169 research outputs found
R-L-MS-L Filter Function for CT Image Reconstruction
In X-ray computer tomography (CT), convolution back projection is a fundamental algorithm for CT image reconstruction. As filtering plays an important part in convolution back projection, the choice of filter has a direct impact upon the quality of reconstructed images. Aim at improving reconstructed image quality, a new mixed filter based on the idea of “first weighted average then linear mixing” is designed in this article, denoted by R-L-MS-L. Here, R-L filter is relied on to guarantee the spatial resolution of reconstructed image and S-L filter is processed via 3-point weighted averaging to improve the density resolution, thus enhancing the overall reconstruction quality. Gaussian noise of different coefficients is added to the projection data to contrast the noise performance of the new and traditional mixed filters. The simulation and experiment results show that the new filter is better in anti-noise performance and produces reconstructed images with notably improved quality
On-axis spectroscopy of the host galaxies of 20 optically luminous quasars at z~0.3
We present the analysis of a sample of 20 bright low-redshift quasars
(M_B<-23 and z < 0.35) observed spectroscopically with the VLT. The FORS1
spectra, obtained in Multi Object Spectroscopy (MOS) mode, allow to observe
simultaneously the quasars and several reference stars used to spatially
deconvolve the data. Applying the MCS deconvolution method, we are able to
separate the individual spectra of the quasar and of the underlying host
galaxy. Contrary to some previous claims, we find that luminous quasars are not
exclusively hosted by massive ellipticals. Most quasar host galaxies harbour
large amounts of gas, irrespective of their morphological type. Moreover, the
stellar content of half of the hosts is a young Sc-like population, associated
with a rather low metallicity interstellar medium. A significant fraction of
the galaxies contain gas ionized at large distances by the quasar radiation.
This large distance ionization is always associated with signs of gravitational
interactions (as detected from images or disturbed rotation curves). The
spectra of the quasars themselves provide evidence that gravitational
interactions bring dust and gas in the immediate surrounding of the super
massive black hole, allowing to feed it. The quasar activity might thus be
triggered (1) in young gas-rich spiral galaxies by local events and (2) in more
evolved galaxies by gravitational interactions or collisions. The latter
mechanism gives rises to the most powerful quasars. Finally, we derive mass
models for the isolated spiral host galaxies and we show that the most reliable
estimators of the systemic redshift in the quasar spectrum are the tips of the
Ha and Hb lines.Comment: 30 pages, 19 figures, 9 tables, accepted for publication in MNRAS,
major revisio
Registration-based multi-orientation tomography
We propose a combination of an experimental approach and a reconstruction technique that leads to reduction of artefacts in X-ray computer tomography of strongly attenuating objects. Through fully automatic data alignment, data generated in multiple experiments with varying object orientations are combined. Simulations and exp
Multi wavelength study of the gravitational lens system RXS J1131-1231: II Lens model and source reconstruction
High angular resolution images of the complex gravitational lens system RXS
J1131-1231 (a quadruply imaged AGN with a bright Einstein ring) obtained with
the Advanced Camera for Surveys and NICMOS instruments onboard the Hubble Space
Telescope are analysed to determine the lens model and to reconstruct the host
galaxy.
Results: 1- Precise astrometry and photometry of the four QSO lensed images
(A-D) and of the lensing galaxy (G) are obtained. They are found in agreement
with an independent study presented in a companion paper. The position and
colours of the X object seen in projection close to the lens are found to be
only compatible with a satellite galaxy associated with the lens.
2- The Singular Isothermal Ellipsoid plus external shear provides a good fit
of the astrometry of images A-D. The positions of extended substructures are
also well reproduced. However an octupole (m=4) must be added to the lens
potential in order to reproduce the observed lens position, as well as the
IB/IC point-like image flux ratio. The ellipticity and orientation of the mass
quadrupole are found similar to those of the light distribution, fitted by a
Sersic profile. The lens (z=0.295) is found to be a massive elliptical in a
rich environment and showing possible evolution with respect to z=0.
3- The host galaxy (z=0.658) is found to be a substantially magnified (M ~ 9)
luminous Seyfert 1 spiral galaxy. The angular resolution is sufficient to see
regions where stars are intensively forming. Interaction with a closeby
companion is also observed.
4- Finally, in the case of RXS J1131-1231, extended lensed structures do not
help much in constraining the lens model.Comment: 17 pages, 6 figures, accepted in Astronomy and Astrophysics; improved
Latex processing. Version with full resolution figures available at
http://www.astro.ulg.ac.be/~claesken/lens1131_II.pd
Understanding the relations between QSOs and their host galaxies from combined HST imaging and VLT spectroscopy
The host galaxies of six nearby QSOs are studied on the basis of high
resolution HST optical images and spatially resolved VLT slit spectra. The gas
ionization and velocity are mapped as a function of the distance to the central
QSO. In the majority of the cases, the QSO significantly contributes to the gas
ionization in its whole host galaxy, and sometimes even outside.
Reflection or scattering of the QSO \ha line from remote regions of the
galaxy is detected in several instances. The line shifts show that, in all
cases, the matter responsible for the light reflection moves away from the QSO,
likely accelerated by its radiation pressure.
The two faintest QSOs reside in spirals, with some signs of a past
gravitational perturbation. One of the intermediate luminosity QSOs resides in
a massive elliptical containing gas ionized (and probably pushed away) by the
QSO radiation. The other medium-power object is found in a spiral galaxy
displaying complex velocity structure, with the central QSO moving with respect
to the bulge, probably as a result of a galactic collision. The two most
powerful objects are involved in violent gravitational interactions and one of
them has no detected host.
These results suggest that (1) large-scale phenomena, such as galactic
collisions, are closely related to the triggering and the feeding of the QSO
and (2) once ignited, the QSO has significant influence on its large-scale
neighborhood (often the whole host and sometimes further away).Comment: Accepted for publication in ApJ. 31 pages, 17 figures, 3 table
Artefact Reduction Methods for Iterative Reconstruction in Full-fan Cone Beam CT Radiotherapy Applications
A cone beam CT (CBCT) system acquires two-dimensional projection images of an imaging object from multiple angles in one single rotation and reconstructs the object geometry in three dimensions for volumetric visualization. It is mounted on most modern linear accelerators and is routinely used in radiotherapy to verify patient positioning, monitor patient contour changes throughout the course of treatment, and enable adaptive radiotherapy planning.
Iterative image reconstruction algorithms use mathematical methods to iteratively solve the reconstruction problem. Iterative algorithms have demonstrated improvement in image quality and / or reduction in imaging dose over traditional filtered back-projection (FBP) methods. However, despite the advancement in computer technology and growing availability of open-source iterative algorithms, clinical implementation of iterative CBCT has been limited. This thesis does not report development of codes for new iterative image reconstruction algorithms. It focuses on bridging the gap between the algorithm and its implementation by addressing artefacts that are the results of imperfections from the raw projections and from the imaging geometry. Such artefacts can severely degrade image quality and cannot be removed by iterative algorithms alone. Practical solutions to solving these artefacts will be presented and this in turn will better enable clinical implementation of iterative CBCT reconstruction
Computed Tomography of Chemiluminescence: A 3D Time Resolved Sensor for Turbulent Combustion
Time resolved 3D measurements of turbulent flames are required to further understanding
of combustion and support advanced simulation techniques (LES). Computed Tomography
of Chemiluminescence (CTC) allows a flame’s 3D chemiluminescence profile to be
obtained by inverting a series of integral measurements. CTC provides the instantaneous
3D flame structure, and can also measure: excited species concentrations, equivalence
ratio, heat release rate, and possibly strain rate. High resolutions require simultaneous
measurements from many view points, and the cost of multiple sensors has traditionally
limited spatial resolutions. However, recent improvements in commodity cameras makes
a high resolution CTC sensor possible and is investigated in this work.
Using realistic LES Phantoms (known fields), the CT algorithm (ART) is shown to
produce low error reconstructions even from limited noisy datasets. Error from selfabsorption
is also tested using LES Phantoms and a modification to ART that successfully
corrects this error is presented. A proof-of-concept experiment using 48 non-simultaneous
views is performed and successfully resolves a Matrix Burner flame to 0.01% of the domain
width (D). ART is also extended to 3D (without stacking) to allow 3D camera
locations and optical effects to be considered. An optical integral geometry (weighted
double-cone) is presented that corrects for limited depth-of-field, and (even with poorly
estimated camera parameters) reconstructs the Matrix Burner as well as the standard geometry.
CTC is implemented using five PicSight P32M cameras and mirrors to provide 10
simultaneous views. Measurements of the Matrix Burner and a Turbulent Opposed Jet
achieve exposure times as low as 62 ÎĽs, with even shorter exposures possible. With only
10 views the spatial resolution of the reconstructions is low. However, a cosine Phantom
study shows that 20–40 viewing angles are necessary to achieve high resolutions (0.01–
0.04D). With 40 P32M cameras costing ÂŁ40000, future CTC implementations can achieve
high spatial and temporal resolutions
Improving Image Reconstruction for Digital Breast Tomosynthesis
Digital breast tomosynthesis (DBT) has been developed to reduce the issue of overlapping tissue in conventional 2-D mammography for breast cancer screening and diagnosis. In the DBT procedure, the patient’s breast is compressed with a paddle and a sequence of x-ray projections is taken within a small angular range. Tomographic reconstruction algorithms are then applied to these projections, generating tomosynthesized image slices of the breast, such that radiologists can read the breast slice by slice. Studies have shown that DBT can reduce both false-negative diagnoses of breast cancer and false-positive recalls compared to mammography alone.
This dissertation focuses on improving image quality for DBT reconstruction. Chapter I briefly introduces the concept of DBT and the inspiration of my study. Chapter II covers the background of my research including the concept of image reconstruction, the geometry of our experimental DBT system and figures of merit for image quality. Chapter III introduces our study of the segmented separable footprint (SG) projector. By taking into account the finite size of detector element, the SG projector improves the accuracy of forward projections in iterative image reconstruction. Due to the more efficient access to memory, the SG projector is also faster than the traditional ray-tracing (RT) projector. We applied the SG projector to regular and subpixel reconstructions and demonstrated its effectiveness. Chapter IV introduces a new DBT reconstruction method with detector blur and correlated noise modeling, called the SQS-DBCN algorithm. The SQS-DBCN algorithm is able to significantly enhance microcalcifications (MC) in DBT while preserving the appearance of the soft tissue and mass margin. Comparisons between the SQS-DBCN algorithm and several modified versions of the SQS-DBCN algorithm indicate the importance of modeling different components of the system physics at the same time.
Chapter V investigates truncated projection artifact (TPA) removal algorithms. Among the three algorithms we proposed, the pre-reconstruction-based projection view (PV) extrapolation method provides the best performance. Possible improvements of the other two TPA removal algorithms have been discussed. Chapter VI of this dissertation examines the effect of source blur on DBT reconstruction. Our analytical calculation demonstrates that the point spread function (PSF) of source blur is highly shift-variant. We used CatSim to simulate digital phantoms. Analysis on the reconstructed images demonstrates that a typical finite-sized focal spot (~ 0.3 mm) will not affect the image quality if the x-ray tube is stationary during the data acquisition. For DBT systems with continuous-motion data acquisition, the motion of the x-ray tube is the main cause of the effective source blur and will cause loss in the contrast of objects. Therefore modeling the source blur for these DBT systems could potentially improve the reconstructed image quality. The final chapter of this dissertation discusses a few future studies that are inspired by my PhD research.PHDElectrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144059/1/jiabei_1.pd
Spatial decomposition of on-nucleus spectra of quasar host galaxies
In order to study the host galaxies of type 1 (broad-line) quasars, we
present a semi-analytic modelling method to decompose the on-nucleus spectra of
quasars into nuclear and host galaxy channels. The method uses the spatial
information contained in long-slit or slitlet spectra. A routine determines the
best fitting combination of the spatial distribution of the point like nucleus
and extended host galaxy. Inputs are a simultaneously observed PSF, and
external constraints on galaxy morphology from imaging. We demonstrate the
capabilities of the method to two samples of a total of 18 quasars observed
with EFOSC at the ESO 3.6m telescope and FORS1 at the ESO VLT.
~50% of the host galaxies with sucessful decomposition show distortions in
their rotation curves or peculiar gas velocities above normal maximum
velocities for disks. This is consistent with the fraction from optical
imaging. All host galaxies have quite young stellar populations, typically 1-2
Gyr. For the disk dominated hosts these are consistent with their inactive
counterparts, the luminosity weighted stellar ages are much younger for the
bulge dominated hosts, compared to inactive early type galaxies. While this
presents further evidence for a connection of galaxy interaction and AGN
activity for half of the sample, this is not clear for the other half: These
are often undistorted disk dominated host galaxies, and interaction on a
smaller level might be detected in deeper high-resolution images or deeper
spectroscopic data. The velocity information does not show obvious signs for
large scale outflows triggered by AGN feedback - the data is consistent with
velocity fields created by galaxy interaction.Comment: Accepted for publication in MNRAS; 19 pages, 12 figure
- …