Kwok L. Yip

Understanding the relative sensitivity of radiographic screens to scattered radiation

This study compared 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 various 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. In addition, measurements of the scatter degradation factor (SDF) for different screen systems by using the beam stop technique with water phantoms were made to verify the model results. Theoretically calculated values of SDF were in good agreement with experimental data. These results are consistent with the common observation that rare-earth screens generally produce better image quality than calcium tungstate screens and the CR screen.

Sensitivity of radiographic screens to scattered radiation

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 CaWO4 screen pair have higher scatter sensitivity than the rare earth Gd2O2S 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.

Performance analysis of medical x-ray film digitizers

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 observers 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.

Substrate effect on indirect digital radiography system performance

In a typical indirect flat-panel digital radiography detector, a phosphor screen is coupled to an a-Si:H imaging array, whose pixels comprise an a-Si:H photodiode and an a-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 Gd2O2S:Tb phosphor screen and an a-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.

Imaging characteristics of CRT multiformat printers

We have analyzed the imaging characteristics of a cathode-ray tube multiformat printer. These include the dependence of gray scale response on the printers 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.

Modulator-induced streaking artifact in laser film printer images

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.