemical strategy for selective FLT3 inhibition while avoiding KIT inhibition with the staurosporine analog, Star 27. Star 27 maintains potency against FLT3 in proliferation assays of FLT3-transformed cells compared with KIT-transformed cells, shows no toxicity towards normal human hematopoiesis at concentrations that inhibit primary FLT3-mutant AML blast growth, and is active against mutations that confer resistance to clinical inhibitors. As a more complete understanding of kinase networks emerges, it may be possible to define anti-targets such as KIT in the case of AML to allow improved kinase inhibitor design of clinical agents with enhanced efficacy and reduced toxicity
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