239 research outputs found
A direct D-bar reconstruction algorithm for recovering a complex conductivity in 2-D
A direct reconstruction algorithm for complex conductivities in
, where is a bounded, simply connected Lipschitz
domain in , is presented. The framework is based on the
uniqueness proof by Francini [Inverse Problems 20 2000], but equations relating
the Dirichlet-to-Neumann to the scattering transform and the exponentially
growing solutions are not present in that work, and are derived here. The
algorithm constitutes the first D-bar method for the reconstruction of
conductivities and permittivities in two dimensions. Reconstructions of
numerically simulated chest phantoms with discontinuities at the organ
boundaries are included.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in [insert name of journal]. IOP Publishing Ltd is
not responsible for any errors or omissions in this version of the manuscript
or any version derived from it. The Version of Record is available online at
10.1088/0266-5611/28/9/09500
Deep Neural Networks for Inverse Problems with Pseudodifferential Operators: An Application to Limited-Angle Tomography
We propose a novel convolutional neural network (CNN), called \Psi DONet, designed for learning pseudodifferential operators (\Psi DOs) in the context of linear inverse problems. Our starting point is the iterative soft thresholding algorithm (ISTA), a well-known algorithm to solve sparsity-promoting
minimization problems. We show that, under rather general assumptions on the forward operator, the unfolded iterations of ISTA can be interpreted as the successive layers of a CNN, which in turn provides fairly general network architectures that, for a specific choice of the parameters involved, allow us to reproduce ISTA, or a perturbation of ISTA for which we can bound the coefficients of the filters. Our case study is the limited-angle X-ray transform and its application to limited-angle computed tomography (LA-CT). In particular, we prove that, in the case of LA-CT, the operations of upscaling, downscaling, and convolution, which characterize our \Psi DONet and most deep learning schemes, can be exactly determined by combining the convolutional nature of the limited-angle Xray transform and basic properties defining an orthogonal wavelet system. We test two different implementations of \Psi DONet on simulated data from limited-angle geometry, generated from the ellipse data set. Both implementations provide equally good and noteworthy preliminary results, showing the potential of the approach we propose and paving the way to applying the same idea to other convolutional operators which are \Psi DOs or Fourier integral operators
Enhanced second harmonic generation from resonant GaAs gratings
We study second harmonic generation in nonlinear, GaAs gratings. We find
large enhancement of conversion efficiency when the pump field excites the
guided mode resonances of the grating. Under these circumstances the spectrum
near the pump wavelength displays sharp resonances characterized by dramatic
enhancements of local fields and favorable conditions for second harmonic
generation, even in regimes of strong linear absorption at the harmonic
wavelength. In particular, in a GaAs grating pumped at 1064nm, we predict
second harmonic conversion efficiencies approximately five orders of magnitude
larger than conversion rates achievable in either bulk or etalon structures of
the same material.Comment: 8 page
The Novikov-Veselov Equation and the Inverse Scattering Method, Part I: Analysis
The Novikov-Veselov (NV) equation is a
(2+1)-dimensional nonlinear evolution equation that generalizes the
(1+1)-dimensional Korteweg-deVries (KdV) equation. Solution of the NV
equation using the inverse scattering method has been discussed in the
literature, but only formally (or with smallness assumptions in case of nonzero
energy) because of the possibility of exceptional points, or singularities in
the scattering data. In this work, absence of exceptional points is proved at
zero energy for evolutions with compactly supported, smooth and rotationally
symmetric initial data of the conductivity type:
with a strictly positive function
. The inverse scattering evolution is shown to be well-defined,
real-valued, and preserving conductivity-type. There is no smallness assumption
on the initial data
Psychosocial issues need more attention in COPD self-management education
Objective: To find out how regularly the contents of patient education regarded as essential for COPD patients' self-management are provided by healthcare professionals in specialised healthcare (SHC) and primary healthcare (PHC) in Finland. Design: A cross-sectional study based on an e-questionnaire with 42 items on the content of self-management education of COPD patients. Setting: The study sample included all public SHC units with pulmonary outpatient clinics (n = 29) and nine out of 160 health centres in Finland. Subjects: 83 doctors and 162 nurses. Main outcome measures: The respondents' answers on how regularly they included the contents regarded as essential for COPD patients' self-management in their education of COPD patients. Results: COPD patients were educated regularly on medical issues regarding COPD treatment, such as smoking cessation, exercise and pharmacological treatment. However, issues vital for coping with the disease, such as psychological well-being, stress management or fatigue, were often ignored. Patient education in SHC seemed to be more systematic than education in PHC. The education provided by the asthma/COPD nurses (n = 70) was more systematic than the education provided by the other nurses (n = 84). Conclusion: Healthcare professionals' continuous education should cover not only the medical but also the psychosocial aspects of coping with COPD. The role of doctors and nurses should be considered to ensure that there is no gap in COPD patients' education. Training asthma/COPD nurses and promoting specialised nurse-led asthma/COPD clinics in primary care could be beneficial while improving practices of patient education that enhance patients' ability to cope with the disease
Discovery of small molecules that activate RNA-methylation through cooperative binding to the METTL3/14/WTAP complex active site
Chemical modifications of RNA provide an additional, epitranscriptomic, level of control over cellular functions. N-6-methylated adenosines (m6As) are found in several types of RNA, and their amounts are regulated by methyltransferases and demethylases. One of the most important enzymes catalyzing generation of m6A on mRNA is the trimer N-6-methyltransferase METTL3-14-WTAP complex. Its activity has been linked to such critical biological processes as cell differentiation, proliferation, and death.
We used in silico-based discovery to identify small-molecule ligands that bind to METTL3-14-WTAP and determined experimentally their binding affinity and kinetics, as well as their effect on enzymatic function. We show that these ligands serve as activators of the METTL3-14-WTAP complex
Organisational participation and women - an attitude problem?
Employee participation is a dynamic and contested area of organisational behaviour, attracting continuing academic, practitioner and policy interest and debate. This chapter focuses on organisational participation and women
High-dose adenosine versus saline-induced cardioplegic arrest in coronary artery bypass grafting : A randomized double-blind clinical feasibility trial
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