2,787 research outputs found
Computed tomographic imaging characteristics of the normal canine lacrimal glands.
BackgroundThe canine lacrimal gland (LG) and accessory lacrimal gland of the third eyelid (TEG) are responsible for production of the aqueous portion of the precorneal tear film. Immune-mediated, toxic, neoplastic, or infectious processes can affect the glands directly or can involve adjacent tissues, with secondary gland involvement. Disease affecting these glands can cause keratoconjunctivitis sicca, corneal ulcers, and loss of vision. Due to their location in the orbit, these small structures are difficult to evaluate and measure, making cross-sectional imaging an important diagnostic tool. The detailed cross-sectional imaging appearance of the LG and TEG in dogs using computed tomography (CT) has not been reported to date.ResultsForty-two dogs were imaged, and the length, width, and height were measured and the volume calculated for the LGs & TEGs. The glands were best visualized in contrast-enhanced CT images. The mean volume of the LG was 0.14 cm3 and the TEG was 0.1 cm3. The mean height, width, and length of the LG were, 9.36 mm, 4.29 mm, and 9.35 mm, respectively; the corresponding values for the TEG was 2.02 mm, 9.34 mm, and 7.90 mm. LG and TEG volume were positively correlated with body weight (p < 0.05).ConclusionsContrast-enhanced CT is a valuable tool for noninvasive assessment of canine lacrimal glands
Lossy compression and real-time geovisualization for ultra-low bandwidth telemetry from untethered underwater vehicles
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2008Oceanographic applications of robotics are as varied as the undersea environment itself. As
underwater robotics moves toward the study of dynamic processes with multiple vehicles,
there is an increasing need to distill large volumes of data from underwater vehicles and
deliver it quickly to human operators. While tethered robots are able to communicate data
to surface observers instantly, communicating discoveries is more difficult for untethered
vehicles. The ocean imposes severe limitations on wireless communications; light is quickly
absorbed by seawater, and tradeoffs between frequency, bitrate and environmental effects
result in data rates for acoustic modems that are routinely as low as tens of bits per second.
These data rates usually limit telemetry to state and health information, to the exclusion
of mission-specific science data.
In this thesis, I present a system designed for communicating and presenting science
telemetry from untethered underwater vehicles to surface observers. The system's goals
are threefold: to aid human operators in understanding oceanographic processes, to enable
human operators to play a role in adaptively responding to mission-specific data, and to accelerate mission planning from one vehicle dive to the next. The system uses standard lossy
compression techniques to lower required data rates to those supported by commercially
available acoustic modems (O(10)-O(100) bits per second).
As part of the system, a method for compressing time-series science data based upon
the Discrete Wavelet Transform (DWT) is explained, a number of low-bitrate image compression techniques are compared, and a novel user interface for reviewing transmitted
telemetry is presented. Each component is motivated by science data from a variety of
actual Autonomous Underwater Vehicle (AUV) missions performed in the last year.National Science Foundation Center for Subsurface Sensing and Imaging (CenSSIS ERC
Progressively communicating rich telemetry from autonomous underwater vehicles via relays
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2012As analysis of imagery and environmental data plays a greater role in mission construction
and execution, there is an increasing need for autonomous marine vehicles
to transmit this data to the surface. Without access to the data acquired by a
vehicle, surface operators cannot fully understand the state of the mission. Communicating
imagery and high-resolution sensor readings to surface observers remains
a significant challenge – as a result, current telemetry from free-roaming
autonomous marine vehicles remains limited to ‘heartbeat’ status messages, with
minimal scientific data available until after recovery. Increasing the challenge, longdistance
communication may require relaying data across multiple acoustic hops
between vehicles, yet fixed infrastructure is not always appropriate or possible.
In this thesis I present an analysis of the unique considerations facing telemetry
systems for free-roaming Autonomous Underwater Vehicles (AUVs) used in exploration.
These considerations include high-cost vehicle nodes with persistent storage
and significant computation capabilities, combined with human surface operators
monitoring each node. I then propose mechanisms for interactive, progressive
communication of data across multiple acoustic hops. These mechanisms include
wavelet-based embedded coding methods, and a novel image compression scheme
based on texture classification and synthesis. The specific characteristics of underwater
communication channels, including high latency, intermittent communication,
the lack of instantaneous end-to-end connectivity, and a broadcast medium,
inform these proposals. Human feedback is incorporated by allowing operators to
identify segments of data thatwarrant higher quality refinement, ensuring efficient
use of limited throughput. I then analyze the performance of these mechanisms
relative to current practices.
Finally, I present CAPTURE, a telemetry architecture that builds on this analysis.
CAPTURE draws on advances in compression and delay tolerant networking to
enable progressive transmission of scientific data, including imagery, across multiple acoustic hops. In concert with a physical layer, CAPTURE provides an endto-
end networking solution for communicating science data from autonomous marine
vehicles. Automatically selected imagery, sonar, and time-series sensor data
are progressively transmitted across multiple hops to surface operators. Human
operators can request arbitrarily high-quality refinement of any resource, up to an
error-free reconstruction. The components of this system are then demonstrated
through three field trials in diverse environments on SeaBED, OceanServer and
Bluefin AUVs, each in different software architectures.Thanks to the National Science Foundation, and the
National Oceanic and Atmospheric Administration for
their funding of my education and this work
English Learning and Learning Disabilities: Has Research Made Its Way into Practice?
Fundamental to the hypothesis-driven approach to assessing the special-education (SPED) needs of English learners (ELs) is the awareness that behaviors often exhibited by ELs may mirror indicators of learning disabilities (e.g., difficulty comprehending text, difficulty following directions, lack of appropriate classroom behavior, lack of attention). We surveyed 330 school professionals to examine their perceptions about four mirrored behaviors displayed by ELs during instruction, as indicators of either a second-language difficulty or a learning disability. Only 31% of the respondents correctly indicated that none of the behaviors can distinguish these two possibilities. The probability of correct assessment was positively associated with the level of English-as-a-Second Language (ESL) training for those with less than 15 years of teaching experience and negatively associated for those with more than 20 years of experience. Correct assessment was positively associated with years of teaching experience for those with no ESL training but negatively associated for those with any level of ESL training. Correct assessment was not generally associated with the level of SPED training. From these results, we derive recommendations for improving the training of all pre- and in-service teachers in ESL instruction to better equip them to discriminate ELs’ language difficulties from learning disabilities
Exact 1-D Model for Coherent Synchrotron Radiation with Shielding and Bunch Compression
Coherent Synchrotron Radiation has been studied effectively using a
1-dimensional model for the charge distribution in the realm of small angle
approximations and high energies. Here we use Jefimenko's form of Maxwell's
equations, without such approximations, to calculate the exact wake-fields due
to this effect in multiple bends and drifts. It has been shown before that the
influence of a drift can propagate well into a subsequent bend. We show, for
reasonable parameters, that the influence of a previous bend can also propagate
well into a subsequent bend, and that this is especially important at the
beginning of a bend. Shielding by conducting parallel plates is simulated using
the image charge method. We extend the formalism to situations with compressing
and decompressing distributions, and conclude that simpler approximations to
bunch compression usually overestimates the effect. Additionally, an exact
formula for the coherent power radiated by a Gaussian bunch is derived by
considering the coherent synchrotron radiation spectrum, and is used to check
the accuracy of wake-field calculations
Theory and optimisation of radiative recombination in broken-gap InAs/GaSb superlattices
We present a theoretical analysis of mid-infrared radiative recombination in
InAs/GaSb superlattices (SLs). We employ a semi-analytical plane wave expansion
method in conjunction with an 8-band Hamiltonian
to compute the SL electronic structure, paying careful attention to the
identification and mitigation of spurious solutions. The calculated SL
eigenstates are used directly to compute spontaneous emission spectra and the
radiative recombination coefficient . We elucidate the origin of the
relatively large coefficients in InAs/GaSb SLs which, despite the presence
of spatially indirect (type-II-like) carrier confinement, are close to that of
bulk InAs and compare favourably to those calculated for mid-infrared type-I
pseudomorphic and metamorphic quantum well structures having comparable
emission wavelengths. Our analysis explicitly quantifies the roles played by
carrier localisation (specifically, partial delocalisation of bound electron
states) and miniband formation (specifically, miniband occupation and optical
selection rules) in determining the magnitude of and its temperature
dependence. We perform a high-throughput optimisation of the room temperature
coefficient in InAs/GaSb SLs across the 3.5 -- 7 m wavelength range,
quantifying the dependence of on the relative thickness of the
electron-confining InAs and hole-confining GaSb layers. This analysis provides
guidance for the growth of optimised SLs for mid-infrared light emitters. Our
results, combined with the expected low non-radiative Auger recombination rates
in structures having spatially indirect electron and hole confinement,
corroborate recently observed high output power in prototype InAs/GaSb SL
inter-band cascade light-emitting diodes.Comment: Published versio
Equitability revisited: why the “equitable threat score” is not equitable
In the forecasting of binary events, verification measures that are “equitable” were defined by Gandin and Murphy to satisfy two requirements: 1) they award all random forecasting systems, including those that always issue the same forecast, the same expected score (typically zero), and 2) they are expressible as the linear weighted sum of the elements of the contingency table, where the weights are independent of the entries in the table, apart from the base rate. The authors demonstrate that the widely used “equitable threat score” (ETS), as well as numerous others, satisfies neither of these requirements and only satisfies the first requirement in the limit of an infinite sample size. Such measures are referred to as “asymptotically equitable.” In the case of ETS, the expected score of a random forecasting system is always positive and only falls below 0.01 when the number of samples is greater than around 30. Two other asymptotically equitable measures are the odds ratio skill score and the symmetric extreme dependency score, which are more strongly inequitable than ETS, particularly for rare events; for example, when the base rate is 2% and the sample size is 1000, random but unbiased forecasting systems yield an expected score of around −0.5, reducing in magnitude to −0.01 or smaller only for sample sizes exceeding 25 000. This presents a problem since these nonlinear measures have other desirable properties, in particular being reliable indicators of skill for rare events (provided that the sample size is large enough). A potential way to reconcile these properties with equitability is to recognize that Gandin and Murphy’s two requirements are independent, and the second can be safely discarded without losing the key advantages of equitability that are embodied in the first. This enables inequitable and asymptotically equitable measures to be scaled to make them equitable, while retaining their nonlinearity and other properties such as being reliable indicators of skill for rare events. It also opens up the possibility of designing new equitable verification measures
MASE: A New Data--Reduction Pipeline for the Magellan Echellette Spectrograph
We introduce a data reduction package written in Interactive Data Language
(IDL) for the Magellan Echellete Spectrograph (MAGE). MAGE is a
medium-resolution (R ~4100), cross-dispersed, optical spectrograph, with
coverage from ~3000-10000 Angstroms. The MAGE Spectral Extractor (MASE)
incorporates the entire image reduction and calibration process, including bias
subtraction, flat fielding, wavelength calibration, sky subtraction, object
extraction and flux calibration of point sources. We include examples of the
user interface and reduced spectra. We show that the wavelength calibration is
sufficient to achieve ~5 km/s RMS accuracy and relative flux calibrations
better than 10%. A light-weight version of the full reduction pipeline has been
included for real-time source extraction and signal-to-noise estimation at the
telescope.Comment: 10 pages (ApJ format), accepted PAS
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