For digital X-ray detectors, the need to control factory yield and cost invariably leads to the presence of some defective
pixels. Recently, a standard procedure was developed to identify such pixels for industrial applications. However, no
quality standards exist in medical or industrial imaging regarding the maximum allowable number and size of detector
defects. While the answer may be application specific, the minimum requirement for any defect specification is that the
diagnostic quality of the images be maintained. A more stringent criterion is to keep any changes in the images due to
defects below the visual threshold. Two highly sensitive image simulation and evaluation methods were employed to
specify the fraction of allowable defects as a function of defect cluster size in general radiography. First, the most critical
situation of the defect being located in the center of the disease feature was explored using image simulation tools and a
previously verified human observer model, incorporating a channelized Hotelling observer. Detectability index d' was
obtained as a function of defect cluster size for three different disease features on clinical lung and extremity
backgrounds. Second, four concentrations of defects of four different sizes were added to clinical images with subtle
disease features and then interpolated. Twenty observers evaluated the images against the original on a single display
using a 2-AFC method, which was highly sensitive to small changes in image detail. Based on a 50% just-noticeable
difference, the fraction of allowed defects was specified vs. cluster size.
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.
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.
We have analyzed the imaging characteristics of a cathode-ray tube multiformat printer. These include the dependence of gray scale response on the printer's settings, the modulation transfer function of the printer, the acutance (or sharpness) and the noise power spectrum of the printed images. A comparison of theoretical results and experimental data is presented.
The visibility of the modulatorinduced streaking artifact in the images from a laser film printer has been observed and analyzed. These streaks which occur in the printing of a light (low density) area after the printing of a dark (high density) area can be ascribed to the reflection of acoustic power in the acoustooptic modulator. By comparing the fractional transmission change in the light area (due to the reflection of acoustic power used for the printing of the dark area) with a visual threshold contrast the visibility of streaks can be determined. These results are found to be in good agreement with experiments. Specifications on the maximum acoustic reflection coefficient for the suppression of streaking visibility are also given. 1.
A system model for analyzing degradation in the image quality of a
radiograph introduced by a film digitizer is presented. The analysis
is an extension of the screen-film model of Shaw and VanMetter (SPIE
454, 128-141(1984)). By combining the screen-film characteristics
for specific exam types with the properties (e.g., MTF and NPS) of a
particular scanner design, the information transfer of the whole
digital system can be determined. As an example, the performance of
two typical film digitizers, a CCD-based scanner and a laser-based
scanner, are evaluated and compared. Image quality descriptors, such
as DQE and NEQ as well as equivalent bandwidth and system aperture,
are used for the evaluation. By incorporating the human observer's
threshold response to changes in noise levels (just noticeable differences),
a criterion for negligible loss of image information can be
established. This can be very useful for system optimization and
determination of design tradeoffs.