6,066 research outputs found
A multi-scale multi-frequency deconvolution algorithm for synthesis imaging in radio interferometry
Aims : We describe MS-MFS, a multi-scale multi-frequency deconvolution
algorithm for wide-band synthesis-imaging, and present imaging results that
illustrate the capabilities of the algorithm and the conditions under which it
is feasible and gives accurate results.
Methods : The MS-MFS algorithm models the wide-band sky-brightness
distribution as a linear combination of spatial and spectral basis functions,
and performs image-reconstruction by combining a linear-least-squares approach
with iterative minimization. This method extends and combines the
ideas used in the MS-CLEAN and MF-CLEAN algorithms for multi-scale and
multi-frequency deconvolution respectively, and can be used in conjunction with
existing wide-field imaging algorithms. We also discuss a simpler hybrid of
spectral-line and continuum imaging methods and point out situations where it
may suffice.
Results : We show via simulations and application to multi-frequency VLA data
and wideband EVLA data, that it is possible to reconstruct both spatial and
spectral structure of compact and extended emission at the continuum
sensitivity level and at the angular resolution allowed by the highest sampled
frequency.Comment: 17 pages, 11 figure
Product Specialization, Efficiency and Productivity Change in the Spanish Insurance Industry
In this paper we analyze the levels of technical efficiency and productivity growth attained by Spanish insurance companies during a period of deregulation. We compute Malmquist productivity indexes using the estimates of parametric distance function for several specialized insurance branches. In this way, we show that branch specialization matters a great deal and that firms combining two or three product lines (Health, Property-Liabilities and Life) perform better than firms operating in one insurance line exclusively. In the light of these results, we recommend that the remaining restrictions coming from the European Third Directives on the operations of multi-branch firms should be removed. Moreover, from a management point of view, it would be appropriate to encourage the creation of multi-branch insurance firms. However, in all cases, the estimated scores indicate low productivity growth (less than 2% per year) compared with a huge increase in insurance activity (premiums were multiplied by nearly 3 in a decade).Efficiency, parametric Malmquist index, output specialization, Spanish insurance
Robust semi-automated path extraction for visualising stenosis of the coronary arteries
Computed tomography angiography (CTA) is useful for diagnosing and planning treatment of heart disease. However, contrast agent in surrounding structures (such as the aorta and left ventricle) makes 3-D visualisation of the coronary arteries difficult. This paper presents a composite method employing segmentation and volume rendering to overcome this issue. A key contribution is a novel Fast Marching minimal path cost function for vessel centreline extraction. The resultant centreline is used to compute a measure of vessel lumen, which indicates the degree of stenosis (narrowing of a vessel). Two volume visualisation techniques are presented which utilise the segmented arteries and lumen measure. The system is evaluated and demonstrated using synthetic and clinically obtained datasets
The detection of globular clusters in galaxies as a data mining problem
We present an application of self-adaptive supervised learning classifiers
derived from the Machine Learning paradigm, to the identification of candidate
Globular Clusters in deep, wide-field, single band HST images. Several methods
provided by the DAME (Data Mining & Exploration) web application, were tested
and compared on the NGC1399 HST data described in Paolillo 2011. The best
results were obtained using a Multi Layer Perceptron with Quasi Newton learning
rule which achieved a classification accuracy of 98.3%, with a completeness of
97.8% and 1.6% of contamination. An extensive set of experiments revealed that
the use of accurate structural parameters (effective radius, central surface
brightness) does improve the final result, but only by 5%. It is also shown
that the method is capable to retrieve also extreme sources (for instance, very
extended objects) which are missed by more traditional approaches.Comment: Accepted 2011 December 12; Received 2011 November 28; in original
form 2011 October 1
Refraction-corrected ray-based inversion for three-dimensional ultrasound tomography of the breast
Ultrasound Tomography has seen a revival of interest in the past decade,
especially for breast imaging, due to improvements in both ultrasound and
computing hardware. In particular, three-dimensional ultrasound tomography, a
fully tomographic method in which the medium to be imaged is surrounded by
ultrasound transducers, has become feasible. In this paper, a comprehensive
derivation and study of a robust framework for large-scale bent-ray ultrasound
tomography in 3D for a hemispherical detector array is presented. Two
ray-tracing approaches are derived and compared. More significantly, the
problem of linking the rays between emitters and receivers, which is
challenging in 3D due to the high number of degrees of freedom for the
trajectory of rays, is analysed both as a minimisation and as a root-finding
problem. The ray-linking problem is parameterised for a convex detection
surface and three robust, accurate, and efficient ray-linking algorithms are
formulated and demonstrated. To stabilise these methods, novel
adaptive-smoothing approaches are proposed that control the conditioning of the
update matrices to ensure accurate linking. The nonlinear UST problem of
estimating the sound speed was recast as a series of linearised subproblems,
each solved using the above algorithms and within a steepest descent scheme.
The whole imaging algorithm was demonstrated to be robust and accurate on
realistic data simulated using a full-wave acoustic model and an anatomical
breast phantom, and incorporating the errors due to time-of-flight picking that
would be present with measured data. This method can used to provide a
low-artefact, quantitatively accurate, 3D sound speed maps. In addition to
being useful in their own right, such 3D sound speed maps can be used to
initialise full-wave inversion methods, or as an input to photoacoustic
tomography reconstructions
- …