9,639 research outputs found
Spatio-Temporal Modelling of Perfusion Cardiovascular MRI
Myocardial perfusion MRI provides valuable insight into how coronary artery and microvascular diseases affect myocardial tissue. Stenosis in a coronary vessel leads to reduced maximum blood flow (MBF), but collaterals may secure the blood supply of the myocardium but with altered tracer kinetics. To date, quantitative analysis of myocardial perfusion MRI has only been performed on a local level, largely ignoring the contextual information inherent in different myocardial segments. This paper proposes to quantify the spatial dependencies between the local kinetics via a Hierarchical Bayesian Model (HBM). In the proposed framework, all local systems are modelled simultaneously along with their dependencies, thus allowing more robust context-driven estimation of local kinetics. Detailed validation on both simulated and patient data is provided
The characteristic stellar mass as a function of redshift
We present a model for the star formation process during the initial collapse
of dark matter haloes at redshifts z=0-30. We derive a simple expression for
the characteristic stellar mass scale during this initial burst of star
formation. In our picture, this characteristic scale reflects both the minimum
temperature to which the gas can cool (determined by the metallicity and the
temperature of the cosmic microwave background) and the pressure of overlying
baryons in the collapsing halo. This prescription reproduces both the large
mass scales found in simulations of Population III star formation and the near
solar values observed for star formation at low redshift.Comment: 8 pages, 4 figures, accepted for publication in MNRA
A General Approach to Regularizing Inverse Problems with Regional Data using Slepian Wavelets
Slepian functions are orthogonal function systems that live on subdomains
(for example, geographical regions on the Earth's surface, or bandlimited
portions of the entire spectrum). They have been firmly established as a useful
tool for the synthesis and analysis of localized (concentrated or confined)
signals, and for the modeling and inversion of noise-contaminated data that are
only regionally available or only of regional interest. In this paper, we
consider a general abstract setup for inverse problems represented by a linear
and compact operator between Hilbert spaces with a known singular-value
decomposition (svd). In practice, such an svd is often only given for the case
of a global expansion of the data (e.g. on the whole sphere) but not for
regional data distributions. We show that, in either case, Slepian functions
(associated to an arbitrarily prescribed region and the given compact operator)
can be determined and applied to construct a regularization for the ill-posed
regional inverse problem. Moreover, we describe an algorithm for constructing
the Slepian basis via an algebraic eigenvalue problem. The obtained Slepian
functions can be used to derive an svd for the combination of the regionalizing
projection and the compact operator. As a result, standard regularization
techniques relying on a known svd become applicable also to those inverse
problems where the data are regionally given only. In particular, wavelet-based
multiscale techniques can be used. An example for the latter case is elaborated
theoretically and tested on two synthetic numerical examples
Knowledge Management What Can Organizational Economics Contribute?
Knowledge management has emerged as a very successful organization practice and has been extensively treated in a large body of academic work. Surprisingly, however, organizational economics (i.e., transaction cost economics, agency theory, team theory and property rights theory) has played no role in the development of knowledge management. We argue that organizational economics insights can further the theory and practice of knowledge management in several ways. Specifically, we apply notions of contracting, team production, complementaries, hold-up, etc. to knowledge management issues (i.e., creating and integration knowledge, rewarding knowledge workers, etc.) , and derive refutable implications that are novel to the knowledge management field from our discussion.Transaction costs, organizational economics
The Spectrum of GRB 930131 (``Superbowl Burst'') from 20 keV to 200 MeV
We have constructed a broad-band spectrum for GRB 930131 (the ``Superbowl
Burst''), ranging from 20 keV to 200 MeV, by combining spectral information
from the Gamma Ray Observatory's BATSE, COMPTEL and EGRET instruments. We
present general methods for combining spectra from different time intervals
obtained by the same instrument as well as for combining spectra from the same
time interval taken by different instruments. The resulting spectrum is
remarkably flat (in nu F_nu-space) up to high energies. We find that the
spectral shape can be successfully fitted by the shocked synchrotron emission
model of Tavani. We present evidence that the flatness of the spectrum at high
energies is not due to spectral time-variability.Comment: ApJ accepted, 10 pages, 1 table, 3 figure
Bayesian Age-Period-Cohort Modeling and Prediction - BAMP
The software package BAMP provides a method of analyzing incidence or mortality data on the Lexis diagram, using a Bayesian version of an age-period-cohort model. A hierarchical model is assumed with a binomial model in the first-stage. As smoothing priors for the age, period and cohort parameters random walks of first and second order, with and without an additional unstructured component are available. Unstructured heterogeneity can also be included in the model. In order to evaluate the model fit, posterior deviance, DIC and predictive deviances are computed. By projecting the random walk prior into the future, future death rates can be predicted.
Strategy Research and the Market Process Perspective
We argue that strategizing fundamentally concerns disequilibrium phenomena, such as discovery, innovation, resource-combination, imagination - in short, entrepreneurship. Therefore, the understanding of strategizing is likely to be led astray by drawing too heavily on equilibrium theories. Arguably, the three dominant economic approaches to strategy - the Porter industry analysis approach, the new industrial organization, and the ressourcebased approach - are characterized precisely by their strong reliance on equilibrium methodology. We argue that the market process approach in its Austrian version offers much inspiration for bringing process issues to bear on strategy issues.Strategy, organization, competitive advantage.
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