3,446 research outputs found
Motion Correction via Locally Linear Embedding for Helical Photon-counting CT
X-ray photon-counting detector (PCD) offers low noise, high resolution, and
spectral characterization, representing a next generation of CT and enabling
new biomedical applications. It is well known that involuntary patient motion
may induce image artifacts with conventional CT scanning, and this problem
becomes more serious with PCD due to its high detector pitch and extended scan
time. Furthermore, PCD often comes with a substantial number of bad pixels,
making analytic image reconstruction challenging and ruling out
state-of-the-art motion correction methods that are based on analytical
reconstruction. In this paper, we extend our previous locally linear embedding
(LLE) cone-beam motion correction method to the helical scanning geometry,
which is especially desirable given the high cost of large-area PCD. In
addition to our adaption of LLE-based parametric searching to helical cone-beam
photon-counting CT geometry, we introduce an unreliable-volume mask to improve
the motion estimation accuracy and perform incremental updating on gradually
refined sampling grids for optimization of both accuracy and efficiency. Our
numerical results demonstrate that our method reduces the estimation errors
near the two longitudinal ends of the reconstructed volume and overall image
quality. The experimental results on clinical photon-counting scans of the
patient extremities show significant resolution improvement after motion
correction using our method, which reveals subtle fine structures previously
hidden under motion blurring and artifacts
Fundamental Performance of a Dispersed Fixed Delay Interferometer In Searching For Planets Around M Dwarfs
We present a new method to calculate fundamental Doppler measurement limits
with a dispersed fixed-delay interferometer (DFDI) in the near infrared
wavelength region for searching for exoplanets around M dwarfs in the coming
decade. It is based on calculating the Q factor, a measure of flux-normalized
Doppler sensitivity in the fringing spectra created with DFDI. We calculate the
Q factor as a function of spectral resolution R, stellar projected rotational
velocity V sini, stellar effective temperature T_eff and optical path
difference (OPD) of the interferometer. We also compare the DFDI Q factor to
that for the popular cross-dispersed echelle spectrograph method (the direct
echelle (DE) method). Given the IR Doppler measurement is likely to be
detector-limited for a while, we introduce new merit functions, which is
directly related to photon-limited RV uncertainty, to evaluate Doppler
performance with the DFDI and DE methods. We find that DFDI has strength in
wavelength coverage and multi-object capability over the DE for a limited
detector resource. We simulate the performance of the InfraRed Exoplanet
Tracker (IRET) based on the DFDI design, being considered for the next
generation IR Doppler measurements. The predicted photon-limited RV uncertainty
suggests that IRET is capable of detecting Earth-like exoplanets in habitable
zone around nearby bright M dwarfs if they exist. A new method is developed to
quantitatively estimate the influence of telluric lines on RV uncertainty. Our
study shows that photon-limited RV uncertainty can be reached if 99% of the
strength of telluric lines can be removed from the measured stellar spectra. At
low to moderate levels of telluric line strength removal (50% to 90%), the
optimal RV uncertainty is typically a factor of 2-3 times larger than
photon-limited RV uncertainty.Comment: 43 pages, 20 figures, 6 tables. Accepted by Ap
Impact of a hospice rapid response service on preferred place of death, and costs
Background: Many people with a terminal illness would prefer to die at home. A new palliative rapid response service (RRS) provided by a large hospice provider in South East England was evaluated (2010) to provide evidence of impact on achieving preferred place of death and costs. The RRS was delivered by a team of trained health care assistants and available 24/7. The purpose of this study was to (i) compare the characteristics of RRS users and non-users, (ii) explore differences in the proportions of users and non-users dying in the place of their choice, (iii) monitor the whole system service utilisation of users and non-users, and compare costs. Methods: All hospice patients who died with a preferred place of death recorded during an 18 month period were included. Data (demographic, preferences for place of death) were obtained from hospice records. Dying in preferred place was modelled using stepwise logistic regression analysis. Service use data (period between referral to hospice and death) were obtained from general practitioners, community providers, hospitals, social services, hospice, and costs calculated using validated national tariffs. Results: Of 688 patients referred to the hospice when the RRS was operational, 247 (35.9 %) used it. Higher proportions of RRS users than non-users lived in their own homes with a co-resident carer (40.3 % vs. 23.7 %); more non-users lived alone or in residential care (58.8 % vs. 76.3 %). Chances of dying in the preferred place were enhanced 2.1 times by being a RRS user, compared to a non-user, and 1.5 times by having a co-resident carer, compared to living at home alone or in a care home. Total service costs did not differ between users and non-users, except when referred to hospice very close to death (users had higher costs). Conclusions: Use of the RRS was associated with increased likelihood of dying in the preferred place. The RRS is cost neutral
A generalization of the q-Saalschutz sum and the Burge transform
A generalization of the q-(Pfaff)-Saalschutz summation formula is proved.
This implies a generalization of the Burge transform, resulting in an
additional dimension of the ``Burge tree''. Limiting cases of our summation
formula imply the (higher-level) Bailey lemma, provide a new decomposition of
the q-multinomial coefficients, and can be used to prove the Lepowsky and Primc
formula for the A_1^{(1)} string functions.Comment: 18 pages, AMSLaTe
A mixed effects model to estimate timing and intensity of pubertal growth from height and secondary sexual characteristics
Abstract Aim: To estimate and compare pubertal growth timing and intensity in height, Tanner stage markers and testis volume. Subjects and methods: Data on height, genital stage, breast stage and pubic hair stage, testis volume and menarche in 103 boys and 74 girls from the Edinburgh Longitudinal Growth Study were analysed. The SITAR model for height and a novel mixed effects logistic model for Tanner stage and testis volume provided estimates of peak velocity (PV, intensity) and age at peak velocity (APV, timing), both overall (from fixed effects) and for individuals (random effects). Results: Based on the six markers, mean APV was 13.0-14.0 years in boys and 12.0-13.1 years in girls, with between-subject standard deviations of ∼1 year. PV for height was 8-9 cm/year by sex and for testis volume 6 ml/year, while Tanner stage increased by 1.2-1.8 stages per year at its peak. The correlations across markers for APV were 0.6-0.8 for boys and 0.8-0.92 for girls, very significantly higher for girls (p = 0.005). Correlations for PV were lower, -0.2-0.6. Conclusions: The mixed effects models perform well in estimating timing and intensity in individuals across several puberty markers. Age at peak velocity correlates highly across markers, but peak velocity less so
Detectability of Exoplanetary Transits from Radial Velocity Surveys
Of the known transiting extra-solar planets, a few have been detected through
photometric follow-up observations of radial velocity planets. Perhaps the best
known of these is the transiting exoplanet HD 209458b. For hot Jupiters
(periods less than ~5 days), the a priori information that 10% of these planets
will transit their parent star due to the geometric transit probability leads
to an estimate of the expected transit yields from radial velocity surveys. The
radial velocity information can be used to construct an effective photometric
follow-up strategy which will provide optimal detection of possible transits.
Since the planet-harbouring stars are already known in this case, one is only
limited by the photometric precision achieveable by the chosen
telescope/instrument. The radial velocity modelling code presented here
automatically produces a transit ephemeris for each planet dataset fitted by
the program. Since the transit duration is brief compared with the fitted
period, we calculate the maximum window for obtaining photometric transit
observations after the radial velocity data have been obtained, generalising
for eccentric orbits. We discuss a typically employed survey strategy which may
contribute to a possible radial velocity bias against detection of the very hot
Jupiters which have dominated the transit discoveries. Finally, we describe how
these methods can be applied to current and future radial velocity surveys.Comment: 11 pages, 8 figures, accepted for publication in MNRAS, minor
correctio
Transgenic expression of phytase and acid phosphatase genes in alfalfa (Medicago sativa) leads to improved phosphate uptake in natural soils
Alfalfa (Medicagosativa L.) is one of the most widely grown crops in the USA. Phosphate (P) deficiency is common in areas where forage crops are grown. To improve the use of organic phosphate by alfalfa, two Medicagotruncatula genes, phytase (MtPHY1) and purple acid phosphatase (MtPAP1), were overexpressed in alfalfa under the control of the constitutive CaMV35S promoter or the root-specific MtPT1 promoter. Root enzyme activity analyses revealed that although both genes lead to similar levels of acid phosphatase activities, overexpression of the MtPHY1 gene usually results in a higher level of phytase activity than overexpression of the MtPAP1 gene. The MtPT1 promoter was more effective than the CaMV35S promoter in regulating gene expression and extracellular secretion under P-deficient conditions. Measurement of growth performance of the transgenic lines further proved that the best promoter–gene combination is the MtPHY1 gene driven by the MtPT1 promoter. Compared to the control, the plants with high levels of transgene expression showed improved growth. The biomass of several transgenic lines was three times that of the control when plants were grown in sand supplied with phytate as the sole P source. When the plants were grown in natural soils without additional P supplement, the best performing transgenic lines produced double the amount of biomass after 12 weeks (two cuts) of growth. Transgene effects were more obvious in soil with lower pH and lower natural P reserves than in soil with neutral pH and relatively higher P storage. The total P concentration in leaf tissues of the high-expressing transgenic lines was significantly higher than that of the control. The transgenes have great potential for improving plant P acquisition and biomass yield in P-deficient agricultural soils. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11032-011-9628-0) contains supplementary material, which is available to authorized users
The Frequency of Hot Jupiters Orbiting Nearby Solar-Type Stars
We determine the fraction of F, G, and K dwarfs in the Solar Neighborhood
hosting hot jupiters as measured by the California Planet Survey from the Lick
and Keck planet searches. We find the rate to be 1.2\pm0.38%, which is
consistent with the rate reported by Mayor et al. (2011) from the HARPS and
CORALIE radial velocity surveys. These numbers are more than double the rate
reported by Howard et al. (2011) for Kepler stars and the rate of Gould et al.
(2006) from the OGLE-III transit search, however due to small number statistics
these differences are of only marginal statistical significance. We explore
some of the difficulties in estimating this rate from the existing radial
velocity data sets and comparing radial velocity rates to rates from other
techniques.Comment: 6pp emulateapj, 2 tables, ApJ accepte
Quantum oscillations of nitrogen atoms in uranium nitride
The vibrational excitations of crystalline solids corresponding to acoustic
or optic one phonon modes appear as sharp features in measurements such as
neutron spectroscopy. In contrast, many-phonon excitations generally produce a
complicated, weak, and featureless response. Here we present time-of-flight
neutron scattering measurements for the binary solid uranium nitride (UN),
showing well-defined, equally-spaced, high energy vibrational modes in addition
to the usual phonons. The spectrum is that of a single atom, isotropic quantum
harmonic oscillator and characterizes independent motions of light nitrogen
atoms, each found in an octahedral cage of heavy uranium atoms. This is an
unexpected and beautiful experimental realization of one of the fundamental,
exactly-solvable problems in quantum mechanics. There are also practical
implications, as the oscillator modes must be accounted for in the design of
generation IV nuclear reactors that plan to use UN as a fuel.Comment: 25 pages, 10 figures, submitted to Nature Communications,
supplementary information adde
Porosity detection in electron beam-melted Ti-6Al-4V using high-resolution neutron imaging and grating-based interferometry
© 2017, Springer International Publishing Switzerland. A high-resolution neutron tomography system and a grating-based interferometer are used to explore electron beam-melted titanium test objects. The high-resolution neutron tomography system (attenuation-based imaging) has a pixel size of 6.4 µm, appropriate for detecting voids near 25 µm over a (1.5 cm)3 volume. The neutron interferometer provides dark-field (small-angle scattering) images with a pixel size of 30 µm. Moreover, the interferometer can be tuned to a scattering length, in this case, 1.97 µm, with a field-of-view of (6 cm)3. The combination of high-resolution imaging with grating-based interferometry provides a way for nondestructive testing of defective titanium samples. A chimney-like pore structure was discovered in the attenuation and dark-field images along one face of an electron beam-melted (EBM) Ti-6Al-4V cube. Tomographic reconstructions of the titanium samples are utilized as a source for a binary volume and for skeletonization of the pores. The dark-field volume shows features with dimensions near and smaller than the interferometer auto-correlation scattering length
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