Ping-chin Cheng

Iterative X-ray Cone-Beam Tomography for Metal Artifact Reduction and Local Region Reconstruction

: X-ray cone-beam reconstruction from incomplete projection data has important practical applications, especially in microtomography. We developed expectation maximization (EM)-type and algebraic reconstruction technique (ART)-type iterative cone-beam reconstruction algorithms for metal artifact reduction and local reconstruction from truncated data. These iterative algorithms are adapted from the emission computerized tomography (CT) EM formula and the ART. A key step in our iterative algorithms is introduction of a projection mask and computation of a 3-D spatially varying relaxation factor that allows compensation for beam divergence and data incompleteness. The algorithms are simulated with projection data synthesized from mathematical phantoms. In simulation, the EM-type and ART-type iterative algorithms are demonstrated to be effective for metal artifact reduction and local region reconstruction. They perform similarly in terms of visual quality, image noise, and discrepancy between measured and reprojected data. The EM-type and ART-type iterative cone-beam reconstruction algorithms have potential for metal artifact reduction and local region reconstruction in X-ray CT.

Multi-photon fluorescence microscopy — the response of plant cells to high intensity illumination

Multi-photon fluorescence microscopy has been cited for its advantage in increased depth penetration due to low linear absorption and scattering coefficient of biological specimen in the near infrared (NIR) range. Because of the need of high peak power for efficiently exciting two-photon fluorescence, the relationship between cell damage and peak power has become an interesting and much debated topic in the applications of multi-photon fluorescence microscopy. It is conceivable that at high illumination intensity, non-linear photochemical processes have impacts on cell physiology and viability in ways much different from low illumination in the linear domain. In this article, we discuss some of the issues in two-photon fluorescence microscopy, including the degree of transparency of the specimen, a comparison of single- and two-photon excited fluorescence spectra, and the cell damage under high intensity illumination, using plant cells as a model.

Morphology and Development of the Tassel and Ear

As a monoecious plant, maize develops unisexual male and female flowers in physically separated parts of the plant. The tassel (staminate or male inflorescence) arises from the shoot apical meristem, while the ears (pistillate or female inflorescences) originate from the axillary bud apices. Initially, both inflorescences contain bisexual flowers. During the course of development, however, they become unisexual through abortion of gynoecia in the tassel flowers and stamens in the ear flowers. As a result, the tassels develop male flowers and the ears develop female flowers. (See descriptions of Weatherwax 1916; Bonnett 1948, 1953, 1966; Kiesselbach 1949; Cheng et al. 1983; Stevens et al. 1986; Hanway and Ritchie 1987.)

X-Ray Microradiography and Shadow Projection X-Ray Microscopy

The fundamental advantages of soft X rays over electrons in the examination of fine structures have been described in detail by various authors.(1−5) The oretically, x-ray microscopy provides higher resolution than light microscopy, higher penetration ability than electron microscopy, and, most importantly, x-ray microscopy promises the potential for imaging hydrated specimens. Therefore, x-ray microscopy could occupy a niche in biological research in three-dimensional imaging of samples in the resolution range of the electron microscopy but of substantially greater thickness. This would greatly simplify the observation and interpretation of three-dimensional ultrastructures of living specimens beyond the resolution limit of the light microscope.

Photobleaching under single photon and multi-photon excitation: chloroplasts in protoplasts from Arabidopsis thaliana

Abstract Photobleaching as a result of intense laser irradiation has been an important issue in the applications of confocal microscopy. In this study we are employing time-lapsed micro-spectroscopy to compare the effects of photobleaching under single photon and multi-photon excitation. A model is derived to account for photobleaching in both cases and protoplasts from Arabidopsis thaliana are employed as the sample. We have found that multi-photon excitation have impacts on the samples in ways much different from cw illumination in the linear domain.

Confocal Microscopy of Botanical Specimens

In general, plant cells are highly heterogeneous with reference to their optical properties, i.e.,absorbance, refractive index, fluorescence, phosphorescence, and birefringence. Cellular organelles and extracellular structures such as light absorbing chloroplasts and pigments, the cuticular layer and waxes found on the surface of epidermal cells, the cell wall, the exine of the pollen grains, and starch, lipid, and protein granules commonly found in the storage tissues all differ significantly from each other and the surrounding aqueous medium in terms of optical properties. This optical heterogeneity creates a major problem in confocal microscopy of plant cells.

Anther development of maize (Zea mays) and longstamen rice (Oryza longistaminata) revealed by cryo-SEM, with foci on locular dehydration and pollen arrangement.

AbstractKey messagePollen maturation in Poaceae. Another development has been extensively examined by various imaging tools, including transmission electron microscopy, scanning electron microscopy, and light microscopy, but none is capable of identifying liquid water. Cryo-scanning electron microscopy with high-pressure rapid freeze fixation is excellent in preserving structures at cellular level and differentiating gas- versus liquid-filled space, but rarely used in anther study. We applied this technique to examine anther development of Poaceae because of its economic importance and unusual peripheral arrangement of pollen. Maize and longstamen rice were focused on. Here, we report for the first time that anthers of Poaceae lose the locular free liquid during late-microspore to early pollen stages; the majority of pollen grains arranged in a tight peripheral whorl develops normally and reaches maturity in the gas-filled loculus. Occasionally, pollen grains are found situated in the locular cavity, but they remain immature or become shrunk at anthesis. At pollen stage, microchannels and cytoplasmic strands are densely distributed in the entire pollen exine and intine, respectively, suggesting that nutrients are transported into the pollen from the entire surface. We propose that in Poaceae, the specialized peripheral arrangement of pollen grains is crucial for pollen maturation in the gas-filled loculus, which enables pollen achieving large surface contact area with the tapetum and neighboring grains to maintain sufficient nutrient flow. This report also shows that the single aperture of pollen in Poaceae usually faces the tapetum, but other orientation is also common; pollen grains with different aperture orientations show no morphological differences.

Micro-spectroscopy of chloroplasts in protoplasts from Arabidopsis thaliana under single- and multi-photon excitations

Ultrafast laser pulses are known to generate impacts on specimens in ways much different from cw illumination in the applications of confocal microscopy. In this study, protoplasts from Arabidopsis thaliana are employed as the samples to compare the effects induced by single- and multi-photon excitations under confocal imaging configuration. Through time-lapsed micro-spectroscopy we are able to characterize the differences in photo-bleaching and the evolution of fluorescence spectra of chloroplasts thus induced. These differences may be attributed to the disruption of chained reaction as a result of multi-photon excitation in photosynthesis.

Fast Algorithm for X-ray Cone-beam Microtomography

Cone-beam X-ray microtomography attracts increasing attention due to its applications in biomedical sciences, material engineering, and industrial nondestructive evaluation. Rapid volumetric image reconstruction is highly desirable in all these areas for prompt visualization and analysis of complex structures of interest. In this article, we reformulate a generalized Feldkamp cone-beam image reconstruction algorithm, utilize curved voxels and mapping tables, improve the reconstruction efficiency by an order of magnitude relative to a direct implementation of the standard algorithm, and demonstrate the feasibility with numerical simulation and experiments using a prototype cone-beam X-ray microtomographic system. Our fast algorithm reconstructs a 256-voxel cube from 100 projections within 2 min on an Intel Pentium II(R) 233 MHz personal computer, produces satisfactory image quality, and can be further accelerated using special hardware and/or parallel processing techniques.

Experimental System for X-ray Cone-Beam Microtomography

: A laboratory test of X-ray tomography employing a diverging beam of X-rays rather than the usual parallel X-ray beam is described. We chose to test and demonstrate the advantages of divergent beam tomography by imaging an extracted juvenile human premolar using an ordinary dental X-ray source and a cooled CCD camera. Experiments with a three-piece cover-glass sample and with the human tooth demonstrated that three-dimensional reconstruction can be achieved at 34 µm per pixel resolution employing an X-ray tube spot 800 µm in its smallest direction without requiring close contact with the fluorescent screen. Reconstruction of a 256 x 256 pixel single-plane image from 100 projection images took only 45 sec on a personal computer with a Pentium 166 MHz processor. We have also demonstrated a volume reconstruction of 256 x 256 x 256 voxels from the data. Successful extension of this work to submicrometer projection X-ray microscopy is predicted. Improved resolution of medical tomography is another possible application.