Image Amplifier Research Articles

MO-E-I-609-04: New Detector for Low Dose Cone Beam Computed Tomographic (CT) Mammography Based On Microchannel Plate Image Amplifiers

Purpose: It is becoming recognized that tomosynthesis and/or full cone‐beam CTmammography may provide more diagnostic information than standard two view projection mammography; however, current flat panel digital image receptors have limitations on image quality and low dose due to excessive lag and detectornoise. Maintaining the same or lower total procedure dose can severely restrict the per‐view exposure, or the number of views. A new high‐resolution detector with adjustable high gain is being investigated to overcome these limitations. Method and Materials: A detector design consisting of a CsI x‐ray converter, fiber‐optic taper, microchannel‐plate‐based image amplifier with variable gain of 6 orders of magnitude, and optical coupling to a high resolution rapid sequence, low noise, 10242–20482 matrix pixel CCD camera is being investigated for this application. Results: A detector with a 16 cm diameter FOV can be achieved with the capability for total scan times of a few seconds. Additionally, off‐center magnification modes of full‐field‐corrected truncated cone‐beam CT are possible involving either geometric or optical magnification to improve spatial resolution. Initial tests of a small field‐of‐view prototype detector of the proposed design with 10242 matrix demonstrated linear response (quantum limited performance) for a range of detector entrance exposures from less than 0.2 microR to greater than 1 mR per view for pixels below 35 microns. Conclusion: The proposed detector design has advantages over flat panel detectors of exhibiting virtually no ghosting or lag and of providing more than 10× lower exposure per view capability due to the huge gain dynamic range and low noise available. Further investigations involving optimization of voxel size, number of views, reconstruction algorithm, exposure per view, exposing x‐ray spectrum, equalization filters, scatter reduction, patient positioning, and total scan times are proceeding. (Partial support: NIH grants R01EB002873 and R01NS43924)

Optical image of squeezed coherent states in the parametric image amplifier

The properties of the output optical image are investigated by taking squeezed c oherent states as the input image in the parametric image amplifier.Our results show that the squeezing degree and squeezing direction of the output optical ima ge can be controlled by the parametric image amplifier,namely,the increase or de crease of the squeezing degree and the change of the squeezing direction are dep endent on the parameter selection of the parametric image amplifier.

Closed reduction and percutaneous fixation offemoral neck fractures in children

Fracture of the femoral neck is a rare injury in children but remains difficult problem and challenge with high rate of complications. Having in mind recommendations of the American Academy of Orthopaedic Surgeons, our effort was to improve treatment approach. In the period 1999 to 2004, five patients, aged 5 to 14 years, were treated by closed reduction and percutaneous pinning. All patients sustained high energy trauma and displaced femoral neck fractures, out of which two were transcervical, two cervicotrochanteric and one intertrochanteric. Procedures were performed under image amplifier. Hip decompression was achieved by needle aspiration via a subadductor approach. Three smooth Kirschner wires appeared to be stable and least aggressive fixation device, easy for applying and subsequent removal. We insisted on urgent treatment, anatomic alignment, stabile fixation and hip decompression. Excellent results were obtained in four patients who had undergone immediate treatment. Poor outcome with vascular necrosis in a five-year-old girl was attributed to five-day treatment delay. Suggested treatment, simple and applicable in practice, is aimed at reducing pathologic mechanisms crucial for development of complications.

Investigation and optimization of a pulsed ultraviolet image amplifier within a micro-marking system

We present an all optical image amplifier in the pulsed ultraviolet (355 nm) based on photorefractive two wave mixing and its implementation in an optical micro-lithographic system. Due to the high possible amplification factor it is possible to use masks with low damage threshold such as polymeric slides or an amplitude modulator based on liquid crystals. This leads to a versatile system which is independent of expensively prepared masks. The amplified image is de-magnified to the sub-millimetre range and projected onto the target. The main application for our system is the authentication of artifacts by imprinting an individual code onto the item surface.

Realization of an all optical image amplifier in the pulsed UV range and implementation to a micro-marking system

We report on the implementation of an all optical image amplifier by use of two wave mixing based on the photorefractive effect, within a micro-lithographic authentification system. The combination of a computer driven spatial light modulator and a holographic image amplifier enables the parallel engraving of information without the need of previously prepared masks. The necessity for an amplification of the information carrying beam rises from the damage threshold of the spatial light modulation device, which is lower than the energy density required for imprinting the information onto the objects surface. This work is embedded in a European Growth project, Holographic Authenticity Sensors, GRD1 –1999 –11078.

Pattern recognition in the automatic inspection of aluminium castings

∑ Image Formation: The images are obtained by X-ray irradiation of the test-piece. The X-rays are then converted to a visible image by means of an image amplifier or a flat-panel detector that are sensitive to X-rays. The sensor is bi-dimensional (or unidimensional in motion) in order to capture the two dimensions of the image. An A/D converter turns the electrical signal into binary code that can be interpreted by a computer to form a digital image of the study object. ∑ Pre-processing: This stage is devoted to improving the quality of the image in order to better recognise flaws. Some of the techniques used in this stage are elimination of noise by means of digital filters or integration, improvement of contrast, and restoration. ∑ Segmentation: The segmentation process divides the digital image into disjoint regions with the purpose of separating the parts of interest from the rest of the scene. Over the last few decades, diverse segmentation techniques have been developed. These can largely be divided into three groups: pixel, edge and region orientated techniques. The present investigation uses the segmentation process oriented towards the detection of edges by employing the LoG filter (Mery and Filbert, 2002b). As can be seen in Figure 2, this technique searches for changes in the grey values of the image (edges), thus identifying zones delimited by edges that indicate flaws. ∑ Feature extraction: In the inspection of cast pieces, segmentation detects regions that are denominated as ‘hypothetical defects’, which may be flaws or structural features of the object. Subsequently the feature extraction is centred principally around

Bipolar two-dimensional analysis of grating dynamics in photorefractive thin films

A two-dimensional analytical model is developed to describe transient grating formation for bipolar transport in photorefractive devices. This approach is suitable for structures in which the film thickness is comparable with or smaller than the grating vector. A crucial feature of the model is that it takes into account the edge effects by including both parallel and perpendicular components of the fields and the currents. The treatment is entirely general and valid for any geometry, in particular, for parallel (image amplifiers, etc) and for perpendicular (Pockels readout optical modulators, etc) photorefractive devices. The role of bulk and surface charges in each configuration is discussed. The results are illustrated in a few examples showing the effect the material and geometrical parameters have on the device resolution thus imposing more stringent limits upon the resolution than those found in previous studies.

A Microscope for Two-Dimensional Measurements of In Vivo Chlorophyll Fluorescence Kinetics Using Pulsed Measuring Radiation, Continuous Actinic Radiation, and Saturating Flashes

Transients of chlorophyll fluorescence in photosynthetic objects are often measured using short pulses of exciting radiation, which has recently been employed to capture kinetic images of fluorescence at the macroscopic level. Here we describe an instrument introducing this principle to recording of two dimensional fluorescence transients in microscopic objects. A modified fluorescence microscope is equipped with a CCD camera intensified by a micro-channel plate image amplifier. The microscopic field is irradiated simultaneously by three types of radiation: actinic radiation, saturating flashes, and pulsed measuring radiation. The measuring pulses are generated by a light-emitting diode and their duration is between 10 to 250 µs. The detection of fluorescence images (300×400 pixels, 8 bit) has a maximum time resolution of 40 ms and is gated in synchrony with the exciting pulses. This allows measuring on a background of a continuous actinic radiation up to irradiance that can elicit the maximal fluorescence yield (FM). On the other hand, the integral irradiance of the objects by the measuring radiation is very low, e.g., 0.08 µmol m−2 s−1 at 0.05 µm spatial resolution and 0.006 µmol m−2 s−1 at 4 µm spatial resolution. This allows a reliable recording of F0 even in very short time intervals (e.g., 5×80 ms). The software yields fluorescence kinetic curves for objects in user-selected areas as well as complete false-colour maps of the essential fluorescence kinetics parameters (FM, FO, FV, FV/FM, etc.) showing a two-dimensional distribution of their values. Several examples demonstrate that records of fluorescence kinetics can be obtained with a reasonable signal-to-noise ratio with all standard microscope objectives and with object sizes reaching from segments of leaf tissue to individual algal cells or chloroplasts.

Image amplifier based on a photoelectret-electro-optic medium sandwich-type structure

This paper presents theoretically the architecture, working principle and computed performance for an image amplifier based on a sandwich-type structure containing both photoelectret and electro-optic medium layers. The device is versatile and allows image amplification (both intensity and/or contrast), image attenuation (both intensity and/or contrast) as well as real-time image reversal. The device input and output are both optical. The device can also be used as an image converter from γ-ray, x-ray or ultraviolet radiation to visible light.

Parametric image amplifiers with correction of the gain inhomogeneity

A longitudinal shift of the amplified and converted images of a beam focussed in a nonlinear component are determined. This shift is related to a change in the effective wave number in the course of parametric amplification. When the gain is considerable, the dependence of this shift on the pump intensity is weak and it does not reduce the resolving power. It is also shown that an inhomogeneity of the distribution of the gain over the field of view can be corrected in the second stage of an amplifier which amplifies the angular spectrum of the radiation transmitted by the first stage.

Transient laser-induced thermochemical processes on metal surfaces and their visualisation with a laser image amplifier

Laser oxidation of metals and alloys (steel, copper, brass, titanium, etc.) was investigated with a laser image amplifier based on a Cu laser making use of computer image processing. A method was developed for measuring the growth of an oxide film by recording the motion of interference minima (maxima) of the reflectivity for copper laser radiation when the oxide distribution was nonuniform. The results were compared with theoretical models.

Bright Future Seen as Possible for Digitized X‐Ray Image Amplifiers

Bright Future Seen as Possible for Digitized X‐Ray Image Amplifiers

In situ calibration of microchannel-plate-based x-ray pinhole camera for observation of magnetically trapped plasma

The electronic amplification gain of a microchannel plate (MCP), as employed for detector and image amplifier of an x-ray pinhole camera, tends to decrease as the output current increases, posing problems both in quantitative analyses and in construction of a three-dimensional emissivity distribution. We report that the output-current dependent MCP gain is described in a simple empirical formula that is determined by an in situ calibration experiment using a steady-state low-temperature discharge plasma. We examine the validity of the formula affirmatively in correcting raw data of x-ray images of magnetically trapped hot plasma. It is also demonstrated that the correction leads to a three-dimensional distribution of soft x-ray emissivity in a quadrupole-mirror-trapped hot plasma that is consistent with other indirect measurements.

Unified two-dimensional model for grating dynamics in photorefractive materials

A two-dimensional treatment suitable both for perpendicular (Pockels readout optical modulators, etc.) and for parallel (image amplifiers, etc.) photorefractive devices is presented. A partial differential equation of the third order, valid for both families of devices, is set up and solved analytically for the relevant initial and boundary conditions in the full time domain. The results are illustrated in a few examples showing the relative significance of bulk and surface charges, and some new features of the solution for the perpendicular configuration.

Panoramic observation of Thomson scattering of laser radiation by a nonrelativistic electron beam

Thomson scattering of ruby laser radiation by a nonrelativistic beam was investigated with the aid of a television-type detector based on an image dissector. The video signal was amplified first in an image amplifier consisting of two image converters. Panoramic investigation of the parameters of an electron beam with a density of the order of 1010 cm-3 is shown to be possible by recording the radiation emitted by the residual gas under the action of the electron beam and calibration of the Thomson scattering results.

X-ray kinematography as a tool for investigations of distal limb movements of the cat

X-ray kinematography was used to investigate the kinematics of cat distal forelimb joints during motor behaviour. These joints are not accessible for instrumentation with external markers normally used in conventional motion analysis systems. To trace the movements in space two X-ray systems positioned rectangularly to each other illuminated the forelimb quasi-simultaneously with pulsed X-ray shots (time resolution: 20 ms). A mathematical model was developed for 3-dimensional reconstruction of the object and includes procedures for correction of image distortions, e.g., pincushion distortions and rotation of the image due to interaction of the earth's magnetic field with the electron optics of the image amplifiers. Accuracy of image correction and object reconstruction is ±1 pixel, corresponding to ±0.5 mm in space which is sufficient for investigation of the kinematics of cat distal forelimb joints. The approach described is of general relevance and useful in kinematic investigations where the structures under study are not directly accessible to external instrumentation with markers.

Photorefractive Materials

There is a growing demand for nonlinear optical materials for a variety of applications—lasers and coherent sources, electrooptic devices, communication technologies, and optical processors and computers. Nonlinear optics is a vast field requiring materials with diverse performance features. Photorefractive (PR) materials, which experience a change in the refractive index under the effect of inhomogeneous illumination, constitute a relevant branch of the field. They behave as third-order nonlinear materials, which can be considered, in general, as photorefractive. However, the materials more commonly designated as photorefractives involve a charge-transport-induced nonlinearity, and it is these materials which are the object of this issue of the MRS Bulletin.At variance with conventional (often designated as Kerr) nonlinear materials, photorefractives are sensitive not to the local light intensity but to its spatial variation; i.e., they are nonlocal materials. This feature makes them more complicated to deal with than their conventional counterparts, since a χ(3) susceptibility cannot be properly defined (except as a k-dependent function). On the other hand, this sensitivity gives them some unique and interesting features. In particular, an interference light pattern illuminating the crystal and the generated index grating are phase-shifted, leading to remarkable beam coupling and amplification effects. The coupling gain can be markedly enhanced by applying alternating electric fields or by oscillating the interference fringes with a piezoelectric mirror. Efficient image amplifiers have been made using this effect.

Noise reduction in photorefractive image amplifiers and applications

We propose and experimentally demonstrate two original methods for suppressing the beam fanning noise of photorefractive amplifiers. The first one involves two wave mixing in a slowly rotating crystal, while the second requires to tilt the crystal with respect to the recording beams. This method applies to materials which exhibit a sharp resonance of the gain versus the pump-probe beam angle. With both methods a large improvement of the S/N ratio is achieved and virtually noise free image amplifiers are demonstrated. In Bi 12 SiO 20 , subpicowatt optical signals are detected and amplified. By using combinations of LiNbO 3 /Bi 12 SiO 20 , as storage and amplification crystals respectively a long term readout (more than 20 hours) of a photorefractive memory is achieved

Multibeam projection microinterferometer with an image amplifier

An experimental investigation was made of the principal properties of a multibeam projection microinterferometer with a copper-vapor image amplifier. The optical system and the operating principle of the microinterferometer are described. It is shown that the lower limit of the height of a microstructure which can be determined is less than 10 nm. If the edges of the microstructure are sharp, the upper limit can reach 5λ, where λ = 0.5782 μm is the wavelength of the radiation of the copper-vapor image amplifier.

Time-gated imaging through scattering media using stimulated Raman amplification

We propose the use of stimulated Raman scattering for time-gated image amplification and demonstrate its use for the detection of images through a strongly scattering material. Using 30-ps pulses from a frequency-doubled Nd:YAG laser, we have amplified and detected images through a suspension of nondairy creamer with a spatial resolution of less than 300 microm and at concentrations such that the nonscattered extinction ratio was e(-33). Our time-gated image amplifier can produce images under conditions in which the scattering medium is sufficiently dense that an image cannot be seen by using multiple exposures on a streak camera or time-integrated exposures on a sensitive, low-noise CCD camera.