16,850 research outputs found
Learning-based Ensemble Average Propagator Estimation
By capturing the anisotropic water diffusion in tissue, diffusion magnetic
resonance imaging (dMRI) provides a unique tool for noninvasively probing the
tissue microstructure and orientation in the human brain. The diffusion profile
can be described by the ensemble average propagator (EAP), which is inferred
from observed diffusion signals. However, accurate EAP estimation using the
number of diffusion gradients that is clinically practical can be challenging.
In this work, we propose a deep learning algorithm for EAP estimation, which is
named learning-based ensemble average propagator estimation (LEAPE). The EAP is
commonly represented by a basis and its associated coefficients, and here we
choose the SHORE basis and design a deep network to estimate the coefficients.
The network comprises two cascaded components. The first component is a
multiple layer perceptron (MLP) that simultaneously predicts the unknown
coefficients. However, typical training loss functions, such as mean squared
errors, may not properly represent the geometry of the possibly non-Euclidean
space of the coefficients, which in particular causes problems for the
extraction of directional information from the EAP. Therefore, to regularize
the training, in the second component we compute an auxiliary output of
approximated fiber orientation (FO) errors with the aid of a second MLP that is
trained separately. We performed experiments using dMRI data that resemble
clinically achievable -space sampling, and observed promising results
compared with the conventional EAP estimation method.Comment: Accepted by MICCAI 201
Electronic Band Structure In A Periodic Magnetic Field
We analyze the energy band structure of a two-dimensional electron gas in a
periodic magnetic field of a longitudinal antiferromagnet by considering a
simple exactly solvable model. Two types of states appear: with a finite and
infinitesimal longitudinal mobility. Both types of states are present at a
generic Fermi surface. The system exhibits a transition to an insulating regime
with respect to the longitudinal current, if the electron density is
sufficiently low.Comment: 8 pages, 5 figures; to appear in Phys. Rev. B '9
Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope
We present a study of the intrinsic absorption lines in the ultraviolet
spectra of Seyfert 1 galaxies. We find that the fraction of Seyfert 1 galaxies
that show absorption associated with their active nuclei is more than one-half
(10/17), which is much higher than previous estimates (3 - 10%) . There is a
one-to-one correspondence between Seyferts that show intrinsic UV absorption
and X-ray ``warm absorbers''. The intrinsic UV absorption is generally
characterized by high ionization: C IV and N V are seen in all 10 Seyferts with
detected absorption (in addition to Ly-alpha), whereas Si IV is present in only
four of these Seyferts, and Mg II absorption is only detected in NGC 4151. The
absorption lines are blueshifted (or in a few cases at rest) with respect to
the narrow emission lines, indicating that the absorbing gas is undergoing net
radial outflow. At high resolution, the absorption often splits into distinct
kinematic components that show a wide range in widths (20 - 400 km/s FWHM),
indicating macroscopic motions (e.g., radial velocity subcomponents or
turbulence) within a component. The strong absorption components have cores
that are much deeper than the continuum flux levels, indicating that the
regions responsible for these components lie completely outside of the broad
emission-line regions. The covering factor of the absorbing gas in the line of
sight, relative to the total underlying emission, is C > 0.86, on average. The
global covering factor, which is the fraction of emission intercepted by the
absorber averaged over all lines of sight, is C > 0.5.Comment: 56 pages, Latex, includes 4 figures (encapsulated postscript), Fig. 1
has 2 parts and Fig. 2 has 3 parts, to appear in the Astrophysical Journa
Synthetic Physiology
Optogenetic tools are DNA-encoded molecules that, when genetically targeted to cells, enable the control of specific physiological processes within those cells through exposure to light. These tools can pinpoint how these specific processes affect the emergent properties of a complex biological system, such as a mammalian organ or even an entire animal. They can also allow control of a biological system for therapeutic or bioengineering purposes. Many of the optical control tools explored to date are single-component reagents containing a photoactive signaling domain. An interesting question is raised by comparing optogenetics to synthetic biology. In the latter, interchangeable and modular DNA-encoded parts are assembled into complex biological circuits, thus enabling sophisticated logic and computation as well as the production of biologics and reagents (1, 2). Is it possible to devise strategies for the temporally precise cell-targeted optical control of complex engineered biological computational or chemical-synthetic pathways? Such a marriage of optogenetics and synthetic biologyâwhich one might call synthetic physiologyâwould open up the ability to use optogenetics to trigger and regulate engineered synthetic biology systems, which in turn could execute computational and biological programs of great complexity (3). On page 1565 of this issue, Ye et al. (4) explore such a hybrid approach to controlling a biological system, as well as the bioengineering and preclinical capabilities opened up by such an approach
Frequency-dependent magnetotransport and particle dynamics in magnetic modulation systems
We analyze the dynamics of a charged particle moving in the presence of
spatially-modulated magnetic fields. From Poincare surfaces of section and
Liapunov exponents for characteristic trajectories we find that the fraction of
pinned and runaway quasiperiodic orbits {\em vs}. chaotic orbits depends
strongly on the ratio of cyclotron radius to the structure parameters, as well
as on the amplitude of the modulated field. We present a complete
characterization of the dynamical behavior of such structures, and investigate
the contribution to the magnetoconductivity from all different orbits using a
classical Kubo formula. Although the DC conductivity of the system depends
strongly on the pinned and runaway trajectories, the frequency response
reflects the topology of all different orbits, and even their unusual temporal
behavior.Comment: Submitted to PRB - 14 figure files - REVTEX tex
A Systematic Approach for Inertial Sensor Calibration of Gravity Recovery Satellites and Its Application to Taiji-1 Mission
High-precision inertial sensors or accelerometers can provide us references
of free-falling motions in gravitational field in space. They serve as the key
payloads for gravity recovery missions such as the CHAMP, the GRACE-type
missions, and the planned Next Generation Gravity Missions. In this work, a
systematic method of electrostatic inertial sensor calibrations for gravity
recovery satellites is suggested, which is applied to and verified with the
Taiji-1 mission. With this method, the complete operating parameters including
the scale factors, the center of mass offset vector and the intrinsic biased
acceleration can be precisely calibrated with only two sets of short-term
in-orbit experiments. Taiji-1 is the first technology demonstration satellite
of the "Taiji Program in Space", which, in its final extended phase in 2022,
could be viewed as operating in the mode of a high-low satellite-to-satellite
tracking gravity mission. Based on the calibration principles, swing maneuvers
with time span about 200 s and rolling maneuvers for 19 days were conducted by
Taiji-1 in 2022. The inertial sensor's operating parameters are precisely
re-calibrated with Kalman filters and are updated to the Taiji-1 science team.
Data from one of the sensitive axis is re-processed with the updated operating
parameters, and the performance is found to be slightly improved compared with
former results. This approach could be of high reference value for the
accelerometer or inertial sensor calibrations of the GFO, the Chinese
GRACE-type mission, and the Next Generation Gravity Missions. This could also
shed some light on the in-orbit calibrations of the ultra-precision inertial
sensors for future GW space antennas because of the technological inheritance
between these two generations of inertial sensors.Comment: 24 pages, 19 figure
In ovo serial skeletal muscle diffusion tractography of the developing chick embryo using DTI: feasibility and correlation with histology
Abstract
Background
Magnetic resonance imaging is a noninvasive method of evaluating embryonic development. Diffusion tensor imaging (DTI), based on the directional diffusivity of water molecules, is an established method of evaluating tissue structure. Yet embryonic motion degrades the in vivo acquisition of long-duration DTI. We used a dual-cooling technique to avoid motion artifact and aimed to investigate whether DTI can be used to monitor chick embryonic skeletal muscle development in ovo, and to investigate the correlation between quantitative DTI parameters fractional anisotropy (FA) and fiber length and quantitative histologic parameters fiber area percentage (FiberArea%) and limb length.
Results
From 84 normally developing chick embryos, 5 were randomly chosen each day from incubation days 5 to 18 and scanned using 3.0 Tesla magnetic resonance imaging. A dual-cooling technique is used before and during imaging. Eggs were cracked for making histological specimen after imaging. 3 eggs were serially imaged from days 5 to 18. We show that skeletal muscle fibers can be tracked in hind limb in DTI beginning with incubation day 8. Our data shows a good positive correlation between quantitative DTI and histologic parameters (FA vs FiberArea%: r= 0.943, p\u3c0.0001; Fiber_length vs Limb_length: r=0.974, p\u3c0.0001). The result of tracked fibers in DTI during incubation corresponds to the development of chick embryonic skeletal muscle as reported in the literature.
Conclusion
Diffusion tensor imaging can provide a noninvasive means of evaluating skeletal muscle development in ovo
One at a time, live tracking of NGF axonal transport using quantum dots
Retrograde axonal transport of nerve growth factor (NGF) signals is critical for the survival, differentiation, and maintenance of peripheral sympathetic and sensory neurons and basal forebrain cholinergic neurons. However, the mechanisms by which the NGF signal is propagated from the axon terminal to the cell body are yet to be fully elucidated. To gain insight into the mechanisms, we used quantum dot-labeled NGF (QD-NGF) to track the movement of NGF in real time in compartmentalized culture of rat dorsal root ganglion (DRG) neurons. Our studies showed that active transport of NGF within the axons was characterized by rapid, unidirectional movements interrupted by frequent pauses. Almost all movements were retrograde, but short-distance anterograde movements were occasionally observed. Surprisingly, quantitative analysis at the single molecule level demonstrated that the majority of NGF-containing endosomes contained only a single NGF dimer. Electron microscopic analysis of axonal vesicles carrying QD-NGF confirmed this finding. The majority of QD-NGF was found to localize in vesicles 50â150 nm in diameter with a single lumen and no visible intralumenal membranous components. Our findings point to the possibility that a single NGF dimer is sufficient to sustain signaling during retrograde axonal transport to the cell body
- âŠ