4,198 research outputs found
Insight into the fundamental trade-offs of diffusion MRI from polarization-sensitive optical coherence tomography in ex vivo human brain
In the first study comparing high angular resolution diffusion MRI (dMRI) in the human brain to axonal orientation measurements from polarization-sensitive optical coherence tomography (PSOCT), we compare the accuracy of orientation estimates from various dMRI sampling schemes and reconstruction methods. We find that, if the reconstruction approach is chosen carefully, single-shell dMRI data can yield the same accuracy as multi-shell data, and only moderately lower accuracy than a full Cartesian-grid sampling scheme. Our results suggest that current dMRI reconstruction approaches do not benefit substantially from ultra-high b-values or from very large numbers of diffusion-encoding directions. We also show that accuracy remains stable across dMRI voxel sizes of 1 ​mm or smaller but degrades at 2 ​mm, particularly in areas of complex white-matter architecture. We also show that, as the spatial resolution is reduced, axonal configurations in a dMRI voxel can no longer be modeled as a small set of distinct axon populations, violating an assumption that is sometimes made by dMRI reconstruction techniques. Our findings have implications for in vivo studies and illustrate the value of PSOCT as a source of ground-truth measurements of white-matter organization that does not suffer from the distortions typical of histological techniques.Published versio
Power-line Interference Removal from ECG in Case of Power-line Frequency Variations
The original version of the most successful approach for power-line (PL) interference removal from ECG, called subtraction procedure, is based on linear segment detection in the signal and hardware synchronised analogue-to-digital conversion to cope with the PL frequency variations. However, this is not feasible for battery supplied devices and some computer-aided ECG systems. Recent improvements of the procedure apply software measurement of the frequency variations that allow a re-sampling of the contaminated signal with the rated PL frequency followed by interference removal and back re-sampling for restoration of the original time intervals. This study deals with a more accurate software frequency measurement and introduces a notch filtration as alternative to the procedure when no linear segments are encountered for long time, e.g. in cases of ventricular fibrillation or tachycardia. The result obtained with large PL frequency variations demonstrate very small errors, usually in the range of ± 20 μV for the subtraction procedure and ± 60 μV for the notch filtration, the last values strongly depending on the frequency contents of the QRS complexes
Estimation of tri-axial walking ground reaction forces of left and right foot from total forces in real-life environments
This is the final version of the article. Available from MDPI via the DOI in this record.Continuous monitoring of natural human gait in real-life environments is essential in many applications including disease monitoring, rehabilitation, and professional sports. Wearable inertial measurement units are successfully used to measure body kinematics in real-life environments and to estimate total walking ground reaction forces GRF(t) using equations of motion. However, for inverse dynamics and clinical gait analysis, the GRF(t) of each foot is required separately. Using an
experimental dataset of 1243 tri-axial separate-foot GRF(t) time histories measured by the authors across eight years, this study proposes the ‘Twin Polynomial Method’ (TPM) to estimate the tri-axial left and right foot GRF(t) signals from the total GRF(t) signals. For each gait cycle, TPM fits polynomials of degree five, eight, and nine to the known single-support part of the left and right foot vertical, anterior-posterior, and medial-lateral GRF(t) signals, respectively, to extrapolate the unknown double-support parts of the corresponding GRF(t) signals. Validation of the proposed method both with force plate measurements (gold standard) in the laboratory, and in real-life environment showed a peak-to-peak normalized root mean square error of less than 2.5%, 6.5% and 7.5% for the estimated GRF(t) signals in the vertical, anterior-posterior and medial-lateral directions, respectively. These values show considerable improvement compared with the currently available GRF(t) decomposition methods in the literature.The authors acknowledge the financial support provided by the UK Engineering and Physical
Sciences Research Council (EPSRC) for the following research grants: Frontier Engineering Grant EP/K03877X/1 (Modelling complex and partially identified engineering problems: Application to the individualized multiscale simulation of the musculoskeletal system); and Platform Grant EP/G061130/2 (Dynamic performance of large civil engineering structures: an integrated approach to management, design and assessment)
Multi-Target Tracking in Distributed Sensor Networks using Particle PHD Filters
Multi-target tracking is an important problem in civilian and military
applications. This paper investigates multi-target tracking in distributed
sensor networks. Data association, which arises particularly in multi-object
scenarios, can be tackled by various solutions. We consider sequential Monte
Carlo implementations of the Probability Hypothesis Density (PHD) filter based
on random finite sets. This approach circumvents the data association issue by
jointly estimating all targets in the region of interest. To this end, we
develop the Diffusion Particle PHD Filter (D-PPHDF) as well as a centralized
version, called the Multi-Sensor Particle PHD Filter (MS-PPHDF). Their
performance is evaluated in terms of the Optimal Subpattern Assignment (OSPA)
metric, benchmarked against a distributed extension of the Posterior
Cram\'er-Rao Lower Bound (PCRLB), and compared to the performance of an
existing distributed PHD Particle Filter. Furthermore, the robustness of the
proposed tracking algorithms against outliers and their performance with
respect to different amounts of clutter is investigated.Comment: 27 pages, 6 figure
Evaluating the accuracy of diffusion MRI models in white matter
Models of diffusion MRI within a voxel are useful for making inferences about
the properties of the tissue and inferring fiber orientation distribution used
by tractography algorithms. A useful model must fit the data accurately.
However, evaluations of model-accuracy of some of the models that are commonly
used in analyzing human white matter have not been published before. Here, we
evaluate model-accuracy of the two main classes of diffusion MRI models. The
diffusion tensor model (DTM) summarizes diffusion as a 3-dimensional Gaussian
distribution. Sparse fascicle models (SFM) summarize the signal as a linear sum
of signals originating from a collection of fascicles oriented in different
directions. We use cross-validation to assess model-accuracy at different
gradient amplitudes (b-values) throughout the white matter. Specifically, we
fit each model to all the white matter voxels in one data set and then use the
model to predict a second, independent data set. This is the first evaluation
of model-accuracy of these models. In most of the white matter the DTM predicts
the data more accurately than test-retest reliability; SFM model-accuracy is
higher than test-retest reliability and also higher than the DTM, particularly
for measurements with (a) a b-value above 1000 in locations containing fiber
crossings, and (b) in the regions of the brain surrounding the optic
radiations. The SFM also has better parameter-validity: it more accurately
estimates the fiber orientation distribution function (fODF) in each voxel,
which is useful for fiber tracking
Observations of Detailed Structure in the Solar Wind at 1 AU with STEREO/HI-2
Heliospheric imagers offer the promise of remote sensing of large-scale
structures present in the solar wind. The STEREO/HI-2 imagers, in particular,
offer high resolution, very low noise observations of the inner heliosphere but
have not yet been exploited to their full potential. This is in part because
the signal of interest, Thomson scattered sunlight from free electrons, is
~1000 times fainter than the background visual field in the images, making
background subtraction challenging. We have developed a procedure for
separating the Thomson-scattered signal from the other background/foreground
sources in the HI-2 data. Using only the Level 1 data from STEREO/HI-2, we are
able to generate calibrated imaging data of the solar wind with sensitivity of
a few times 1e-17 Bsun, compared to the background signal of a few times 1e-13
Bsun. These images reveal detailed spatial structure in CMEs and the solar wind
at projected solar distances in excess of 1 AU, at the instrumental motion-blur
resolution limit of 1-3 degree. CME features visible in the newly reprocessed
data from December 2008 include leading-edge pileup, interior voids,
filamentary structure, and rear cusps. "Quiet" solar wind features include V
shaped structure centered on the heliospheric current sheet, plasmoids, and
"puffs" that correspond to the density fluctuations observed in-situ. We
compare many of these structures with in-situ features detected near 1 AU. The
reprocessed data demonstrate that it is possible to perform detailed structural
analyses of heliospheric features with visible light imagery, at distances from
the Sun of at least 1 AU.Comment: Accepted by Astrophysical Journa
Topology of non-linear structure in the 2dF Galaxy Redshift Survey
We study the evolution of non-linear structure as a function of scale in
samples from the 2dF Galaxy Redshift Survey, constituting over 221 000 galaxies
at a median redshift of z=0.11. The two flux-limited galaxy samples, located
near the southern galactic pole and the galactic equator, are smoothed with
Gaussian filters of width ranging from 5 to 8 Mpc/h to produce a continuous
galaxy density field. The topological genus statistic is used to measure the
relative abundance of overdense clusters to void regions at each scale; these
results are compared to the predictions of analytic theory, in the form of the
genus statistic for i) the linear regime case of a Gaussian random field; and
ii) a first-order perturbative expansion of the weakly non-linear evolved
field. The measurements demonstrate a statistically significant detection of an
asymmetry in the genus statistic between regions corresponding to low- and
high-density volumes of the universe. We attribute the asymmetry to the
non-linear effects of gravitational evolution and biased galaxy formation, and
demonstrate that these effects evolve as a function of scale. We find that
neither analytic prescription satisfactorily reproduces the measurements,
though the weakly non-linear theory yields substantially better results in some
cases, and we discuss the potential explanations for this result.Comment: 13 pages, matching proof to be published in MNRAS; new version adds
reference and corrects figure
An Independent Analysis of Kepler-4b through Kepler-8b
We present two independent, homogeneous, global analyses of the transit light
curves, radial velocities and spectroscopy of Kepler-4b through Kepler-8b, with
numerous differences over the previous methods. These include: 1) improved
decorrelated parameter fitting set, 2) new limb darkening coefficients, 3)
time-stamps modified to BJD for consistency with RV, 4) two different methods
for compensating for the integration-time of Kepler LC-data, 5) best-fit
secondary-eclipse depths and excluded upper limits, 6) fitted mid-transit
times, durations, depths and baseline fluxes for individual transits. We make
several new determinations: 1) We detect a secondary eclipse for Kepler-7b of
depth (47+/-14)ppm and significance 3.5-sigma. We conclude reflected light is a
more plausible origin than thermal emission and determine a geometric albedo of
Ag=(0.38+/-0.12). 2) An eccentric-orbit model for the Neptune-mass planet
Kepler-4b is marginally detected to 2-sigma confidence with e=(0.25+/-0.12). If
confirmed, this would place Kepler-4b in a similar category as GJ436b and
HAT-P-11b as an eccentric, Neptune-mass planet. 3) We find marginal (2-sigma)
evidence for Kepler-5b's secondary eclipse of depth (26+/-17)ppm. The most
plausible explanation is reflected light caused by a planet with
Ag=(0.15+/-0.10). 4) A 2.6-sigma peak in Kepler-6b TTV periodogram is detected
and is not easily explained as an aliased frequency. We find that a
resonant/non-resonant perturber, Trojan or exomoon all provide inadequate
explanations and the most likely source is stellar rotation. 5) We find
different impact parameters relative to the discovery papers in most cases, but
generally self-consistent. 6) We constrain the presence of resonant planets,
exomoons and Trojans using transit timing.Comment: Revised version accepted for publication in ApJ. 34 pages, 26
figures, 12 table
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