Megavoltage X-Ray Imaging Based on Cerenkov Effect: A New Application of Optical Fibres to Radiation Therapy

A Monte Carlo simulation was used to study imaging and dosimetric characteristics of a novel design of megavoltage (MV) X-ray detectors for radiotherapy applications. The new design uses Cerenkov effect to convert X-ray energy absorbed in optical fibres into light for MV X-ray imaging. The proposed detector consists of a matrix of optical fibres aligned with the incident X rays and coupled to an active matrix flat-panel imager (AMFPI) for image readout. Properties, such as modulation transfer function, detection quantum efficiency (DQE), and energy response of the detector, were investigated. It has been shown that the proposed detector can have a zero-frequency DQE more than an order of magnitude higher than that of current electronic portal imaging device (EPID) systems and yet a spatial resolution comparable to that of video-based EPIDs. The proposed detector is also less sensitive to scattered X rays from patients than current EPIDs.

On the experimental violation of Mermin's inequality with imperfect measurements

We investigate theoretically the feasibility of an experimental test of the Bell-type inequality derived by Mermin for correlated spins larger than ${1}/{2}$. Using the Schwinger representation, we link the output fields of two two-mode squeezers in order to create correlated effective spins between two observers. Spin measurements will be performed by photon-number-resolved photodetection, which has recently come of age. We examine the effect of nonideal detection quantum efficiency -and any other optical loss - on the violation margin of Mermin's inequality. We find that experimental violation is well accessible for spins larger than 1, for quantum efficiencies compatible with the current state of the art.

Quantitative determination of modulation transfer function and detection quantum efficiency for standard and anti-reflection YAG scintillator slow-scan CCD-camera

Introduction The modulation transfer function (MTF) of a slow-scan CCD camera (Gatan modell 679 attached to our Zeiss EM 912 Omega) has been determined with high precision for the standard and the new anti-reflection YAG scintillator. It is shown that deconvolution of experimental patterns allows the reconstruction of image details down to pixel size. From the analysis of deconvoluted noise patterns we found that for this camera the detection quantum efficiency (DQE) is 0.6 in the typical working range of 300 to 3000 electrons per pixel and does not depend on the scintillator. A full discussion of this work can be found in.Experiment To determine the point spread function (PSF) we punched a hole in a sheet film holder which then was partially covered by one or two knife edges (slit). These masks were projected onto the scintillator. About 600 line scans perpendicular to the edge or slit were extracted and averaged to reduce noise (Fig. 1).

Thin-foil based TEM screens with electrophoretic deposition of phosphors

The availability of 2k×2k or larger format CCD chips has made direct digital imaging more practical in electron microscopy. But the suboptimal performance of scintillating screens, particularly their inferior resolution as compared to film, is still an obstacle to a broader adaptation of digital imaging technology to electron microscopy. Thin-foil substrate screens have improved brightness and resolution over the more commonly used glass substrate screens, particularly at higher operating voltages, but further improvements are required for the optimum performance of the CCD imaging systems. Self-supporting single crystal YAG screens, approximately 30 μm in thickness, provide better resolution and detection-quantum-efficiency as compared to the phosphor screens, but their lower brightness, about 4 - 8 times lower, makes them less than ideal at least for low dose applications. Until brighter single crystal scintillators become available, powder phosphor screens may offer performance advantages for applications where radiation damage to the specimen is a concern.

On-line image readout in CTEM

Quantitative recording of electron patterns and their rapid conversion into digital information is an outstanding goal which the photoplate fails to solve satisfactorily. For a long time, LLL-TV cameras have been used for EM adjustment but due to their inferior pixel number they were never a real alternative to the photoplate. This situation has changed with the availability of scientific grade slow-scan charged coupled devices (CCD) with pixel numbers exceeding 106, photometric accuracy and, by Peltier cooling, both excellent storage and noise figures previously inaccessible in image detection technology. Again the electron image is converted into a photon image fed to the CCD by some light optical transfer link. Subsequently, some technical solutions are discussed using the detection quantum efficiency (DQE), resolution, pixel number and exposure range as figures of merit.A key quantity is the number of electron-hole pairs released in the CCD sensor by a single primary electron (PE) which can be estimated from the energy deposit ΔE in the scintillator,

State and Future of Electronic Image Read-OUT in TEM

The electron microscope is becoming a link in a highly sophisticated data processing system. The acquisition of image data supplied as a spatial distribution of current density requires a position sensitive electron detector which converts the current into digital information to be processed by image storages and computers to retrieve the information in which the user is interested. The ultimate goal of this interface is a lossless conversion with respect to both the number of pixels and the detection quantum efficiency (DQE) as well as high speed, minimum distortion, and linearity. I shall try to outline the present state of image read-out using both the conventional photoplate with spatial digitizing equipment and conventional TV sets. Subsequently it will be discussed how the future CCD technology (charge coupled device) may overcome some restrictions of present solutions.Every spatial recording device combines a conversion with a storage function in order to build up a signal-to-noise ratio (SNR) sufficient for detection. The following characteristics describe their performance: