5,554 research outputs found
Fast and adaptive fractal tree-based path planning for programmable bevel tip steerable needles
© 2016 IEEE. Steerable needles are a promising technology for minimally invasive surgery, as they can provide access to difficult to reach locations while avoiding delicate anatomical regions. However, due to the unpredictable tissue deformation associated with needle insertion and the complexity of many surgical scenarios, a real-time path planning algorithm with high update frequency would be advantageous. Real-time path planning for nonholonomic systems is commonly used in a broad variety of fields, ranging from aerospace to submarine navigation. In this letter, we propose to take advantage of the architecture of graphics processing units (GPUs) to apply fractal theory and thus parallelize real-time path planning computation. This novel approach, termed adaptive fractal trees (AFT), allows for the creation of a database of paths covering the entire domain, which are dense, invariant, procedurally produced, adaptable in size, and present a recursive structure. The generated cache of paths can in turn be analyzed in parallel to determine the most suitable path in a fraction of a second. The ability to cope with nonholonomic constraints, as well as constraints in the space of states of any complexity or number, is intrinsic to the AFT approach, rendering it highly versatile. Three-dimensional (3-D) simulations applied to needle steering in neurosurgery show that our approach can successfully compute paths in real-time, enabling complex brain navigation
Internal Anisotropy of Collision Cascades
We investigate the internal anisotropy of collision cascades arising from the
branching structure. We show that the global fractal dimension cannot give an
adequate description of the geometrical structure of cascades because it is
insensitive to the internal anisotropy. In order to give a more elaborate
description we introduce an angular correlation function, which takes into
account the direction of the local growth of the branches of the cascades. It
is demonstrated that the angular correlation function gives a quantitative
description of the directionality and the interrelation of branches. The power
law decay of the angular correlation is evidenced and characterized by an
exponent and an angular correlation length different from the radius of
gyration. It is demonstrated that the overlapping of subcascades has a strong
effect on the angular correlation.Comment: RevteX, 8 pages, 6 .eps figures include
Effects of the Charge-Dipole Interaction on the Coagulation of Fractal Aggregates
A numerical model with broad applications to complex (dusty) plasmas is
presented. The self-consistent N-body code allows simulation of the coagulation
of fractal aggregates, including the charge-dipole interaction of the clusters
due to the spatial arrangement of charge on the aggregate. It is shown that not
only does a population of oppositely charged particles increase the coagulation
rate, the inclusion of the charge-dipole interaction of the aggregates as well
as the electric dipole potential of the dust ensemble decreases the gelation
time by a factor of up to twenty. It is further shown that these interactions
can also stimulate the onset of gelation, or "runaway growth," even in a
population of particles charged to a monopotential where previously it was
believed that like-charged grains would inhibit coagulation. Gelation is
observed to occur due to the formation of high-mass aggregates with fractal
dimensions greater than two which act as seeds for runaway growth.Comment: 9 page
Fractal Reconnection in Solar and Stellar Environments
Recent space based observations of the Sun revealed that magnetic
reconnection is ubiquitous in the solar atmosphere, ranging from small scale
reconnection (observed as nanoflares) to large scale one (observed as long
duration flares or giant arcades). Often the magnetic reconnection events are
associated with mass ejections or jets, which seem to be closely related to
multiple plasmoid ejections from fractal current sheet. The bursty radio and
hard X-ray emissions from flares also suggest the fractal reconnection and
associated particle acceleration. We shall discuss recent observations and
theories related to the plasmoid-induced-reconnection and the fractal
reconnection in solar flares, and their implication to reconnection physics and
particle acceleration. Recent findings of many superflares on solar type stars
that has extended the applicability of the fractal reconnection model of solar
flares to much a wider parameter space suitable for stellar flares are also
discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and
Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016),
33 pages, 18 figure
Attracted Diffusion-Limited Aggregation
In this paper, we present results of extensive Monte Carlo simulations of
diffusion-limited aggregation (DLA) with a seed placed on an attractive plane
as a simple model in connection with the electrical double layers. We compute
the fractal dimension of the aggregated patterns as a function of the
attraction strength \alpha. For the patterns grown in both two and three
dimensions, the fractal dimension shows a significant dependence on the
attraction strength for small values of \alpha, and approaches to that of the
ordinary two-dimensional (2D) DLA in the limit of large \alpha. For
non-attracting case with \alpha=1, our results in three dimensions reproduce
the patterns of 3D ordinary DLA, while in two dimensions our model leads to
formation of a compact cluster with dimension two. For intermediate \alpha, the
3D clusters have quasi-2D structure with a fractal dimension very close to that
of the ordinary 2D-DLA. This allows one to control morphology of a growing
cluster by tuning a single external parameter \alpha.Comment: 6 pages, 6 figures, to appear in Phys. Rev. E (2012
Soft set theory based decision support system for mining electronic government dataset
Electronic government (e-gov) is applied to support performance and create more efficient and
effective public services. Grouping data in soft-set theory can be considered as a decision-making
technique for determining the maturity level of e-government use. So far, the uncertainty of the data
obtained through the questionnaire has not been maximally used as an appropriate reference for the
government in determining the direction of future e-gov development policy. This study presents
the maximum attribute relative (MAR) based on soft set theory to classify attribute options. The
results show that facilitation conditions (FC) are the highest variable in influencing people to use
e-government, followed by performance expectancy (PE) and system quality (SQ). The results provide
useful information for decision makers to make policies about their citizens and potentially provide
recommendations on how to design and develop e-government systems in improving public services
Biomimetic flow fields for proton exchange membrane fuel cells: A review of design trends
Bipolar Plate design is one of the most active research fields in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) development. Bipolar Plates are key components for ensuring an appropriate water management within the cell, preventing flooding and enhancing the cell operation at high current densities. This work presents a literature review covering bipolar plate designs based on nature or biological structures such as fractals, leaves or lungs. Biological inspiration comes from the fact that fluid distribution systems found in plants and animals such as leaves, blood vessels, or lungs perform their functions (mostly the same functions that are required for bipolar plates) with a remarkable efficiency, after millions of years of natural evolution. Such biomimetic designs have been explored to date with success, but it is generally acknowledged that biomimetic designs have not yet achieved their full potential. Many biomimetic designs have been derived using computer simulation tools, in particular Computational Fluid Dynamics (CFD) so that the use of CFD is included in the review. A detailed review including performance benchmarking, time line evolution, challenges and proposals, as well as manufacturing issues is discussed.Ministerio de Ciencia, Innovación y Universidades ENE2017-91159-EXPMinisterio de Economía y Competitividad UNSE15-CE296
Fractal and multifractal analysis of PET-CT images of metastatic melanoma before and after treatment with ipilimumab
PET/CT with F-18-Fluorodeoxyglucose (FDG) images of patients suffering from
metastatic melanoma have been analysed using fractal and multifractal analysis
to assess the impact of monoclonal antibody ipilimumab treatment with respect
to therapy outcome. Our analysis shows that the fractal dimensions which
describe the tracer dispersion in the body decrease consistently with the
deterioration of the patient therapeutic outcome condition. In 20 out-of 24
cases the fractal analysis results match those of the medical records, while 7
cases are considered as special cases because the patients have non-tumour
related medical conditions or side effects which affect the results. The
decrease in the fractal dimensions with the deterioration of the patient
conditions (in terms of disease progression) are attributed to the hierarchical
localisation of the tracer which accumulates in the affected lesions and does
not spread homogeneously throughout the body. Fractality emerges as a result of
the migration patterns which the malignant cells follow for propagating within
the body (circulatory system, lymphatic system). Analysis of the multifractal
spectrum complements and supports the results of the fractal analysis. In the
kinetic Monte Carlo modelling of the metastatic process a small number of
malignant cells diffuse throughout a fractal medium representing the blood
circulatory network. Along their way the malignant cells engender random
metastases (colonies) with a small probability and, as a result, fractal
spatial distributions of the metastases are formed similar to the ones observed
in the PET/CT images. In conclusion, we propose that fractal and multifractal
analysis has potential application in the quantification of the evaluation of
PET/CT images to monitor the disease evolution as well as the response to
different medical treatments.Comment: 38 pages, 9 figure
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