This paper describes and evaluates two new approaches to longitudinal interpolation in single-slice helical CT that represent a step toward the goal of achieving essentially isotropic resolution and noise properties in reconstructed helical CT volumes. Both approaches exploit the fast Fourier transform and the Fourier shift theorem to generate from the helical projection data a set of fan-beam sinograms corresponding to equispaced transverse slices. Slice-by-slice reconstruction is then performed by use of two-dimensional fan-beam algorithms. The first approach, called 360FT, makes use only of the directly measured projection data, but the second approach, called 180FT, exploits the redundancy of fan-beam data acquired over 360° to generate a second set of longitudinal samples at each projection angle and bin. These approaches, and particularly the 180FT approach, have been shown under certain conditions to produce reconstructed volumes with more isotropic resolution and aliasing properties than do existing approaches based on the use of linear interpolation. We present evidence that the approaches also have favorable noise uniformity properties relative to currently used approaches
[Show abstract][Hide abstract] ABSTRACT: 1] In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0°N to 60°N at intervals of 5°. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40°), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40°), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.
Journal of Geophysical Research Atmospheres 07/2009; 114(A7). DOI:10.1029/2009JA014072 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Although analyses of in-plane aliasing have been done for conventional computed tomography (CT) images, longitudinal aliasing in spiral CT has not been properly investigated. We propose a mathematical model of the three-dimensional (3-D) sampling scheme in spiral CT and analyze its effects on longitudinal aliasing. We investigated longitudinal aliasing as a function of the helical-interpolation algorithm, pitch, and reconstruction interval using CT simulations and actual phantom scans. Our model predicts, and we verified, that for a radially uniform object at the isocenter, the spiral sampling scheme results in spatially varying cancellation of the aliased spectral islands which, in turn, results in spatially varying longitudinal aliasing. The aliasing is minimal at the scanner isocenter, but worsens with distance from it and rapidly becomes significant. Our results agree with published results observed at the isocenter of the scanner and further provide new insight into the aliasing conditions at off-isocenter locations with respect to the pitch, interpolation algorithm, and reconstruction interval. We conclude that longitudinal aliasing at off-isocenter locations can be significant, and that its magnitude and effects cannot be predicted by measurements made only at the scanner isocenter.
IEEE Transactions on Medical Imaging 02/1999; 18(1):43-58. DOI:10.1109/42.750254 · 3.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The low-latitude ionospheric tomography network (LITN) consists of a chain of six Naval Navigation Satellite System (NNSS) receiving stations established along 121øE longitude from a geographic latitude of 14.6øN to 3 IøN. It is specifically designed to observe large-scale ionospheric variations over the equatorial anomaly region by using tomographic imaging techniques. Recently, the network LITN was applied to observations of the October 24, 1995, solar eclipse. Two-dimensional images of ionospheric electron density during the eclipse period were reconstructed. These images and the corresponding results from a nearby ionosonde were compared with those for a reference day. It is shown that during the eclipse day the ionosphere experienced some large-scale changes. In particular, four episodes of electron density enhancement or depression have been identified. (1) The maximum enhancement occurred before the maximum phase of the solar eclipse at approximately 7 ø- 10øN geomagnetic latitude at the 275-300 km ionospheric height. (2) The second enhancement appeared roughly 3 1/2 hours after the maximum obscuration at 15 ø- 22øN geomagnetic latitude and 300-325 km ionospheric height. (3) The largest electron density depression occurred roughly 2 hours after the maximum obscuration at approximately 9ø-15øN geomagnetic latitude and on both the bottom and topside ionosphere. (4) The second depression occurred about 4 hours after the maximum obscuration at approximately 5.5øN geomagnetic latitude and mainly on the topside ionosphere. More detailed study suggests that the two enhancements have their origins in the ionospheric day-to-day variations, the first depression is related to the combined photochemical and the equatorial fountain effects, and the second depression may have its origin in geomagnetic coupling between conjugate ionospheres. These observations are interpreted within the framework of ionospheric dynamics in the equatorial anomaly region.
Journal of Geophysical Research Atmospheres 01/1999; 104(A1):79-94. DOI:10.1029/98JA02531 · 3.43 Impact Factor
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