58,436 research outputs found
Investigation of iterative image reconstruction in three-dimensional optoacoustic tomography
Iterative image reconstruction algorithms for optoacoustic tomography (OAT),
also known as photoacoustic tomography, have the ability to improve image
quality over analytic algorithms due to their ability to incorporate accurate
models of the imaging physics, instrument response, and measurement noise.
However, to date, there have been few reported attempts to employ advanced
iterative image reconstruction algorithms for improving image quality in
three-dimensional (3D) OAT. In this work, we implement and investigate two
iterative image reconstruction methods for use with a 3D OAT small animal
imager: namely, a penalized least-squares (PLS) method employing a quadratic
smoothness penalty and a PLS method employing a total variation norm penalty.
The reconstruction algorithms employ accurate models of the ultrasonic
transducer impulse responses. Experimental data sets are employed to compare
the performances of the iterative reconstruction algorithms to that of a 3D
filtered backprojection (FBP) algorithm. By use of quantitative measures of
image quality, we demonstrate that the iterative reconstruction algorithms can
mitigate image artifacts and preserve spatial resolution more effectively than
FBP algorithms. These features suggest that the use of advanced image
reconstruction algorithms can improve the effectiveness of 3D OAT while
reducing the amount of data required for biomedical applications
Modeling the Flow of Yield-Stress Fluids in Porous Media
Yield-stress is a problematic and controversial non-Newtonian flow
phenomenon. In this article, we investigate the flow of yield-stress substances
through porous media within the framework of pore-scale network modeling. We
also investigate the validity of the Minimum Threshold Path (MTP) algorithms to
predict the pressure yield point of a network depicting random or regular
porous media. Percolation theory as a basis for predicting the yield point of a
network is briefly presented and assessed. In the course of this study, a
yield-stress flow simulation model alongside several numerical algorithms
related to yield-stress in porous media were developed, implemented and
assessed. The general conclusion is that modeling the flow of yield-stress
fluids in porous media is too difficult and problematic. More fundamental
modeling strategies are required to tackle this problem in the future.Comment: 27 pages and 5 figure
X-ray luminescence computed tomography using a focused X-ray beam
Due to the low X-ray photon utilization efficiency and low measurement
sensitivity of the electron multiplying charge coupled device (EMCCD) camera
setup, the collimator based narrow beam X-ray luminescence computed tomography
(XLCT) usually requires a long measurement time. In this paper, we, for the
first time, report a focused X-ray beam based XLCT imaging system with
measurements by a single optical fiber bundle and a photomultiplier tube (PMT).
An X-ray tube with a polycapillary lens was used to generate a focused X-ray
beam whose X-ray photon density is 1200 times larger than a collimated X-ray
beam. An optical fiber bundle was employed to collect and deliver the emitted
photons on the phantom surface to the PMT. The total measurement time was
reduced to 12.5 minutes. For numerical simulations of both single and six fiber
bundle cases, we were able to reconstruct six targets successfully. For the
phantom experiment, two targets with an edge-to-edge distance of 0.4 mm and a
center-to-center distance of 0.8 mm were successfully reconstructed by the
measurement setup with a single fiber bundle and a PMT.Comment: 39 Pages, 12 Figures, 2 Tables, In submission (under review) to JB
The Observation of Up-going Charged Particles Produced by High Energy Muons in Underground Detectors
An experimental study of the production of up-going charged particles in
inelastic interactions of down-going underground muons is reported, using data
obtained from the MACRO detector at the Gran Sasso Laboratory. In a sample of
12.2 10^6 single muons, corresponding to a detector livetime of 1.55 y, 243
events are observed having an up-going particle associated with a down-going
muon. These events are analysed to determine the range and emission angle
distributions of the up-going particle, corrected for detection and
reconstruction efficiency. Measurements of the muon neutrino flux by
underground detectors are often based on the observation of through-going and
stopping muons produced in interactions in the rock below the
detector. Up-going particles produced by an undetected down-going muon are a
potential background source in these measurements. The implications of this
background for neutrino studies using MACRO are discussed.Comment: 18 pages, 9 figures. Accepted by Astrop. Physic
Digital predictions of complex cylinder packed columns
A digital computational approach has been developed to simulate realistic structures of packed beds. The underlying principle of the method is digitisation of the particles and packing space, enabling the generation of realistic structures. Previous publications [Caulkin, R., Fairweather, M., Jia, X., Gopinathan, N., & Williams, R.A. (2006). An investigation of packed columns using a digital packing algorithm. Computers & Chemical Engineering, 30, 1178–1188; Caulkin, R., Ahmad, A., Fairweather, M., Jia, X., & Williams, R. A. (2007). An investigation of sphere packed shell-side columns using a digital packing algorithm. Computers & Chemical Engineering, 31, 1715–1724] have demonstrated the ability of the code in predicting the packing of spheres. For cylindrical particles, however, the original, random walk-based code proved less effective at predicting bed structure. In response to this, the algorithm has been modified to make use of collisions to guide particle movement in a way which does not sacrifice the advantage of simulation speed. Results of both the original and modified code are presented, with bulk and local voidage values compared with data derived by experimental methods. The results demonstrate that collisions and their impact on packing structure cannot be disregarded if realistic packing structures are to be obtained
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