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High-brightness and short-pulse-width ( approximately 200 ps) x-ray lasers offer biologists the possibility of high-resolution three-dimensional imaging of specimens in an aqueous environment without the blurring effects associated with natural motions. As a first step toward developing the capabilities of this type of x-ray microscopy we have used a tantalum x-ray laser (lambda = 4.483 nm) together with an x-ray zone plate lens to image a test pattern. The observed image shows a detector-limited resolution of approximately 75 nm and paves the way to three dimensional biological imaging with high spatial resolution (20-30 nm).

Given only the static boundary flux and potential, electrical impedance tomography solves the inverse problem for the conductivity distribution. A Gauss-Newton solution is presented to solve this nonlinear problem when the conductivity distribution is represented by a piecewise polynomial basis function. An efficient method is presented to solve for the Jacobian matrix. This efficiency is made possible because of the local support of the basis functions used to approximate the conductivity distribution and data collection using the four-electrode technique. A method is presented for the local support case to solve for the Jacobian constants, which are needed to assemble the Jacobian matrix. It is shown that when higher than piecewise constant conductivities are desired it is more efficient to model conductivity than resistivity. Results are presented showing simulated reconstructions using a piecewise constant conductivity representation, and a (bi)linear conductivity representation with C° continuity across conductivity elements. These results show that although the Gauss-Newton method performs very well, further work needs to be done in designing meshes that increase the conditioning of the approximate Hessian matrix.

Tomographic x-ray holography may make possible the imaging of biological objects at high resolution in three dimensions. We performed a demonstration experiment with soft x-rays to explore the feasibility of this technique. Coherent 3.2 nm undulator radiation was used to record Fourier transform holograms of a microfabricated test object from various illumination angles. The holograms were numerically reconstructed according to the principles of diffraction tomography, yielding images of the object that are well resolved in three dimensions.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper gives a basic overview of holography as applied to x-ray microscopy. Topics discussed include fourier transform holography, phase recovery from a hologram, reconstruction of scattered fields, and the use of straight ray approximations. Three dimensional spatial resolution below 100 nm is possible. (GH)

The x-ray holography program at the Lawrence Livermore National Laboratory has two principal goals: the development of x-ray diffraction techniques for DNA sequence analysis and the development of x-ray laser holography for structural analysis of intact biological cells and organelles. DNA sequence analysis will be accomplished via x-ray diffraction techniques to determine the ensemble average of the sequence of labels along the individual elements of crystalline DNA. X-ray laser holographic imaging will be accomplished via 3-D x-ray holography to elucidate the structure of few hundred angstrom objects such as 300 A chromatin fibers, nuclear pores and nucleic acid replication complexes in living cells. Existing laboratory x-ray lasers will be used to produce flash x-ray holograms of the biological structures.

In this paper we discuss the potential for application of x-ray holographic imaging techniques to the sequencing of DNA. We formulate an approximate model for the scattering of partially coherent x-rays from an oriented DNA fiber and show the feasibility of reconstruction of heavy atom label positions from the x-ray scattering data. A series of simulations has been done to demonstrate the required reconstruction algorithms. An x-ray experiment is currently in progress to demonstrate the real feasibility of the technique. The potential of x-ray imaging techniques for the sequencing of DNA is attractive because of their inherently parallel nature. Hundreds or thousands of base pairs could be sequenced in a single set of x-ray images. The fundamental idea is to attach heavy atom labels to a selected base type on the DNA fragment to be sequenced. A large number (> 1012) of identical fragments can be constructed as an oriented fiber and illuminated with partially coherent x-rays. The heavy label positions can then be determined from the recorded pattern of scattered x-rays. If this operation is repeated for each of the four bases, the sequence can be reconstructed. The phase determination problem is solved by attaching to each DNA fragment a reference label in a known position. The scattered field then forms a Fourier transform hologram of the averaged DNA fragment. Because of the high photoelectric absorption of DNA relative to its coherent scattering cross-section, a single molecule would be damaged before an image could be formed. We solve this problem by distributing the damage over a large number of identical copies of the DNA fragment. In this paper we model a relatively simple experiment whose objective is to form a Fourier transform hologram of a labelled DNA fiber using 1 .54 A x-rays. We will first describe the hologram formation process and then the method for reconstructing the label positions.

An improved perturbation technique for electrical impedance tomography is proposed. This improvement is based on simultaneous corrections. Previously the perturbation technique limited corrections to a central region of the image model. When the whole model was allowed to vary, the perturbation technique diverged unless simultaneous corrections were used. The size of the perturbation matrix was minimized by using only unique measurements. Some problems associated with the perturbation matrix are discussed. Namely, it is a highly nonlinear function of the resistivity perturbation amount, and it does not hold for profiles other than the one for which it was calculated.

In this paper, model calculations evaluate the usefulness of tomographic image reconstruction from cross-borehole resistivity measurements. The model has two resistive layers with a conductive anomaly imbedded in the lower layer. We calculated the potential distributions along the borehole resulting from a dipole current source located at various places in each borehole. From these data a resistivity map (tomograph) is constructed. For many different cases these tomographs were good representations of both spatial distribution and magnitude of the model resistivity. 6 refs., 4 figs.