Fiber Optic Taper Research Articles

Time-Resolved High Resolution Neutron Imaging Studies at the ORNL Spallation Neutron Source

Our evaluation of a high-resolution digital imaging detector capable of providing millisecond time resolution and high sensitivity for neutrons is presented. This detector is a modified version of the high-resolution gamma-ray imager developed previously at Radiation Monitoring Devices, Inc. (RMD, Inc.), and consists of an Electron Multiplying Charge Coupled Device (EMCCD) attached to a neutron-sensitive scintillator via a fiberoptic taper. By virtue of its internal gain, the EMCCD permits high speed readout without introducing additional noise, thereby enabling high frame rate operation with an enhanced signal-to-noise ratio (SNR). Detector sensitivity is enhanced through the use of a back-thinned EMCCD, which provides high quantum efficiency over a typical emission range for many neutron-sensitive scintillators. Preliminary evaluations conducted at the Liquids Reflectometer beam port of the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) demonstrate that this new detector has the sensitivity to detect individual neutrons and the acquisition speed to perform energy-selective imaging with a temporal resolution of milliseconds. While substantial improvements in timing and imaging performance are planned, this prototype detector has already generated the first ever images of the SNS Liquids Reflectometer beam profile and was also used to demonstrate a technique for obtaining Bragg edge transmission imaging using energy-selective neutrons. The preliminary data, along with the detector design, evaluation, and planned developments are discussed in this paper.

TH‐C‐210A‐04: New High Resolution Dynamic Detectors and Flow Modifying Stents for Neuro‐Endovascular Image Guided Interventions

As the field of minimally invasive endovascular image‐guided interventions (EIGI) advances, there has been progress in the development of new endovascular devices such as the asymmetric vascular stent for treatment of aneurysms as well as progress in the image guidance systems needed to improve both the diagnoses and interventions and the methods used to evaluate the improved imaging performance. High resolution imaging systems with MTFs extending past 8 Lp/mm and with ability to visualize not only radioopaque markers but the detailed structure of devices such as stents have motivated the development of new asymmetric devices such as the blood flow modifying asymmetric vascular stent (AVS). The AVS has now come through a number of generations from balloon expandable stainless steel strutted structures with laser micro‐welded mesh flow diverters to new super‐elastic nitinol closedcell self‐expanding stents with organic material flow diverters. The methods of laser machining, surface finishing, and deployment will be described with progress in animal models reported. In parallel with advances in EIGI devices has come new high resolution detector development. The detectors have a unique combination of features such as far superior spatial resolution compared with conventional dynamic flat panel detectors, large dynamic range of sensitivity with negligible lag and ghosting to enable both fluoroscopy and angiography, and low noise to enable quantum limited performance over the full useful range including during lowdose fluoroscopy. The Micro‐Angiographic Fluoroscope (MAF) consists of an x‐ray converter phosphor sensed by a micro‐channel plate light image intensifier which is in turn coupled to a high performance CCD camera using a fiber optic taper. The MAF is a region of interest (ROI) imager with 4 cm field of view centered at the interventional site and may be moved in front of a larger conventional detector when improved resolution is needed. The Solid State X‐ray Image Intensifier (SSXII) while having much of the benefits of the MAF in superior imaging capability achieves its great sensitivity using only electron multiplying CCD sensors. An array design is being developed so that the imaging FOV may be expanded by adding modules each with its own EMCCD‐based detector. To more fully characterize detectors, new evaluation methods are being explored. For example, the accurate determination of MTF from measurements of noise only, without the need for a slit or edge will be reported. Also from a careful analysis of noise, the exposure range for detector quantum limited performance can be well demarcated by the instrumentation noise equivalent exposure (INEE). Finally, more realistic linear system parameters that include focal‐spot size, geometry, and scatter provide generalized MTFs and DQEs or GMTFs and GDQEs. All told, there is much happening in EIGI.Learning Objectives1. Appreciate the progress being made in improved EIGI devices and in particular flow modifiers such as the asymmetric vascular2. stent (AVS) for aneurysm treatment.3. Understand the operation of new high‐resolution micro‐angiographic systems including the MAF and SSXII.4. Understand new objective image detector evaluations including INEE, GMTF, GDQE, and determination of MTF from noise5. measurements alone.(Supported in part by NIH Grants R01EB002873, R01 NS43924, R01EB008425, and the Toshiba Medical Systems Corp.)

SU‐FF‐I‐170: Control, Acquisition, Processing, and Image Display System (CAPIDS) for the Solid‐State X‐Ray Image Intensifier (SSXII)

Purpose: A new control, acquisition, processing, and image display system (CAPIDS) for the Solid‐State X‐Ray Image Intensifier (SSXII) was developed and implemented using Laboratory Virtual Instrumentation Engineering Workbench (LabVIEW). Method and Materials: The SSXII is an EMCCD‐based x‐ray detector that was designed to satisfy the high‐resolution and real‐time needs of neurovascular imaging. The SSXII includes a CsI(Tl) scintillator and a fiber‐optic taper (FOT) coupled to an EMCCD camera with fiber‐optic input window. The SSXII can be mounted on and used with a clinical fluoroscopic c‐arm unit (Toshiba Infinix). The new CAPIDS provides controls for several radiographic imaging modes including: fluoroscopy, roadmapping, radiography, and digital‐subtraction‐angiography (DSA). Complete functionality of the SSXII is also provided in research mode, including the ability to modify the detector‐element binning, exposure time, video gain, and electron‐multiplying gain of the camera. A camera control‐panel window can be accessed using a keyboard shortcut; this window shows the status parameters of the camera and allows control for all functions. Status words are saved to a text file for reference by the system control program. Results: The CAPIDS provides a user‐friendly interface that allows control of several clinical radiographic imaging modes using the SSXII including: fluoroscopy, roadmapping, radiographic mode, and DSA. Examples of each mode will be demonstrated. Additional features, including recursive filtering, contrast enhancement, patient registry and image archival have been implemented and should facilitate the clinical use of the new EMCCD‐based detector system. Conclusion: The control, acquisition, and display capabilities along with the electron multiplication gain (1–2000×), high‐resolution, high‐sensitivity and real‐time imaging capabilities of the SSXII should provide angiographers and interventionalists with an improved ability to visualize details of small vessels and endovascular devices (such as stents), making diagnoses and image‐guided interventions more accurate.(Research partially supported by: NIH Grants R01‐EB008425, R01‐EB002873)

Component analysis of a new Solid State X-ray Image Intensifier (SSXII) using photon transfer and Instrumentation Noise Equivalent Exposure (INEE) measurements.

The SSXII is a novel x-ray imager designed to improve upon the performance limitations of conventional dynamic radiographic/fluoroscopic imagers related to resolution, charge-trapping, frame-rate, and instrumentation-noise. The SSXII consists of a CsI:Tl phosphor coupled via a fiber-optic taper (FOT) to an electron-multiplying CCD (EMCCD). To facilitate investigational studies, initial designs enable interchangeability of such imaging components. Measurements of various component and configuration characteristics enable an optimization analysis with respect to overall detector performance. Photon transfer was used to characterize the EMCCD performance including ADC sensitivity, read-noise, full-well capacity and quantum efficiency. X-ray sensitivity was measured using RQA x-ray spectra. Imaging components were analyzed in terms of their MTF and transmission efficiency. The EMCCD was measured to have a very low effective read-noise of less than 1 electron rms at modest EMCCD gains, which is more than two orders-of-magnitude less than flat panel (FPD) and CMOS-based detectors. The variable signal amplification from 1 to 2000 times enables selectable sensitivities ranging from 8.5 (168) to over 15k (300k) electrons per incident x-ray photon with (without) a 4:1 FOT; these sensitivities could be readily increased with further component optimization. MTF and DQE measurements indicate the SSXII performance is comparable to current state-of-the-art detectors at low spatial frequencies and far exceeds them at higher spatial frequencies. The instrumentation noise equivalent exposure (INEE) was measured to be less than 0.3 μR out to 10 cycles/mm, which is substantially better than FPDs. Component analysis suggests that these improvements can be substantially increased with further detector optimization.

Wavelength-shifting-fibre-based neutron image detector with a fibre-optic taper to increase the spatial resolution

The effective pixel size of a two-dimensional wavelength-shifting-fibre (WLS-fibre)-based neutron image detector was improved from 0.5 to 0.16 mm by including a fibre-optic taper (FOT) between the scintillator screen and the WLS fibre. The WLS-fibre-based detector consisted of a thin ZnS/ 6LiF screen, a FOT and WLS ribbons crossed in the x and y directions. The demonstrator detector had 16 fibre channels in each direction, and the light signals in each fibre were read out individually. The FOT was constructed from fine glass fibres with a taper ratio of 3.1 and served as an image magnifier. The prototype detector equipped with the FOT exhibited a spatial resolution of 0.3 mm, compared to the spatial resolution of 0.8 mm of the original detector without the FOT.

基于熔融拉锥光纤的液体折射率传感器

基于熔融拉锥光纤的液体折射率传感器

Development of an advanced X-ray detector for inspecting inner microscopic structural details in industrial applications

A new charge coupled device (CCD)-based X-ray detector, which uses a high-gain microchannel plate image intensifier for upfront optical gain, is proposed. The intensifier is directly coupled with the photosensitive surface of a CCD by a fiber optic taper and acts as both a converter and intensifier. Experimental results for the detector in X-ray imaging show that the detector currently has an intrinsic resolution of approximately 83 μm (12 lp/mm).

Characterization and preparation of focused laser beams

We discuss the possibilities of a far-field beam profiling technique for the determination of the near-focal profiles of laser beams focused beyond the paraxial approximation. The analysis is based on the rigorous vectorial Fourier synthesis of the fields and includes polarization effects. The wave-vector distribution can be calculated either a priori using the radius of the impinging laser beam and the data of the objective, or by obtaining its parameters from profiling at different z positions far behind the objective. For such measurements we designed a profiling system suitable for in situ application in microscopy. The device consists of a fiber optic taper bonded onto the chip of a CMOS camera, allowing taking pictures in the working medium in planes sufficiently close to the focus. The near-focal profiles obtained by these two approaches show good agreement, but only if the measured effective numerical aperture NAeff is used. We have further readdressed the effect of filling the objective pupil on the near-focal fields. The small improvement in focusing by the commonly practiced overfilling is hardly worth the significant loss of power. Moreover, overfilling above an optimal beam radius aopt results in an increase of aberrations when focusing through an optical interface.

Novel fast-arrayed CCD X-ray detector using interline transfer-type CCD for time-resolved X-ray diffraction measurements

A novel fast-arrayed fiber-optic taper (FOT) coupled with a charge-coupled device (CCD) X-ray detector was developed. The interline transfer-type CCDs are used as image sensor. The detector was developed for the simultaneous time-resolved small- and wide-angle X-ray diffraction measurement at SPring-8 BL45XU. The detector consists of an array of two identical units, each comprising a phosphor, a demagnifying FOT, and a large format interline transfer-type CCD as an image sensor. The image stored in the CCD can be read out through the dual readout channels with 12 bits ADC within 220 ms (4.5 fps) without binning and within 65 ms (15 fps) with 8×8 binning. Moreover, this detector does not need to stop the incoming X-rays during the CCD readout and can simultaneously execute both exposure and readout on the CCD with a small dead time of ∼1 μs so that a duty-cycle ratio of almost 100% is achieved. Therefore, it allows the continuous rotation method. The basic performance characteristics of the detector are described.

WE‐C‐L100J‐04: The Solid State X‐Ray Image Intensifier (SSXII): A Next‐Generation High‐Resolution Fluoroscopic Detector System

Purpose: Present the design for the new SSXII high‐resolution fluoroscope capable of overcoming lag, noise, and resolution limitations of current flat‐panel devices (FPDs). Method and Materials: The SSXII consists of an array of modules each featuring an electron multiplying (EM) CCD which views a structured phosphor through a fiber‐optic taper (FOT). The EMCCD operates like a standard frame‐transfer CCD; however, an additional row of multiplication elements enables on‐chip signal gains up to 2000X to overcome subsequent instrumentation‐noise degradation. The SSXII therefore has quantum‐limited performance at both fluoroscopic exposures with moderate gain, and radiographic exposures with low gain. The SSXII array design, through pixel binning and module selection, will enable rapid sequence and fluoroscopic imaging for either the full field‐of‐view (FOV) or high‐resolution regions‐of‐interest (ROIs). Results: A SSXII module was assembled with direct fiber‐optic coupling of the 350 micron thick CsI(Tl) phosphor and the EMCCD (Texas Instruments TC285SPD chip with 1004×1002 pixels). Operation at fluoroscopic and angiographic exposure levels was verified experimentally for gains of ∼80X and 1X, respectively, demonstrating sequences of a moving stent with no lag and bar‐pattern resolution up to 20 1p/mm with a 1:1 FOT. An array of four modules each with 6:1 FOTs will have an effective pixel size of 48 microns covering a FOV of 10×10 cm, sufficient for region‐of‐interest and neurovascular imaging. Larger arrays may be constructed to satisfy both cardiac imaging and general fluoroscopic applications. Module alignment, digital stitching, and distortion correction issues are being addressed. Conclusion: When assembled in an array of sufficient size, the new SSXII can be used in the same applications as current FPD's with advantages of high frame rates with no lag, high resolution due to the smaller pixels possible, and low‐effective‐noise, quantum‐limited fluoroscopic performance due to the on‐chip gain.(Support: UB Foundation, NIH grants R01‐EB002873, R01‐NS43924).

WE‐C‐L100J‐06: Linear Systems Analysis for a New Solid State X‐Ray Image Intensifier (SSXII) Based On Electron‐Multiplying Charge‐Coupled Devices (EMCCDs)

Purpose: To objectively evaluate and optimize performance of a new SSXII using linear‐systems analysis with varying constituent elements. Method and Materials: An imaging module of the SSXII consists of a fluorescent phosphor, a minifying fiber‐optic taper (FOT), and a fiber‐optic plate (FOP) coupled directly to the sensor of an EMCCD camera. An array of such modules is used to achieve the desired field‐of‐view (FOV). Linear‐systems analysis was applied to a module to determine system performance for various combinations of components whose properties were estimated using known physical constants and manufacturer specifications. A 350μm thick CsI(Tl) structured phosphor deposited on an additional FOP was considered in this analysis; however the phosphor may be grown directly on the FOT for improved optical transfer efficiency. New back‐thinned sensors, offering high optical quantum efficiencies (>90%), are also considered. Various FOT minifications were studied to evaluate the tradeoffs between a larger field‐of‐view (FOV) per module and potential DQE degradation. Results: Initial calculations indicate elimination of a FOP in the imaging chain could improve the integral DQE by 30–80%, depending on the FOT minification. Use of a back‐thinned sensor could offer additional improvements of 10–25% over a front‐illuminated EMCCD. Increasing the FOT minification factor from 3:1 to 6:1 would tend to decrease the integral DQE by ∼50%, which could be more than compensated for by making the above two design improvements. Calculated MTFs and DQEs will be presented for various minifications, exposures, and gains. A prototype SSXII (16μm pixels) demonstrated 20 lp/mm bar‐pattern resolution radiographically and fluoroscopic image sequences with exposures of <3 μR/frame. Conclusion: The linear‐systems analysis indicates current designs for the SSXII will provide high‐resolution, low‐noise, real‐time imaging. With component optimization, an acceptable DQE can be maintained with taper minifications as large as 6:1.(Partial support: the UB Foundation and NIH grants R01‐EB002873, R01‐NS43924.)

WE‐C‐L100J‐03: The New Solid State X‐Ray Image Intensifier (SSXII): A Demonstration of Operation Over a Range of Angiographic and Fluoroscopic Exposure Levels

Purpose: To demonstrate the SSXII's capability to image at high angiographic to low fluoroscopic exposure levels using the built‐in adjustable gain. Method and Materials: The prototype SSXII is composed of a 1k×1k, 8μm pixel EMCCD camera system coupled to a 350 μm thick CsI(Tl) structured phosphor via a 1:1 fiber‐optic taper (FOT) and is designed for easy interchange of these front‐end components for task‐based optimization. Variable signal amplification (in solid state) from 1x to 2000x effectively eliminates image degradation due to read‐out noise over all input x‐ray intensities. Images were acquired of a stent crimped on a 1 mm diameter balloon‐tip catheter, for various x‐ray spectra, using both radiographic and fluoroscopic exposures ranging from 6 mR to 145 μR and 80 to 2.7 μR, respectively. The EMCCD gain was adjusted in order to maintain a constant output signal of ∼90% of maximum over these exposures (∼3700 digital units). Results: A series of images, obtained by halving the exposure while doubling the gain, offers a near textbook demonstration of the effect of incident SNR on the resulting image quality. Fine structures of a crimped stent are visible for the entire range of radiographic exposures tested, demonstrating the very high‐resolution capabilities of the SSXII. In pulsed fluoroscopic mode, resolving the crimped stent structures becomes difficult below 40 μR. Using high gain, the object was visible down to 2.7 μR, which corresponds to ∼0.1 incident x‐ray photons per pixel, demonstrating both tremendous sensitivity and extremely low effective instrumentation noise. Conclusion: Images obtained with the prototype SSXII, over a vast range of exposure levels, indicate this new dynamic x‐ray imager has the potential to provide significant improvements over current state‐of‐the‐art detectors (both FPDs and XIIs) for high‐resolution angiographic and low‐noise, real‐time fluoroscopic imaging.(Partial support from the UB Foundation and NIH grants R01‐EB002873, R01‐NS43924.)

A 3 × 6 arrayed CCD X-ray detector for continuous rotation method in macromolecular crystallography

A 3 x 6 arrayed charge-coupled device (CCD) X-ray detector has been developed for the continuous-rotation method in macromolecular crystallography at the Photon Factory. The detector has an area of 235.9 mm x 235.9 mm and a readout time of 1.9 s. The detector is made of a 3 x 6 array of identical modules, each module consisting of a fiber-optic taper (FOT), a CCD sensor and a readout circuit. The outputs from 18 CCDs are read out in parallel and are then digitized by 16-bit analog-to-digital converters. The advantage of this detector over conventional FOT-coupled CCD detectors is the unique CCD readout scheme (frame transfer) which enables successive X-ray exposures to be recorded without interruption of the sample crystal rotation. A full data set of a lysozyme crystal was continuously collected within 360 s (180 degrees rotation, 3 s/1.5 degrees frame). The duty-cycle ratio of the X-ray exposure to the data collection time was almost 100%. The combination of this detector and synchrotron radiation is well suited to rapid and continuous data collection in macromolecular crystallography.

Electronic portal imaging based on Cerenkov radiation: A new approach and its feasibility

Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays and convert their energies into light, and the light image is then read out. The main problem with this approach is that the Cu plate/phosphor screen must be thin (approximately 2 mm thick) in order to obtain a high spatial resolution, resulting in a low x-ray absorption or low quantum efficiency for megavoltage x rays (typically 2-4%). In addition, the phosphor screen contains high atomic number (high-Z) materials, resulting in an over-response of the detector to low-energy x rays in dosimetric verification. In this paper, we propose a new approach that uses Cerenkov radiation to convert x-ray energy absorbed by the detector into light for portal imaging applications. With our approach, a thick (approximately 10-30 cm) energy conversion layer made of a low-Z dielectric medium, such as a large-area, thick fiber-optic taper consisting of a matrix of optical fibers aligned with the incident x rays, is used to replace the thin Cu plate/phosphor screen. The feasibility of this approach has been investigated using a single optical fiber embedded in a solid material. The spatial resolution expressed by the modulation transfer function (MTF) and the sensitivity of the detector at low doses (approximately one Linac pulse) have been measured. It is predicted that, using this approach, a detective quantum efficiency of an order of magnitude higher at zero frequency can be obtained while maintaining a reasonable MTF, as compared to current EPIDs.

Evaluation of a EMCCD Detector for Emission-Transmission Computed Tomography

A prototype electron multiplying charged coupled device (EMCCD) based gamma camera is evaluated for emission-transmission computed tomography. The detector has an 8.2times8.2 mm2 active sensor area with a 512times512 pixel array, and can be operated with a 3:1 fiber optic taper to expand the effective imaging area to approximately 25times25 mm2 for small animal studies. We demonstrate the possibility of operating the camera in either charge integrating or single photon counting mode for pinhole SPECT with 125I and 99mTc sources. In photon counting mode, we show that energy discrimination can be used to distinguish between these two sources for dual isotope imaging. X-ray CT images of phantoms are taken without a collimator to evaluate the quality of the camera for transmission imaging

SU-FF-I-64: X-Ray Micro-Angiographic and Fluoroscopic Image-Guided Localization of Asymmetric Endovascular Stents (AES): Study of Accuracy Versus Exposure

Purpose: Image-guided neuro-endovascular interventions, such as treatment of intracranial aneurysms using asymmetric endovascular stents (AES), can be lengthy procedures with high patient dose, yet the treatment success may be highly dependent upon the accuracy of placement of the AES at the aneurysm to occlude the orifice. We investigated the variation of AES localization accuracy for angiographic to fluoroscopic exposures using a high-resolution micro-angiographic (MA) system described previously. Method and Materials: We acquired a sequence of images of a simulated AES placed in a head-equivalent phantom for different calibrated orientations every two degrees. We tracked the 150 μm gold-sphere AES markers with images acquired using the MA detector with 43 μm pixels in angiographic mode and a detector exposure of 1.2 mR/frame. The MA detector uses a fiber-optic taper to optically couple an x-ray converter phosphor to a high performance 1024×1024 matrix CCD camera to enable imaging over a region of interest at the interventional site. Lower dose acquisitions corresponding to fluoroscopy were simulated by adding various amounts of Gaussian noise to achieve a SNR range from ∼80 to 8 corresponding to a 100X reduction in dose. The AES marker position determined by software image analysis was plotted vs. known rotation angle. An inverse function was fit, and the fit residuals calculated in degrees to gauge localization inaccuracy. Results: The results indicated that the accuracy of the stent placement was maintained within 6 degrees for an SNR between 79 and 22. However, for a SNR below 10, the stent localization accuracy deteriorated to 30 – 40 degrees. Conclusion: Intervention using AES with radio-opaque markers can be done at fluoroscopic dose rates during the initial part of the procedure followed by higher exposures to accurately guide the final stent orientation during deployment. (Partial support from NIH grants R01EB002873 and R01NS43924)

Photon-counting versus an integrating CCD-based gamma camera: important consequences for spatial resolution

Charge-coupled devices (CCDs) coupled to scintillation crystals can be used for high resolution imaging with x-rays and gamma-rays. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by real-time image analysis of scintillation light flashes (‘photon counting mode’). We tested a set-up in which an electron-multiplying CCD was coupled to a 1 mm thick columnar CsI crystal by means of a fibre-optic taper. We found that, compared to light integration, photon counting improves the intrinsic spatial resolution by a factor of about 3 to 6. Applying our set-up to Tc-99m and I-125 imaging, we were able to obtain intrinsic resolutions below 60 μm (full width at half maximum). Counting losses due to overlapping of light flashes are negligible for event rates typical for biomedical radio-nuclide imaging and do strongly depend on energy window settings. Energy resolution was estimated to be approximately 35 keV FWHM for a 1:1 taper. We conclude that CCD-based gamma cameras have great potential for applications such as in vivo imaging of gamma emitters.

A comparison of x-ray detectors for mouse CT imaging

There is significant interest in using computed tomography (CT) for in vivo imaging applications in mouse models of disease. Most commercially available mouse x-ray CT scanners utilize a charge-coupled device (CCD) detector coupled via fibre optic taper to a phosphor screen. However, there has been little research to determine if this is the optimum detector for the specific task of in vivo mouse imaging. To investigate this issue, we have evaluated four detectors, including an amorphous selenium (a-Se) detector, an amorphous silicon (a-Si) detector with a gadolinium oxysulphide (GOS) screen, a CCD with a 3:1 fibre taper and a GOS screen, and a CCD with a 2:1 fibre taper and both GOS and thallium-doped caesium iodide (CsI:Tl) screens. The detectors were evaluated by measuring the modulation transfer function (MTF), noise power spectrum (NPS), detective quantum efficiency (DQE), stability over multiple exposures, and noise in reconstructed CT images. The a-Se detector had the best MTF and the highest DQE (0.6 at 0 lp mm−1) but had the worst stability (45% reduction after 2000 exposure frames). The a-Si detector and the CCD with the 3:1 fibre, both of which used the GOS screen, had very similar performance with a DQE of approximately 0.30 at 0 lp mm−1. For the CCD with the 2:1 fibre, the CsI:Tl screen resulted in a nearly two-fold improvement in DQE over the GOS screen (0.4 versus 0.24 at 0 lp mm−1). The CCDs both had the best stability, with less than a 1% change in pixel values over multiple exposures. The pixel values of the a-Si detector increased 5% over multiple exposures due to the effects of image lag. Despite the higher DQE of the a-Se detector, the reconstructed CT images acquired with the a-Si detector had lower noise levels, likely due to the blurring effects from the phosphor screen.

High-speed X-ray imaging camera for time-resolved diffraction studies

We report here on a high-speed X-ray imaging camera, specifically developed for time resolved diffraction studies using synchrotron and laboratory X-ray sources. This camera is capable of acquiring six X-ray images at speeds of up to 2300 frames per second (f/s). The system is based on a modified architecture charge coupled device (CCD) optically coupled to a fiber-optic taper via an image intensifier. The front end of the taper is coupled to a specially designed microstructured CsI(Tl) scintillator screen capable of providing high light output, very high-detection efficiency, and excellent spatial resolution. In addition to the time resolved diffraction studies, this detector will be extremely valuable in applications such as dynamic imaging of small animals, X-ray microtomography, and materials science applications. This paper discusses the design and performance characterization of the imaging system. Additionally, we present some preliminary high-speed X-ray imaging data obtained using laboratory X-ray sources.

Analysis of the effects arising from the near-field optical microscopy of homogeneous dielectric slabs

The term “near-field optical microscopy” refers to a range of techniques capable of producing images of surfaces at a resolution far exceeding the Rayleigh criterion for conventional microscopes. In the illumination mode scanning near-field optical microscope, the sample is placed in the near field of an optical source formed from the aperture at the end of a fibre-optic taper. In normal operation, the aperture is raster scanned over the sample and, by recording the resultant far-field intensity transmitted through the sample as a function of the source coordinates, an image is assembled whose appearance derives from a non-trivial combination of the optical and topological composition of the sample. In this paper a numerical analysis of the near-field interaction of the source and sample is performed using a three-dimensional finite-difference time-domain method. A propagation technique is applied to the near-field values to form a far-field signal that is representative of the signal generated by a real instrument. In this way, the microscope output for a variety of very simple samples is simulated. It is shown that the magnitude of the transmitted signal can be increased both by decreasing the separation between source and sample and, in some circumstances, by increasing the sample permittivity. Simulated microscope output from samples containing topological variation is compared with that produced by samples containing a variation in refractive index and it is demonstrated that contrast inversion may occur at different scan heights in the latter case, leading to the possibility of distinguishing between the different contrast mechanisms experimentally.