D. M. Shinozaki

A general cone-beam reconstruction algorithm

Considering the characteristics of the X-ray microscope system being developed at SUNY at Buffalo and the limitations of available cone-beam reconstruction algorithms, a general cone-beam reconstruction algorithm and several special versions of it are proposed and validated by simulation. The cone-beam algorithm allows various scanning loci, handles reconstruction of rod-shaped specimens which are common in practice, and facilitates near real-time reconstruction by providing the same computational efficiency and parallelism as L.A. Feldkamp et al.s (1984) algorithm. Although the present cone-beam algorithm is not exact, it consistently gives satisfactory reconstructed images. Furthermore, it has several nice properties if the scanning locus meets some conditions. First, reconstruction within a midplane is exact using a planar scanning locus. Second, the vertical integral of a reconstructed image is equal to that of the actual image. Third, reconstruction is exact if an actual image is independent of rotation axis coordinate z. Also, the general algorithm can uniformize and reduce z-axis artifacts, if a helix-like scanning locus is used.

Scanning cone-beam reconstruction algorithms for x-ray microtomography

An x-ray shadow projection microscope using a scannable point source of x-rays is under development at SUNY. The point source is generated by a focused electron beam that can be steered electromagnetically in a plane perpendicular to the optical axis of the microscope. The specimen is mounted on a rotatable mechanical stage for microtomography. An elaborate feedback system is being implemented to measure and correct the motion error of the mechanical stage. The conventional cone-beam image reconstruction algorithm suffers from two constraints. Firstly, the specimen must be contained in a sphere-like reconstruction region. Secondly, the x-ray source must be moved along a circle in the specimen coordinate system. Considering the characteristics of the x-ray microscope system of ARTS-AMIL and the limitations of the conventional cone-beam reconstruction algorithm, a general cone-beam image reconstruction algorithm is presented. The general algorithm can be applied to various scanning geometries, such as polygonal, helical, or random scanning patterns, for different reasons. In order to study the general cone-beam reconstruction algorithm, a computer simulation has been performed. With various scanning parameters, projection data were generated mathematically and then the general cone-beam reconstruction algorithm tested with the simulated data. The experimental results shows that the different kinds of scanning loci of the x-ray source consistently resulted in satisfactory 3-D reconstructed images.

Orientation Effect of Clay Platelets in Transfer Molded Polymer Nanocomposites

Transfer molding has been increasingly used to process polymer composites with various shaped nanoparticles, including platelet type nanoparticles. Platelet nanoparticles exhibit high aspect ratios (length to thickness); therefore their distributions in polymer matrix can be greatly affected by the flow trajectories in the mold. In present study, the clay platelet-polyethylene nanocomposite was prepared by transfer molding. The orientation of clay platelets developed during molding process was analyzed and measured with wide angle X-ray diffraction. Due to large velocity and shear stress gradients in the mold, the platelets caught in surface regions were rotated towards the flow direction and formed the oriented morphologies. With weaker shear stresses at the central region, the platelets were mostly randomly distributed. The shear stresses may be amplified locally at regions near the mold walls, which can further lead to or accelerate the orientations of clay particles. The orientation distribution was found to depend upon the clay fraction and sample size. The oriented platelets can be re-randomized through the annealing process. The orientation of clay platelets enhanced the orientation of polyethylene lamellae and caused shear band formation in composites when deformed.

Preliminary error analysis of the general cone-beam reconstruction algorithm

An x-ray microscope system for microtomography is under development at SUNY/Buffalo, New York. Considering the characteristics of the x-ray microscope system and the limitations of current cone-beam reconstruction algorithms, a general cone-beam image reconstruction algorithm has been developed at AMIL-ARTS. In order to study the reconstruction error characteristics of the general cone-beam algorithm, a preliminary error analysis on the algorithm is performed in this paper. The most important error source in cone-beam reconstruction is the theoretical precision limitation. Like many cone-beam reconstruction algorithms, the general cone-beam algorithm is not exact in nature. Thus, an analytic reconstruction error formula is derived which relates the error to the specimen structure and various imaging parameters. Approximately, the reconstruction error is proportional to either the distance from a voxel to the midplane or the pitch of a helix-like scanning locus, and inversely proportional to the size of the scanning locus. The reconstruction error also depends on the specimen structure. The faster the structure varies along the z direction, the larger the reconstruction error will be. Specimens are modeled as stochastic fields. Typical simulation results are then depicted and discussed.

X-Ray Projection Microscopy and Cone-beam Microtomography

A laboratory sized X-ray projection microscope and microtomographic imaging system is being developed at SUNYAB (AMIL-ARTS). High resolution two dimensional images can be recorded rapidly using a high resolution phosphor and a cooled slow scan CCD camera. The effective source size is kept small to maximize the resolution. For this purpose the electron beam is focused onto a thin film target. Three dimensional information can be derived from the high resolution two dimensional images by generating stereo pairs or by reconstructing the complete tomographic image. Real time stereo pairs can be produced by rapidly moving the position of the electron spot between two locations on the thin film target thus irradiating the specimen with X-rays from two spatially distinct directions. For high resolution tomography or microtomography the two dimensional images are recorded for a large number of different orientations of the specimen. A cone beam reconstruction scheme based on a convolution and back projection algorithm is being developed. To optimize the experimental parameters in microtomography a variety of mathematical phantoms have been examined using computer simulation technique. The experimental results reveal the quantitative relationship between the accuracy of the reconstructed image and the experimental parameters such as object shape orientation and position. 1.

X-Ray Microscopy — Its Application to Biological Sciences

It is not easy to examine bulky or hydrated biological material with an electron microscope (EM). In contrast, x-ray microscopy (XM) provides great penetration power that electron microscopy can not offer, and high resolution with which light microscopy can not compete. Therefore, x-ray microscopy could occupy a niche in biological research in the form of 3D imaging of thick (in comparison to the sample thickness of standard electron microscopy) and living specimens.

Statistical Noise in Soft X-Ray Images Stored in PMMA Resist

At the limit of resolution in recording media such as polymethylmethacrylate (PMMA) resist, the inhomogeneous distribution of photons over the image recording plane results in a background noise in the image. The nature of such noise, and its manifestation in lightly developed resists, is examined. Replicas suitable for examination in the transmission electron microscope have been prepared from PMMA exposed at the Canadian Synchrotron Radiation Facility at the University of Wisconsin. This method is extremely sensitive to the very fine modulations of the resist surface.