For various reasons, a projection dataset acquired on a computed tomography (CT) scanner can be truncated. That is, a portion of the scanned object is positioned outside the scan field-of-view (SFOV) and the line integrals corresponding to those regions are not measured. A projection truncation problem causes imaging artifacts that lead to suboptimal image quality. In this paper, we propose a reconstruction algorithm that enables an adequate estimation of the projection outside the SFOV. We make use of the fact that the total attenuation of each ideal projection in a parallel sampling geometry remains constant over views. We use the magnitudes and slopes of the projection samples at the location of truncation to estimate water cylinders that can best fit to the projection data outside the SFOV. To improve the robustness of the algorithm, continuity constraints are placed on the fitting parameters. Extensive phantom and patient experiments were conducted to test the robustness and accuracy of the proposed algorithm.
A setup is described for simultaneous three-dimensional manipulation and imaging inside a concentrated colloidal dispersion using (time-shared) optical tweezers and confocal microscopy. The use of two microscope objectives, one above and one below the sample, enables imaging to be completely decoupled from trapping. The instrument can be used in different trapping (inverted, upright, and counterpropagating) and imaging modes. Optical tweezers arrays, dynamically changeable and capable of trapping several hundreds of micrometer-sized particles, were created using acousto-optic deflectors. Several schemes are demonstrated to trap three-dimensional colloidal structures with optical tweezers. One combined a Pockels cell and polarizing beam splitters to create two trapping planes at different depths in the sample, in which the optical traps could be manipulated independently. Optical tweezers were used to manipulate collections of particles inside concentrated colloidal dispersions, allowing control over colloidal crystallization and melting. Furthermore, we show that selective trapping and manipulation of individual tracer particles inside a concentrated dispersion of host particles is possible as well. The tracer particles had a core-shell geometry with a high refractive index material core and a lower index material shell. The host particles consisted of the same material as the lower index shells and were fluorescently labeled. The tracer particles could be manipulated without exerting forces on the host particles because the mixture was dispersed in a solvent with the same refractive index as that of the host particles. Using counterpropagating tweezers strongly scattering particles that could not be trapped by conventional single-beam optical tweezers were trapped and manipulated.
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