7,320 research outputs found
Four-dimensional Cone Beam CT Reconstruction and Enhancement using a Temporal Non-Local Means Method
Four-dimensional Cone Beam Computed Tomography (4D-CBCT) has been developed
to provide respiratory phase resolved volumetric imaging in image guided
radiation therapy (IGRT). Inadequate number of projections in each phase bin
results in low quality 4D-CBCT images with obvious streaking artifacts. In this
work, we propose two novel 4D-CBCT algorithms: an iterative reconstruction
algorithm and an enhancement algorithm, utilizing a temporal nonlocal means
(TNLM) method. We define a TNLM energy term for a given set of 4D-CBCT images.
Minimization of this term favors those 4D-CBCT images such that any anatomical
features at one spatial point at one phase can be found in a nearby spatial
point at neighboring phases. 4D-CBCT reconstruction is achieved by minimizing a
total energy containing a data fidelity term and the TNLM energy term. As for
the image enhancement, 4D-CBCT images generated by the FDK algorithm are
enhanced by minimizing the TNLM function while keeping the enhanced images
close to the FDK results. A forward-backward splitting algorithm and a
Gauss-Jacobi iteration method are employed to solve the problems. The
algorithms are implemented on GPU to achieve a high computational efficiency.
The reconstruction algorithm and the enhancement algorithm generate visually
similar 4D-CBCT images, both better than the FDK results. Quantitative
evaluations indicate that, compared with the FDK results, our reconstruction
method improves contrast-to-noise-ratio (CNR) by a factor of 2.56~3.13 and our
enhancement method increases the CNR by 2.75~3.33 times. The enhancement method
also removes over 80% of the streak artifacts from the FDK results. The total
computation time is ~460 sec for the reconstruction algorithm and ~610 sec for
the enhancement algorithm on an NVIDIA Tesla C1060 GPU card.Comment: 20 pages, 3 figures, 2 table
Spontaneous edge-defect formation and defect-induced conductance suppression in graphene nanoribbons
We present a first-principles study of the migration and recombination of
edge defects (carbon adatom and/or vacancy) and their influence on electrical
conductance in zigzag graphene nanoribbons (ZGNRs). It is found that at room
temperature, the adatom is quite mobile while the vacancy is almost immobile
along the edge of ZGNRs. The recombination of an adatom-vacancy pair leads to a
pentagon-heptagon ring defect structure having a lower energy than the perfect
edge, implying that such an edge-defect can be formed spontaneously. This edge
defect can suppresses the conductance of ZGNRs drastically, which provides some
useful hints for understanding the observed semiconducting behavior of the
fabricated narrow GNRs.Comment: 6 pages, 4 figures, to appear in PR
High temperature ferrimagnetic semiconductors by spin-dependent doping in high temperature antiferromagnets
To realize room temperature ferromagnetic (FM) semiconductors is still a
challenge in spintronics. Many antiferromagnetic (AFM) insulators and
semiconductors with high Neel temperature are obtained in experiments,
such as LaFeO, BiFeO, etc. High concentrations of magnetic impurities
can be doped into these AFM materials, but AFM state with very tiny net
magnetic moments was obtained in experiments, because the magnetic impurities
were equally doped into the spin up and down sublattices of the AFM materials.
Here, we propose that the effective magnetic field provided by a FM substrate
could guarantee the spin-dependent doping in AFM materials, where the doped
magnetic impurities prefer one sublattice of spins, and the ferrimagnetic (FIM)
materials are obtained. To demonstrate this proposal, we study the Mn-doped AFM
insulator LaFeO with FM substrate of Fe metal by the density functional
theory (DFT) calculations. It is shown that the doped magnetic Mn impurities
prefer to occupy one sublattice of AFM insulator, and introduce large magnetic
moments in La(Fe,Mn)O. For the AFM insulator LaFeO with high =
740 K, several FIM semiconductors with high Curie temperature 300 K and
the band gap less than 2 eV are obtained by DFT calculations, when 1/8 or 1/4
Fe atoms in LaFeO are replaced by the other 3d, 4d transition metal
elements. The large magneto-optical Kerr effect (MOKE) is obtained in these
LaFeO-based FIM semiconductors. In addition, the FIM semiconductors with
high are also obtained by spin-dependent doping in some other AFM
materials with high , including BiFeO, SrTcO, CaTcO, etc. Our
theoretical results propose a way to obtain high FIM semiconductors by
spin-dependent doping in high AFM insulators and semiconductors
Dirac Fermion in Strongly-Bound Graphene Systems
It is highly desirable to integrate graphene into existing semiconductor
technology, where the combined system is thermodynamically stable yet maintain
a Dirac cone at the Fermi level. Firstprinciples calculations reveal that a
certain transition metal (TM) intercalated graphene/SiC(0001), such as the
strongly-bound graphene/intercalated-Mn/SiC, could be such a system. Different
from free-standing graphene, the hybridization between graphene and Mn/SiC
leads to the formation of a dispersive Dirac cone of primarily TM d characters.
The corresponding Dirac spectrum is still isotropic, and the transport behavior
is nearly identical to that of free-standing graphene for a bias as large as
0.6 V, except that the Fermi velocity is half that of graphene. A simple model
Hamiltonian is developed to qualitatively account for the physics of the
transfer of the Dirac cone from a dispersive system (e.g., graphene) to an
originally non-dispersive system (e.g., TM).Comment: Apr 25th, 2012 submitte
- β¦