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Biomedical subjects

G Y Fan

Publications and source records attributed to G Y Fan.

13 recordsLinked to original sources
PubMed1999

Video-rate scanning two-photon excitation fluorescence microscopy and ratio imaging with cameleons.

A video-rate (30 frames/s) scanning two-photon excitation microscope has been successfully tested. The microscope, based on a Nikon RCM 8000, incorporates a femtosecond pulsed laser with wavelength tunable from 690 to 1050 nm, prechirper optics for laser pulse-width compression, resonant galvanometer for video-rate point scanning, and a pair of nonconfocal detectors for fast emission ratioing. An increase in fluorescent emission of 1.75-fold is consistently obtained with the use of the prechirper optics. The nonconfocal detectors provide another 2.25-fold increase in detection efficiency. Ratio imaging and optical sectioning can therefore be performed more efficiently without confocal optics. Faster frame rates, at 60, 120, and 240 frames/s, can be achieved with proportionally reduced scan lines per frame. Useful two-photon images can be acquired at video rate with a laser power as low as 2.7 mW at specimen with the genetically modified green fluorescent proteins. Preliminary results obtained using this system confirm that the yellow "cameleons" exhibit similar optical properties as under one-photon excitation conditions. Dynamic two-photon images of cardiac myocytes and ratio images of yellow cameleon-2.1, -3.1, and -3.1nu are also presented.

Biophysical Phenomena↗
PubMed1998

ASIC-based event-driven 2D digital electron counter for TEM imaging.

A two-dimensional application specific integrated circuit (ASIC) based detector, designed for X-ray protein crystallography, has been tested to determine its suitability as a direct electron detector for TEM imaging in the voltage range of 20-400 keV. Several markedly different properties of this device distinguish it from the charge coupled device (CCD) detectors: (1) the ASIC detector can be used directly under electron bombardment in the voltage range stated above, therefore requiring no scintillator screen; (2) each active pixel of the device is an electron counter and generates digital output independently; (3) the readout of the device is frameless and event driven; (4) the device can be operated at the room temperature and is nearly noise free; and (5) the counting dynamic range of the device is virtually unlimited. It appears that an imaging system based on this type of device would be ideal for low-dose TEM imaging and online diffraction observation and recording, as well as more conventional imaging, providing the many advantages of direct digital readout for almost all applications.

Crystallography, X-Ray↗
PubMed1996

In vivo calcineurin crystals formed using the baculovirus expression system.

Calcineurin is a heterodimeric phosphatase involved in the signal transduction of antigen-activated T cells. Coexpression of its two subunits, the regulatory subunit from human and the catalytic subunit from Neurospora crassa in cultured insect cells using the baculovirus expression system results in the formation of very large crystals in the cytoplasm. The crystals are formed initially in vesicles, but their subsequent growth appears to be uninhibited and continues without the need of an enclosing membrane until the host cell lyses. Although these in vivo crystals are low in population, ranging only 0-3 per cell, they are extremely large, over 10 mu m in some cases. Biochemical assays confirm their calcineurin origin, with the regulatory subunit incorporated being myristoylated, although both the myristoylated and unmyristoylated forms are expressed. The lattice structure of the in vivo crystals, with a spacing of 5.5 nm, is preserved with the regular electron microscopic (EM) specimen preparation procedure.

Animals↗
PubMed1996

Optimization of thin-foil based phosphor screens for CCD imaging in TEM in the voltage range of 80-400 kV.

The voltage dependence characteristics of thin-foil based phosphor screens in the thickness range of approximately 10-60 microns are examined for CCD imaging in transmission electron microscopy (TEM) in the voltage range of 80-400 kV. The brightest screen is obtained with a P20 layer of about 12 microns at 80 kV, and a thicker screen lowers both the screen brightness and resolution. The thickness of the brightest screen is higher at higher voltage, but other considerations for a practical CCD imaging system suggest that the P20 layer should not be greater than approximately 18 microns for the voltage range stated above.

Microscopy, Electron↗
PubMed1995

Conditions for electron tomographic data acquisition.

The conventional algorithms employed in electron microscope tomography require that the series of images obtained from different orientations of the specimen each represent a parallel orthographic projection under uniform magnification. Electron microscope optics can produce distortions in images that may affect the accuracy of a tomographic reconstruction. These distortions result in images with differential rotation and magnification of regions of a thick or highly tilted specimen located at different distances with respect to the plane of focus. The distortions increase in magnitude and may significantly affect the accuracy of the tomographic data when images are acquired under current center misalignment or non-eucentric positioning of the specimen. An additional source of error can be introduced if the condenser lenses are adjusted to compensate for the intensity attenuation due to the increased beam path length as the specimen is tilted. The change in beam crossover position due to adjustment of the condensers alters the beam divergence and may introduce systematic changes in magnification over the tilt series. The process of alignment of the tilt series will propagate these errors to more central regions of the image. In most cases, with proper consideration, it is possible to minimize these errors to levels where they will have negligible effects on the resolution of the tomographic reconstruction.

Algorithms↗
PubMed1994

Performance of thin foil scintillating screen for transmission electron microscopy.

Scintillating screens made by coating a thin metal foil with a layer of phosphor appear to be attractive for transmission electron microscopy applications. We report here the brightness and resolution in the voltage range of 100-400 kV of such a screen made of a 10 microns layer of P20 phosphor on a 2 microns Al foil. Both brightness and resolution are superior to that of a screen made by coating a glass plate with a similar layer of phosphor. An exciting property of such a device is that its resolution improves slightly at higher voltages. This, combined with its excellent resistivity to radiation damage and stability under the electron beam, makes it a good candidate for high-voltage applications.

Animals↗
PubMed1994

Stereoscopy by tilted illumination in transmission electron microscopy.

We describe a new technique for stereoscopic observation in transmission electron microscopy, employing tilted illumination. A triple-hole objective aperture is used so that the bright beam can pass through with or without tilt. Stereo views can be acquired by tilting the illumination such that the bright beam passes through a pair of symmetrically arranged apertures alternately. The advantages of this technique as compared to the commonly used method of single-axis tilt are: (i) greater speed, potentially at or close to video rate so that live 3D observation is possible; (ii) elimination of specimen movement associated with stage tilt; and (iii) perspective views corresponding to multiple tilting axes which can be realized by installing more aperture-holes and choosing their positions properly. The main limitation is that the angle of tilt is limited by the extent to which the astigmatism, introduced by beam tilt, can be compensated by the objective lens stigmators.

Animals↗
PubMed1993

High-sensitivity lens-coupled slow-scan CCD camera for transmission electron microscopy.

A lens-coupled slow-scan CCD camera has been built for transmission electron microscopy (TEM) applications. In this design, a leaded glass window, which is coated with a 20 microns layer of red P20 phosphor, is mounted on the bottom of the microscope. A lens assembly and mirror prism, located outside the microscope vacuum below the leaded glass, relays the image onto a back-thinned 1kx1k charge-coupled device (CCD) detector. This CCD is electronically cooled to below -30 degrees C during operation. It is found that X-ray irradiation, generally found to be annoying in fiber-optically coupled CCD cameras, is completely eliminated by this configuration. The collection efficiency of this system, although not as high as some of the fiber-optically coupled CCD cameras, is high enough to achieve single-electron sensitivity under a high-gain mode.

Equipment Design↗
PubMed1993

Telemicroscopy.

The feasibility of operating an electron microscope from a remote location using a high-speed computer network has been investigated. A 400 kV electron microscope was modified so that most microscope parameters and stage movement (x, y, z and tilt), can be controlled by a local host computer. The host computer also controls acquisition of digital images from the microscope from either a TV or a slow-scan CCD camera. A computer located at a remote site may send commands to the host computer via a fiber optic network to receive digital images and information concerning the status of microscope. Such a system has been demonstrated at the SIGGRAPH 1992 conference in Chicago and the Supercomputing 92 conference in Minneapolis, where we remotely collected three-dimensional image data sets using the electron microscope in San Diego. This approach, referred to as "telemicroscopy", promises to make possible live microscopy teleconferencing and thereby increase the accessibility and effective usage of the limited number of specialized microscopy resources.

Computer Communication Networks↗
PubMed1993

Performance characteristics of radioluminescent fiber optics as electron scintillators.

Terbium-activated radioluminescent fiber-optic face plates have been studied for their suitability as electron scintillators. The energy conversion efficiency of such material is determined to be about 2.5% in the electron energy range of 100-400 keV tested. The resolution is approximately 40 microns at electron energy of 100 keV, as measured by the rise width of an edge. The main advantage of such a device is the potential simplification of CCD camera designs, since it functions both as a scintillator as well as a light guide so that it can be coupled direct to a CCD detector.

Animals↗
PubMed1991

[Reversion of eye position after surgical correction of exotropia].

The authors analysed the reversion of eye position in 62 postoperative patients of concomitant exotropia. The results indicated the presence of a correlation with the preoperative condition of binocular vision, the type of strabismus, and the age of patient. The authors opened that the amplitude of surgical correction should depend on these factors, and prudence should be exercised in recommending surgical intervention for mild intermittent exotropia in children.

Adolescent↗