Thomas E. Kocher

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.

High-resolution, high-performance radiographic film scanner

In this paper, we discuss the requirenients, design approach, and features for a high-performance, laser-based radiographic film digitizer. The scanner is capable of digitizing a radiograph over a 3.5 diffuse density range in 15-30 seconds while not significantly reducing the noise-equivalent-quanta (NEQ) relative to the original analog screen-film image. Three resolutions are possible, corresponding to maximum film sizes of 35 x 43, 24 x 30, and 18 x 24 cm, with pixel sizes of 86, 59, and 43 microns; respectively. Preliminary results of side-by-side comparisons of scanned data reconstructed on pnnted film with the original analog film are discussed, along with future directions for research.

Monitor simulations for the optimization of medical soft copies

We investigate a simulation tool for the optimization of soft copy displays in radiology. The digital image is traced through the individual components of a black and white cathode ray tube (CRT) monitor and the luminance image observed at the glass faceplate is simulated. The simulated images can be evaluated numerically or rendered on film by a high-resolution printer for viewing. We validated the program simulating a real existing monitor by comparing the results with measured values, as well as by visually comparing the actual image with the simulated one. The gross properties like luminance, dynamic range, and spatial resolution are sufficiently well described. The visual impression of the simulated image is very similar to that of the real soft copy. We investigate the influence of individual parameters on image quality. We find that the bandwidth of the video amplifier has to be larger than half the pixel rate. We demonstrate the influence of the electron beam spot size on spatial resolution. It is shown how the spatial resolution depends on phosphor luminous efficiency and on glass transmission. Furthermore, for a given target display curve, it is found that the current-to- voltage relationship of the electron gun influences the number of perceived gray values. Finally we discuss phosphor noise in context with dynamic range.

High-resolution teleradiology applications within the hospital

Many of the commercial applications for teleradiology have involved the transmission of reduced resolution x-ray images over modest bandwidth telecommunications lines for the purpose of making a preliminary diagnosis. In order to study the technical and operational requirements for future teleradiology applications, the authors have focused on the demanding requirements for teleradiology within the hospital and medical center. Applications within the hospital often require x-ray images of primary diagnostic quality transmitted with a minimum of delay. An experimental, high-resolution film scan/print system designed by Health Sciences Division, Eastman Kodak Company, has been developed for installation in a working clinical environment. Images scanned at a spatial resolution of 4K X 5K can be delivered over a fiber optic link to a laser film printer at a rate of two films per minute. Preliminary plans to install this device in a variety of clinical settings have led to rethinking the requirements for automatic film loading, film and patient identification, throughput requirements, and image display formats. As an initial implementation, and application is being developed which allows chest radiographs taken in the admission area to be interpreted at a remote site within the hospital. Images can be viewed on high resolution monitors, or film replicates can be produced on a nearby laser printer. Tight coupling with a radiology information system provides access to relevant diagnostic information including prior radiology reports, and prompt electronic reporting and signature can be accomplished.

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.

Design considerations for a high-resolution film scanner for teleradiology applications

Successful implementation of a teleradiology system in a clinical setting places stringent requirements on the system configuration and the performance of individual components. To evaluate possible approaches for high-quality teleradiology, a research system has been developed and is being tested at the Mallinckrodt Institute of Radiology. It consists of a high- performance laser-based film digitizer linked by fiber optics to a remote controlling node that can produce film replicas at a rate of one per minute. In this paper, the design considerations and performance of the research system are presented.

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.

Visual study of perceptually optimized displays

Perceptually linear displays have been proposed as a standard for medical imaging. Current displays (display driver/monitor) have intrinsic display characteristics that differ from this proposal. Visual comparisons of the proposed perceptually linear displays and current technology have not been made to date. The subjective assessment presented in this paper is the first such comparison. Clinical images were printed on a 12-bit laser printer to simulate the display characteristics of perceptually linear and currently available 8-bit medium-resolution gray-scale displays. Images were compared subjectively and by means of a 4-alternative forced choice (4-AFC) protocol. In addition, predictions of visible differences were made with Dalys Visible Differences Predictor model. We find that currently available displays can produce clinical images that are visually indistinguishable from those that would be displayed on a perceptually linear display when viewed at currently available monitor luminance levels (200 nits). Therefore, intrinsic display functions may be sufficiently close to perceptually optimized performance that the expense associated with the design and fabrication of special perceptually linear display cards and/or monitors would not be justified. In any case, substantial deviation from perceptual linearity may be tolerable before visible differences will be discerned as long as the image is correctly mapped to the appropriate display function. Further study of the diagnostic benefits claimed for perceptually linear displays would be prudent before human visual models are adopted as the basis for display standardization.