In a typical indirect flat-panel digital radiography detector, a phosphor screen is coupled to an <i>a</i>-Si:H imaging array, whose pixels comprise an <i>a</i>-Si:H photodiode and an <i>a</i>-Si:H TFT switch. This two-dimensional array is fabricated on a thin glass substrate that usually contains a rather high concentration of heavy elements such as barium. In previous system performance analyses, only the effect of K-fluorescence reabsorption in the phosphor screen was included. The effect of K-fluorescence from heavy elements in the glass substrate of the array was not taken into account. This K-fluorescence may be excited directly by primary x-rays that penetrate the overlying phosphor and interact in the glass, or by K-fluorescence x-rays that escape from the phosphor into the glass. In this paper, we extend the parallel-cascaded linear systems model to include the effect of K-fluorescence from heavy elements in the glass substrate. As an example, the MTF, NPS, and DQE of an indirect flat-panel imager consisting of a Gd<sub>2</sub>O<sub>2</sub>S:Tb phosphor screen and an <i>a</i>-Si:H photodiode/TFT array fabricated on a glass substrate containing barium, are calculated. Degradations in MTF and DQE as a result of the K-fluorescence from the substrate are presented and discussed.
The image quality of Gd<SUB>2</SUB>O<SUB>2</SUB>S:Tb phosphor screens used in a flat-panel photodiode array system is examined. The presampled Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum surface (NNPS) and the resulting Detective Quantum Efficiency (DQE) are discussed for a variety of screens used in such a system. A technique for extracting the limiting DQE for a system with significant electronic noise is described. This allows for the examination of the imaging performance of the X-ray converter (or phosphor screen) and removes issues of photodiode array and readout electronics performance. It is shown that depending on the metric being used to judge the imaging performance (i.e., MTF or DQE), it is possible to design a more optimal screen than those currently available for use in screen-film imaging. Evidence is also presented for a significant degradation of the DQE at higher spatial frequencies due to the variation in the light spread MTF through the depth of the screen.
Results of an investigation into the limiting spatial resolution of a flat-panel amorphous silicon (a-Si:H) X-ray imaging system are reported. The system was comprised of a 127 micrometer pixel pitch a-Si:H array used in conjunction with an overlying Gd<SUB>2</SUB>O<SUB>2</SUB>S:Tb (GOS) phosphor screen. The pre- sampled modulation transfer function (psMTF) of the system was measured at diagnostic X-ray energies and compared to the value predicted from a knowledge of the spatial resolution of the individual system components. A reproducible drop in the measured psMTF is seen at low spatial frequencies. Measurements of the magnitude of X-ray backscatter from the array substrate, along with the results of a theoretical model for K-fluorescence X-ray scatter, indicate that a significant fraction of this low-frequency drop is due to K-fluorescence from heavy elements in the glass substrate of the array. This K-fluorescence may be excited directly by primary X-rays that penetrate the overlying phosphor and interact in the glass, or by gadolinium K-fluorescence X-rays that escape from the phosphor into the glass. The measurements indicate that the spatial resolution of such an X-ray imaging system may be improved by the use of a substrate containing as low a concentration of heavy elements as possible.
The purpose of this work is to investigate the potential image quality of scintillator/CCD-based direct digital medical x-ray imaging systems. The x-ray detector is composed of a scintillating screen to convert x-ray photons into lower energy radiation (UV/visible/NIR) that is then collected by a lens or a fiberoptic taper and converted to an electrical signal by a CCD. The DQE (on-axis geometry) was modeled by extending the analysis used for storage phosphor systems. The effect of various system parameters on the system DQE has been investigated. Two coupling approaches, an array of lenses, and an array of fiberoptic tapers, have been studied. For each coupling approach, two applications, chest radiography and mammography, were examined and the DQE was modeled to be comparable to screen/film systems.
This study compares the relative response of various screen-film and computed radiography (CR) systems to diagnostic radiation exposure. An analytic model was developed to calculate the total energy deposition within the depth of screen and the readout signal generated from this energy for the x-ray detection system. The model was used to predict the relative sensitivity of several screen-film and CR systems to scattered radiation as a function of selected parameters, such as x-ray spectra, phantom thickness, phosphor composition, screen thickness, screen configuration (single front screen, single back screen, screen pair), and readout conditions. Measurements of scatter degradation factor (SDF) for different screen systems were made by using the beam stop technique with water phantoms. Calculated results were found to be consistent with experimental observations, namely, both the BaFBr screen used in a CR system and the CaWO<SUB>4</SUB> screen pair have higher scatter sensitivity than the rare earth Gd<SUB>2</SUB>O<SUB>2</SUB>S screen pair; the BaFBr screen in the CR front-screen configuration is less sensitive to scatter radiation than in the normal back-screen configuration; and these screens have higher scatter sensitivity as x-ray tube voltage increases.
The x-ray excited emission spectra of some well-known phosphor materials is examined using different x-ray beam qualities. Very thick powder samples and thinner coated screens are examined. It is observed that the emission spectra are influenced by the x-ray beam quality used if the samples are very thick. A simple light diffusion model is developed and used to understand the observed effects both qualitatively and quantitatively. It is found that very small changes in the total reflectance of the powder samples is correlated with observed spectral changes.
In this paper we examine the imaging performance of an experimental mechanically-flexible antiscatter grid for use with computed radiography. Bucky factors, contrast improvement factors, and signal-to-noise improvement factors were computed from transmittance measurements made with 5-, 10-, 15-, 20-, and 25-cm uniform-thickness water phantoms. These phantoms were selected to span the range of scatter fractions found in chest radiography.
Two years ago in these proceedings1 we reported on a new method for measuring the noise associated
with the variation in light output of x-ray intensifying screens caused by absorption of x-ray
quanta of equal energy, together with data for the Kodak Lanex intensifying screens. We have extended
our measurements to screens in the back-screen configuration, in addition to the front-screen
configuration previously reported. By back-screen configuration we mean that the x-rays are incident
from the same side as that from which the emission will be measured. This has been realized by means
of an integrating sphere, which allows screens to be mounted as back or front screens; or even as a
pair. The light emission statistics (including the mean light output and the Swank I factor) for some
Kodak Lanex intensifying screens in the front and back-screen configurations are given and compared.
These data can provide a basis for understanding the depth dependent emission probability which in
turn provides a useful test of theories of light propagation within the screen.