85,678 research outputs found
A new approach to the assessment of lumen visibility of coronary artery stent at various heart rates using 64-slice MDCT
Coronary artery stent lumen visibility was assessed as a function of cardiac movement and temporal resolution with an automated objective method using an anthropomorphic moving heart phantom. Nine different coronary stents filled with contrast fluid and surrounded by fat were scanned using 64-slice multi-detector computed tomography (MDCT) at 50–100 beats/min with the moving heart phantom. Image quality was assessed by measuring in-stent CT attenuation and by a dedicated tool in the longitudinal and axial plane. Images were scored by CT attenuation and lumen visibility and compared with theoretical scoring to analyse the effect of multi-segment reconstruction (MSR). An average increase in CT attenuation of 144 ± 59 HU and average diminished lumen visibility of 29 ± 12% was observed at higher heart rates in both planes. A negative correlation between image quality and heart rate was non-significant for the majority of measurements (P > 0.06). No improvement of image quality was observed in using MSR. In conclusion, in-stent CT attenuation increases and lumen visibility decreases at increasing heart rate. Results obtained with the automated tool show similar behaviour compared with attenuation measurements. Cardiac movement during data acquisition causes approximately twice as much blurring compared with the influence of temporal resolution on image quality
Variations of the high-level Balmer line spectrum of the helium-strong star Sigma Orionis E
Using the high-level Balmer lines and continuum, we trace the density
structure of two magnetospheric disk segments of the prototypical Bp star sigma
Ori E (B2p) as these segments occult portions of the star during the rotational
cycle. High-resolution spectra of the Balmer lines >H9 and Balmer edge were
obtained on seven nights in January-February 2007 at an average sampling of
0.01 cycles. We measured equivalent width variations due to the star
occultations by two disk segments 0.4 cycles apart and constructed differential
spectra of the migrations of the corresponding absorptions across the Balmer
line profiles. We first estimated the rotational and magnetic obliquity angles.
We then simulated the observed Balmer jump variation using the model atmosphere
codes synspec/circus and evaluated the disk geometry and gas thermodynamics. We
find that the two occultations are caused by two disk segments. The first of
these transits quickly, indicating that the segment resides in a range of
distances, perhaps 2.5-6R_star, from the star. The second consists of a more
slowly moving segment situated closer to the surface and causing two
semi-resolved absorbing maxima. During its transit this segment brushes across
the star's "lower" limb. Judging from the line visibility up to H23-H24 during
the occultations, both disk segments have mean densities near 10^{12} cm^{-3}
and are opaque in the lines and continuum. They have semiheights less than 1/2
of a stellar radius, and their temperatures are near 10500K and 12000K,
respectively. In all, the disks of Bp stars have a much more complicated
geometry than has been anticipated, as evidenced by their (sometimes)
non-coplanarity, de-centerness, and from star to star, differences in disk
height.Comment: Accepted by Astron. Astrophys, 13 pages, 4 embedded figure
Visibility Graphs, Dismantlability, and the Cops and Robbers Game
We study versions of cop and robber pursuit-evasion games on the visibility
graphs of polygons, and inside polygons with straight and curved sides. Each
player has full information about the other player's location, players take
turns, and the robber is captured when the cop arrives at the same point as the
robber. In visibility graphs we show the cop can always win because visibility
graphs are dismantlable, which is interesting as one of the few results
relating visibility graphs to other known graph classes. We extend this to show
that the cop wins games in which players move along straight line segments
inside any polygon and, more generally, inside any simply connected planar
region with a reasonable boundary. Essentially, our problem is a type of
pursuit-evasion using the link metric rather than the Euclidean metric, and our
result provides an interesting class of infinite cop-win graphs.Comment: 23 page
Trajectory Range Visibility
We study the problem of Trajectory Range Visibility, determining the
sub-trajectories on which two moving entities become mutually visible.
Specifically, we consider two moving entities with not necessarily equal
velocities and moving on a given piece-wise linear trajectory inside a simple
polygon. Deciding whether the entities can see one another with given constant
velocities, and assuming the trajectories only as line segments, was solved by
P. Eades et al. in 2020. However, we obtain stronger results and support
queries on constant velocities for non-constant complexity trajectories.
Namely, given a constant query velocity for a moving entity, we specify all
visible parts of the other entity's trajectory and all possible constant
velocities of the other entity to become visible. Regarding line-segment
trajectories, we obtain time to specify all pairs of
mutually visible sub-trajectories s.t. is the number of vertices of the
polygon. Moreover, our results for a restricted case on non-constant complexity
trajectories yield time, in which is
the overall number of vertices of both trajectories. Regarding the unrestricted
case, we provide running time. We
offer query time for line segment trajectories and
for the non-constant complexity ones s.t. is the
number of velocity ranges reported in the answer. Interestingly, our results
require only space for non-constant complexity
trajectories
A Distributed Algorithm for Gathering Many Fat Mobile Robots in the Plane
In this work we consider the problem of gathering autonomous robots in the
plane. In particular, we consider non-transparent unit-disc robots (i.e., fat)
in an asynchronous setting. Vision is the only mean of coordination. Using a
state-machine representation we formulate the gathering problem and develop a
distributed algorithm that solves the problem for any number of robots.
The main idea behind our algorithm is for the robots to reach a configuration
in which all the following hold: (a) The robots' centers form a convex hull in
which all robots are on the convex, (b) Each robot can see all other robots,
and (c) The configuration is connected, that is, every robot touches another
robot and all robots together form a connected formation. We show that starting
from any initial configuration, the robots, making only local decisions and
coordinate by vision, eventually reach such a configuration and terminate,
yielding a solution to the gathering problem.Comment: 39 pages, 5 figure
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