Kevin S. Kohm

Sensitivity of visual targets for display quality assessment

Assessing the image quality of display devices is becoming an important concern for radiology departments with large numbers of widely distributed image displays. Methods commonly used for laboratory measurements are too costly and cumbersome for routine quality assessment, however, methods that rely on visual assessment of currently available test targets may not have adequate sensitivity. The purpose of this paper is to quantify the sensitivity of commonly used test targets for visual assessment of medical display devices with well-defined changes in sharpness and nose. Two test targets methods were selected form those that have been used for visual assessment of image displays. For each, the assessment is a measure of the size and contrast of the smallest visible pattern in the target. Computer simulation was used to produce images of each of the targets having known sharpness and nose degradation. Viewers were trainee in the use of each target, then asked to score a randomly ordered set of simulation-degraded target images. These data were approximately analyzed for each method and the result evaluated with standard statistical methods. Assessments were found to correlate with sharpness and noise. However, the sensitivity of both targets for single-stimulus assessment was found to be adequate. The practical utility of these methods must therefore be questioned.

Visual CRT sharpness estimation using a fiducial marker set

A visual estimation technique has been developed too quickly, yet quantitatively, determine the sharpness quality of CRT displays. While high-resolution camera measurement equipment accurately characterizes display sharpness, the equipment cost is high and the measurements are time consuming to perform. Previously reported visual sharpness assessment techniques are either qualitative or the quantitative measures do not possess adequate sensitivity. The rating scheme investigated in this study provides a practical solution for tracking monitor sharpness in a clinical environment. The target consists of high frequency, high contrast pattern with an embedded, magnified fiducial marker set based upon a Gaussian model for the CRT spot. The magnification of the marker set allows the reference to remain nearly invariant to the actual sharpness of the display. In this study, three commercially available diagnostic displays were evaluated, each at two luminance levels and seven static focus settings. High-resolution CCD camera measurements were acquired for each display and setting combination. The visual sharpness estimate target was then displayed and scored by observers. High correlation was found between the visual ratings and the photometric measurements. More importantly, the sensitivity of the target produced observer ratings, which distinguish between the measured CRT spot sizes at different focus levels.

Dynamic viewing protocols for diagnostic image comparison

There is an ongoing need to evaluate the impact of various digital image processing and display variables on diagnostic image quality. In most cases, evaluation includes comparison of images, often multiple versions of the same image. In order to improve speed and sensitivity, new protocols were developed to enhance a radiologists ability to detect subtle changes in images and provide a means to quantify differences in a standard fashion. The protocols make use of the rapid sequential display of registered images on a single high- resolution CRT (a.k.a., flicker) and 2X magnification in order to increase observer sensitivity. The flicker technique was implemented in the form of an image comparison workstation (ICW) that was designed to facilitate the evaluation of different image processing options. The ICW was developed with capabilities to interactively control the rate of flicker between image pairs (up to 5 Hz), the degree of image magnification (1X to 4X), and the selection of the region of interest (ROI). Three specific protocols were developed based on the flicker technique, two forms of forced-choice and a rank-ordering protocol employing a reference set comprised of images with varying degrees of spatial-resolution degradation. All three protocols were exercised as part of an observe study whose goal was to establish visually lossless compression levels for JPEG and a wavelet-transform based algorithm. The results indicate that, for high resolution digitally acquired posteroanterior (PA) chest radiographs presented to observers at 2X magnification on a 2K X 2.5K addressable pixel monochrome display, the visually lossless thresholds for both JPEG and wavelet occur in the range of 2.0 to 1.5 bits-per- pixel (approximately equals 10:1). These results are a conservative estimate of the visually lossless threshold because of the sensitive nature of the experimental methodology.