Browsing by Subject "tomography"

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  • Kanclerz, Piotr; Khoramnia, Ramin; Wang, Xiaogang (2021)
    Introduction: Accurate assessment of the corneal shape is important in cataract and refractive surgery, both in screening of candidates as well as for analyzing postoperative outcomes. Although corneal topography and tomography are widely used, it is common that these technologies are confused. The aim of this study was to present the current developments of these technologies and particularly distinguish between corneal topography and tomography. Methods: The PubMed, Web of Science and Embase databases were the main resources used to investigate the medical literature. The following keywords were used in various combinations: cornea, corneal, topography, tomography, Scheimpflug, Pentacam, optical coherence tomography. Results: Topography is the study of the shape of the corneal surface, while tomography allows a three-dimensional section of the cornea to be presented. Corneal topographers can be divided into large- and small-cone Placido-based devices, as well as devices with color-LEDs. For corneal tomography, scanning slit or Scheimpflug imaging and optical coherence tomography may be employed. In several devices, corneal topography and tomography have been successfully combined with tear-film analysis, aberrometry, optical biometry and anterior/posterior segment optical coherence tomography. Conclusion: There is a wide variety of imaging techniques to obtain corneal power maps. As different technologies are used, it is imperative that doctors involved in corneal surgery understand the science and clinical application of devices for corneal evaluation in depth.
  • Tiira, Timo; Janik, Tomasz; Skrzynic, Tymon; Komminaho, Kari; Heinonen, Aku; Veikkolainen, Toni; Väkevä, Sakari; Korja, Annakaisa (2020)
    The Kokkola–Kymi Deep Seismic Sounding profile crosses the Fennoscandian Shield in northwest-southeast (NW–SE) direction from Bothnian belt to Wiborg rapakivi batholith through Central Finland granitoid complex (CFGC). The 490-km refraction seismic line is perpendicular to the orogenic strike in Central Finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013. The campaign resulted in 63 usable seismic record sections. The average perpendicular distance between these and the profile was 14 km. Tomographic velocity models were computed with JIVE3D program. The velocity fields of the tomographic models were used as starting points in the ray tracing modelling. Based on collected seismic sections a layer-cake model was prepared with the ray tracing package SEIS83. Along the profile, upper crust has an average thickness of 22 km average, and P-wave velocities (Vp) of 5.9–6.2 km/s near the surface, increasing downward to 6.25–6.40 km/s. The thickness of middle crust is 14 km below CFGC, 20 km in SE and 25 km in NW, but Vp ranges from 6.6 to 6.9 km/s in all parts. Lower crust has Vp values of 7.35–7.4 km/s and lithospheric mantle 8.2–8.25 km/s. Moho depth is 54 km in NW part, 63 km in the middle and 43 km in SW, yet a 55-km long section in the middle does not reveal an obvious Moho reflection. S-wave velocities vary from 3.4 km/s near the surface to 4.85 km/s in upper mantle, consistently with P-wave velocity variations. Results confirm the previously assumed high-velocity lower crust and depression of Moho in central Finland.
  • Harri, Ari-Matti; Schmidt, Walter; Romero, Pilar; Vázquez, Luis; Barderas, Gonzalo; Kemppinen, Osku; Aguirre, Carlos; Vázquez-Poleti, Jose Luis; Llorente, Ignacio M.; Haukka, Harri; Paton, Mark (2012)
    Raportteja - Rapporter - Reports 2012:2
    Abstract We present a general approach to study solar eclipses by Phobos on Mars: its parameterization and prediction.The validation of the model and the involved parameters is made with the already observed eclipses by previous Mars missions. Eclipse prediction is applied for the past Mars lander missions: Viking, Pathfinder and Phoenix, as well as for the future Mars MetNet Precursor Mission. A successful detection of eclipses could be used for the localization of landers and to study atmospheric properties. We also consider the data analysis, with special emphasis in the tomographic method to identify events which are very localized in space and time. Large computation requirements are needed for the implemented methods. To this propose an efficient Cloud Computing Network Infrastructure has been used.
  • Greenleaf, Allan; Lassas, Matti; Santacesaria, Matteo; Siltanen, Samuli; Uhlmann, Gunther (2018)
    The ill-posedness of Calderon's inverse conductivity problem, responsible for the poor spatial resolution of electrical impedance tomography (EIT), has been an impetus for the development of hybrid imaging techniques, which compensate for this lack of resolution by coupling with a second type of physical wave, typically modeled by a hyperbolic PDE. We show in two dimensions how, using EIT data alone, to use propagation of singularities for complex principal-type PDEs to efficiently detect interior jumps and other singularities of the conductivity. Analysis of variants of the CGO solutions of Astala and Paivarinta (Ann. Math. (2) 163: 1 (2006), 265-299) allows us to exploit a complex principal-type geometry underlying the problem and show that the leading term in a Born series is an invertible nonlinear generalized Radon transform of the conductivity. The wave front set of all higher-order terms can be characterized, and, under a prior, some refined descriptions are possible. We present numerics to show that this approach is effective for detecting inclusions within inclusions.