616 research outputs found
Fast and accurate Slicewise OutLIer Detection (SOLID) with informed model estimation for diffusion MRI data
The accurate characterization of the diffusion process in tissue using diffusion MRI is greatly challenged by the presence of artefacts. Subject motion causes not only spatial misalignments between diffusion weighted images, but often also slicewise signal intensity errors. Voxelwise robust model estimation is commonly used to exclude intensity errors as outliers. Slicewise outliers, however, become distributed over multiple adjacent slices after image registration and transformation. This challenges outlier detection with voxelwise procedures due to partial volume effects. Detecting the outlier slices before any transformations are applied to diffusion weighted images is therefore required. In this work, we present i) an automated tool coined SOLID for slicewise outlier detection prior to geometrical image transformation, and ii) a framework to naturally interpret data uncertainty information from SOLID and include it as such in model estimators. SOLID uses a straightforward intensity metric, is independent of the choice of the diffusion MRI model, and can handle datasets with a few or irregularly distributed gradient directions. The SOLID-informed estimation framework prevents the need to completely reject diffusion weighted images or individual voxel measurements by downweighting measurements with their degree of uncertainty, thereby supporting convergence and well-conditioning of iterative estimation algorithms. In comprehensive simulation experiments, SOLID detects outliers with a high sensitivity and specificity, and can achieve higher or at least similar sensitivity and specificity compared to other tools that are based on more complex and time-consuming procedures for the scenarios investigated. SOLID was further validated on data from 54 neonatal subjects which were visually inspected for outlier slices with the interactive tool developed as part of this study, showing its potential to quickly highlight problematic volumes and slices in large population studies. The informed model estimation framework was evaluated both in simulations and in vivo human data.Peer reviewe
Helium-3 and Helium-4 acceleration by high power laser pulses for hadron therapy
The laser driven acceleration of ions is considered a promising candidate for
an ion source for hadron therapy of oncological diseases. Though proton and
carbon ion sources are conventionally used for therapy, other light ions can
also be utilized. Whereas carbon ions require 400 MeV per nucleon to reach the
same penetration depth as 250 MeV protons, helium ions require only 250 MeV per
nucleon, which is the lowest energy per nucleon among the light ions. This fact
along with the larger biological damage to cancer cells achieved by helium
ions, than that by protons, makes this species an interesting candidate for the
laser driven ion source. Two mechanisms (Magnetic Vortex Acceleration and
hole-boring Radiation Pressure Acceleration) of PW-class laser driven ion
acceleration from liquid and gaseous helium targets are studied with the goal
of producing 250 MeV per nucleon helium ion beams that meet the hadron therapy
requirements. We show that He3 ions, having almost the same penetration depth
as He4 with the same energy per nucleon, require less laser power to be
accelerated to the required energy for the hadron therapy.Comment: 8 pages, 3 figures, 1 tabl
Enhancing proton acceleration by using composite targets
Efficient laser ion acceleration requires high laser intensities, which can
only be obtained by tightly focusing laser radiation. In the radiation pressure
acceleration regime, where the tightly focused laser driver leads to the
appearance of the fundamental limit for the maximum attainable ion energy, this
limit corresponds to the laser pulse group velocity as well as to another limit
connected with the transverse expansion of the accelerated foil and consequent
onset of the foil transparency. These limits can be relaxed by using composite
targets, consisting of a thin foil followed by a near critical density slab.
Such targets provide guiding of a laser pulse inside a self-generated channel
and background electrons, being snowplowed by the pulse, compensate for the
transverse expansion. The use of composite targets results in a significant
increase in maximum ion energy, compared to a single foil target case.Comment: 16 pages, 9 figure
Laser-heater assisted plasma channel formation in capillary discharge waveguides
A method of creating plasma channels with controllable depth and transverse
profile for the guiding of short, high power laser pulses for efficient
electron acceleration is proposed. The plasma channel produced by the
hydrogen-filled capillary discharge waveguide is modified by a ns-scale laser
pulse, which heats the electrons near the capillary axis. This interaction
creates a deeper plasma channel within the capillary discharge that evolves on
a ns-time scale, allowing laser beams with smaller spot sizes than would
otherwise be possible in the unmodified capillary discharge.Comment: 5 pages, 3 figure
Infrahyoid fascio-myocutaneous flap as an alternative to free radial forearm flap in head and neck reconstruction.
The use of microvascular free flaps is currently the favored method for the reconstruction of defects after resection of head and neck cancer. The flap most commonly used for head and neck reconstruction is the free radial forearm flap, but the less popular infrahyoid flap represents a good alternative in selected cases. This flap has proven to be helpful in the reconstruction of a wide range of moderate-sized head and neck defects.We reviewed a series of 13 patients with defects resulting from cancer of the head and neck, who underwent infrahyoid flap reconstruction as an alternative to free radial forearm flap. The series includes 12 squamous cell carcinomas arising from the oral cavity and oropharynx, and 1 Merkel cell carcinoma of the submental skin. In the harvesting of the flap, the technical modifications recently suggested by Dolivet et al were used in all cases. Furthermore, another technical change has been introduced so creating a new infrahyoid facio-myocutaneous flap (IHFMCF). The surgical technique is described in detail.No total or partial flap necrosis was experienced. All reconstructions healed quickly without wound complications and with good functional results. The healing process in the donor site was excellent in every case with good aesthetic results.The IHFMCF is a versatile, reliable, and convenient flap suitable for repairing small and medium-sized defects of the oral cavity and oropharynx and obviates the need for a microvascular reconstruction
On the design of experiments to study extreme field limits
We propose experiments on the collision of high intensity electromagnetic
pulses with electron bunches and on the collision of multiple electromagnetic
pulses for studying extreme field limits in the nonlinear interaction of
electromagnetic waves. The effects of nonlinear QED will be revealed in these
laser plasma experiments.Comment: 7 pages, 3 figures, 1 table; 15th Advanced Accelerator Concepts
Workshop (AAC 2012), Austin, Texas, 10-15 June, 201
On production and asymmetric focusing of flat electron beams using rectangular capillary discharge plasmas
A method for the asymmetric focusing of electron bunches, based on the active
plasma lensing technique is proposed. This method takes advantage of the strong
inhomogeneous magnetic field generated inside the capillary discharge plasma to
focus the ultrarelativistic electrons. The plasma and magnetic field parameters
inside the capillary discharge are described theoretically and modeled with
dissipative magnetohydrodynamic computer simulations enabling analysis of the
capillaries of rectangle cross-sections. Large aspect ratio rectangular
capillaries might be used to transport electron beams with high emittance
asymmetries, as well as assist in forming spatially flat electron bunches for
final focusing before the interaction point.Comment: 16 pages, 7 figures, 1 tabl
Laser beam coupling with capillary discharge plasma for laser wakefield acceleration applications
One of the most robust methods, demonstrated up to date, of accelerating
electron beams by laser-plasma sources is the utilization of plasma channels
generated by the capillary discharges. These channels, i.e., plasma columns
with a minimum density along the laser pulse propagation axis, may optically
guide short laser pulses, thereby increasing the acceleration length, leading
to a more efficient electron acceleration. Although the spatial structure of
the installation is simple in principle, there may be some important effects
caused by the open ends of the capillary, by the supplying channels etc., which
require a detailed 3D modeling of the processes taking place in order to get a
detailed understanding and improve the operation. However, the discharge
plasma, being one of the most crucial components of the laser-plasma
accelerator, is not simulated with the accuracy and resolution required to
advance this promising technology. In the present work, such simulations are
performed using the code MARPLE. First, the process of the capillary filling
with a cold hydrogen before the discharge is fired, through the side supply
channels is simulated. The main goal of this simulation is to get a spatial
distribution of the filling gas in the region near the open ends of the
capillary. A realistic geometry is used for this and the next stage
simulations, including the insulators, the supplying channels as well as the
electrodes. Second, the simulation of the capillary discharge is performed with
the goal to obtain a time-dependent spatial distribution of the electron
density near the open ends of the capillary as well as inside the capillary.
Finally, to evaluate effectiveness of the beam coupling with the channeling
plasma wave guide and electron acceleration, modeling of laser-plasma
interaction was performed with the code INF&RNOComment: 11 pages, 9 figure
Plasma Equilibrium inside Various Cross-Section Capillary Discharges
Plasma properties inside a hydrogen-filled capillary discharge waveguide were
modeled with dissipative magnetohydrodynamic simulations to enable analysis of
capillaries of circular and square cross-sections implying that square
capillaries can be used to guide circularly-symmetric laser beams. When the
quasistationary stage of the discharge is reached, the plasma and temperature
in the vicinity of the capillary axis has almost the same profile for both the
circular and square capillaries. The effect of cross-section on the electron
beam focusing properties were studied using the simulation-derived magnetic
field map. Particle tracking simulations showed only slight effects on the
electron beam symmetry in the horizontal and diagonal directions for square
capillary.Comment: 6 pages, 10 figure
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