355 research outputs found
Can coarse-graining introduce long-range correlations in a symbolic sequence?
We present an exactly solvable mean-field-like theory of correlated ternary
sequences which are actually systems with two independent parameters. Depending
on the values of these parameters, the variance on the average number of any
given symbol shows a linear or a superlinear dependence on the length of the
sequence. We have shown that the available phase space of the system is made up
a diffusive region surrounded by a superdiffusive region. Motivated by the fact
that the diffusive portion of the phase space is larger than that for the
binary, we have studied the mapping between these two. We have identified the
region of the ternary phase space, particularly the diffusive part, that gets
mapped into the superdiffusive regime of the binary. This exact mapping implies
that long-range correlation found in a lower dimensional representative
sequence may not, in general, correspond to the correlation properties of the
original system.Comment: 10 pages including 1 figur
Thermoacoustic tomography with detectors on an open curve: an efficient reconstruction algorithm
Practical applications of thermoacoustic tomography require numerical
inversion of the spherical mean Radon transform with the centers of integration
spheres occupying an open surface. Solution of this problem is needed (both in
2-D and 3-D) because frequently the region of interest cannot be completely
surrounded by the detectors, as it happens, for example, in breast imaging. We
present an efficient numerical algorithm for solving this problem in 2-D
(similar methods are applicable in the 3-D case). Our method is based on the
numerical approximation of plane waves by certain single layer potentials
related to the acquisition geometry. After the densities of these potentials
have been precomputed, each subsequent image reconstruction has the complexity
of the regular filtration backprojection algorithm for the classical Radon
transform. The peformance of the method is demonstrated in several numerical
examples: one can see that the algorithm produces very accurate reconstructions
if the data are accurate and sufficiently well sampled, on the other hand, it
is sufficiently stable with respect to noise in the data
Diffraction microtomography with sample rotation: influence of a missing apple core in the recorded frequency space
Diffraction microtomography in coherent light is foreseen as a promising
technique to image transparent living samples in three dimensions without
staining. Contrary to conventional microscopy with incoherent light, which
gives morphological information only, diffraction microtomography makes it
possible to obtain the complex optical refractive index of the observed sample
by mapping a three-dimensional support in the spatial frequency domain. The
technique can be implemented in two configurations, namely, by varying the
sample illumination with a fixed sample or by rotating the sample using a fixed
illumination. In the literature, only the former method was described in
detail. In this report, we precisely derive the three-dimensional frequency
support that can be mapped by the sample rotation configuration. We found that,
within the first-order Born approximation, the volume of the frequency domain
that can be mapped exhibits a missing part, the shape of which resembles that
of an apple core. The projection of the diffracted waves in the frequency space
onto the set of sphere caps covered by the sample rotation does not allow for a
complete mapping of the frequency along the axis of rotation due to the finite
radius of the sphere caps. We present simulations of the effects of this
missing information on the reconstruction of ideal objects.Comment: 7 pages, 11 figures, presented at Focus On Microscopy 200
Adapting “MOVE” to accelerate VMMC coverage for HIV prevention in priority populations:Implementation experiences from Uganda’s military settings
This paper describes the WHO’s Model of Optimizing Volumes and Efficiencies (MOVE), adapted by the University Research Council (URC) - Department of Defense HIV/AIDS Prevention Program (DHAPP) to rapidly scale up Voluntary Medical Male Circumcision (VMMC) within Uganda’s military health facilities. First, we examine the MOVE model and then present the URC-DHAPP adapted intervention package comprising of: a) a Command-driven approach, b) Mobile theatres c) Quality assurance d) Data strengthening and reflection. To expand VMMC, URC-DHAPP worked with army commanders to create awareness, mobilize their troops and surgeons were assigned daily targets. The mobile theatre involved regular visits to hard-to-reach outposts and placing several mobile camps at health facilities close to deployment sites. All stakeholders were briefed on performance trends of previous medical camps and the program was monitored through VMMC camp reports. URC-DHAPP registered an exponential increase in VMMC coverage from 13% performance at Q2 to over 140% in Q4. The integrated approach led to circumcision of over 22,000 men (15-49 years) in a record four months. Our approach also contributed to health system strengthening and national HIV preventiontargets. We conclude that the MOVE is cost-effective and can be successfully scaled up in resource-limited settings with a high HIV burden when implemented with cognizance of contextual specificities
Rationing tests for drug-resistant tuberculosis - who are we prepared to miss?
BACKGROUND: Early identification of patients with drug-resistant tuberculosis (DR-TB) increases the likelihood of treatment success and interrupts transmission. Resource-constrained settings use risk profiling to ration the use of drug susceptibility testing (DST). Nevertheless, no studies have yet quantified how many patients with DR-TB this strategy will miss. METHODS: A total of 1,545 subjects, who presented to Lima health centres with possible TB symptoms, completed a clinic-epidemiological questionnaire and provided sputum samples for TB culture and DST. The proportion of drug resistance in this population was calculated and the data was analysed to demonstrate the effect of rationing tests to patients with multidrug-resistant TB (MDR-TB) risk factors on the number of tests needed and corresponding proportion of missed patients with DR-TB. RESULTS: Overall, 147/1,545 (9.5%) subjects had culture-positive TB, of which 32 (21.8%) had DR-TB (MDR, 13.6%; isoniazid mono-resistant, 7.5%; rifampicin mono-resistant, 0.7%). A total of 553 subjects (35.8%) reported one or more MDR-TB risk factors; of these, 506 (91.5%; 95% CI, 88.9-93.7%) did not have TB, 32/553 (5.8%; 95% CI, 3.4-8.1%) had drug-susceptible TB, and only 15/553 (2.7%; 95% CI, 1.5-4.4%) had DR-TB. Rationing DST to those with an MDR-TB risk factor would have missed more than half of the DR-TB population (17/32, 53.2%; 95% CI, 34.7-70.9). CONCLUSIONS: Rationing DST based on known MDR-TB risk factors misses an unacceptable proportion of patients with drug-resistance in settings with ongoing DR-TB transmission. Investment in diagnostic services to allow universal DST for people with presumptive TB should be a high priority
Detecting small low emission radiating sources
The article addresses the possibility of robust detection of geometrically
small, low emission sources on a significantly stronger background. This
problem is important for homeland security. A technique of detecting such
sources using Compton type cameras is developed, which is shown on numerical
examples to have high sensitivity and specificity and also allows to assign
confidence probabilities of the detection. 2D case is considered in detail
Combination of fast-ion diagnostics in velocity-space tomographies:Paper
Fast-ion Dα (FIDA) and collective Thomson scattering (CTS) diagnostics provide indirect measurements of fastion velocity distribution functions in magnetically confined plasmas. Here we present the first prescription for
velocity-space tomographic inversion of CTS and FIDA measurements that can use CTS and FIDA measurements
together and that takes uncertainties in such measurements into account. Our prescription is general and could
be applied to other diagnostics. We demonstrate tomographic reconstructions of an ASDEX Upgrade beam ion
velocity distribution function. First, we compute synthetic measurements from two CTS views and two FIDA
views using a TRANSP/NUBEAM simulation, and then we compute joint tomographic inversions in velocity-space from these. The overall shape of the 2D velocity distribution function and the location of the maxima at full and half beam injection energy are well reproduced in velocity-space tomographic inversions, if the noise level in the measurements is below 10%. Our results suggest that 2D fast-ion velocity distribution functions can be directly inferred from fast-ion measurements and their uncertainties, even if the measurements are taken with different diagnostic methods
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