Hans Roehrig

Adaptive image contrast enhancement based on human visual properties

Existing methods for image contrast enhancement focus mainly on the properties of the image to be processed while excluding any consideration of the observer characteristics. In several applications, particularly in the medical imaging area, effective contrast enhancement for diagnostic purposes can be achieved by including certain basic human visual properties. Here the authors present a novel adaptive algorithm that tailors the required amount of contrast enhancement based on the local contrast of the image and the observers Just-Noticeable-Difference (JND). This algorithm always produces adequate contrast in the output image, and results in almost no ringing artifacts even around sharp transition regions, which is often seen in images processed by conventional contrast enhancement techniques. By separating smooth and detail areas of an image and considering the dependence of noise visibility on the spatial activity of the image, the algorithm treats them differently and thus avoids excessive enhancement of noise, which is another common problem for many existing contrast enhancement techniques. The present JND-Guided Adaptive Contrast Enhancement (JGACE) technique is very general and can be applied to a variety of images. In particular, it offers considerable benefits in digital radiography applications where the objective is to increase the diagnostic utility of images. A detailed performance evaluation together with a comparison with the existing techniques is given to demonstrate the strong features of JGACE.

Influence of film and monitor display luminance on observer performance and visual search

RATIONALE AND OBJECTIVES The purpose of this study was to measure the influence of display luminance on detection performance and visual search behavior. The results of the study should be helpful in establishing minimally acceptable display conditions for viewing radiographs on cathode-ray tube (CRT) monitors. MATERIALS AND METHODS Two groups of six radiologists each viewed 50 pairs of mammograms. One group viewed film images on a standard mammographic view box; the other viewed images on a high-resolution CRT monitor. Two luminance levels were studied for each display type. Observers reported on the presence or absence of masses or microcalcification clusters and on their confidence in that decision. Confidence data were analyzed by using alternative free-response receiver operating characteristic (AFROC) techniques. Eye position also was recorded as observers viewed the images. RESULTS For both the film and monitor studies, detection performance (AFROC area under the curve) was not affected significantly by display luminance, but search behavior was. Total viewing and decision dwell times were shorter with the higher-luminance displays, especially for true-negative decisions. Significantly more fixation clusters were generated during the search of lesion-free than of lesion-containing images with the lower-luminance displays. CONCLUSION Display luminance affects visual search performance with both film and monitor displays without affecting detection performance significantly. Higher-luminance displays yield more efficient search performance. The true-negative dwell times and number of clusters are suggestive that lower-luminance levels prolong the search and recognition of normal, lesion-free areas compared with lesion-containing areas.

The influence of a perceptually linearized display on observer performance and visual search

RATIONALE AND OBJECTIVES The purpose of this study was to determine whether perceptual linearization of the tone scale affects the detection and visual search behaviors of radiologists searching mammograms for masses and microcalcifications. A perceptually linearized display is designed to match the capabilities of the human visual system more closely than a nonlinearized display. MATERIALS AND METHODS Six radiologists viewed 50 pairs of mammograms, once on a perceptually linearized cathode-ray tube (CRT) monitor and once on a non-linearized CRT monitor. Eye position also was recorded as the observers searched the images for masses and microcalcifications. RESULTS Observer performance was significantly (P = .003) better with the perceptually linearized display. Dwell times associated with true-negative decisions were significantly longer with use of the nonlinearized display. The number of fixation clusters generated during search was also greater with use of the nonlinearized display for the lesion-free images. CONCLUSION A perceptually linearized display yields better detection performance and a more efficient visual search. Perceptually linearized displays should be used for reading radiographs displayed on CRT monitors.

Pulmonary nodule detection and visual search: P45 and P104 monochrome versus color monitor displays.

RATIONALE AND OBJECTIVES The faceplate of a cathode-ray tube (CRT) display monitor is covered on the vacuum side with a phosphor screen. The different phosphors that can be used for this screen have distinctly different physical properties that can affect the noise properties of the display. Differences in noise affect the signal-to-noise ratio and, hence, may affect diagnostic performance. This study evaluated observer performance and visual search parameters in the detection of pulmonary nodules, comparing two monochrome CRT monitors with different phosphors (P45 and P104) and a color CRT monitor. MATERIALS AND METHODS The receiver operating characteristic paradigm was used to evaluate observer performance with a series of radiographic chest images containing solitary pulmonary nodules. Eye position was recorded as the observers searched the images on each type of monitor. RESULTS Observer performance, as indicated by the area under the receiver operating characteristic curve and compared by means of an analysis of variance test, was best for the P45 monitor, next best for the P104 monitor, and worst for the color monitor. All differences were statistically significant. Eye-position parameters were also affected by monitor type. The time required to fixate the lesion and overall search times were longest with the color monitor. CONCLUSION The type of phosphor used in the CRT monitor faceplate can affect diagnostic performance and visual search parameters. Care should be taken in the selection of monitors for use in clinical radiology.

Signal, noise, and detective quantum efficiency in CCD-based x-ray imaging systems for use in mammography

This paper presents results of experiments performed to find parameters affecting signal, noise and Detective Quantum Efficiency of x-ray imaging systems with potential for use in mammography.

Use of a Human Visual System Model to Predict Observer Performance with CRT vs LCD Display of Images

This Project evaluated a human visual system model (JNDmetrix) based on just noticeable difference (JND) and frequency-channel vision-modeling principles to assess whether a Cathode ray tube (CRT) or a liquid crystal display (LCD) monochrome display monitor would yield better observer performance in radiographic interpretation. Key physical characteristics, such as veiling glare and modulation transfer function (MTF) of the CRT and LCD were measured. Regions of interest from mammographic images with masses of different contrast levels were shown once on each display to six radiologists using a counterbalanced presentation order. The images were analyzed using the JNDmetrix model. Performance as measured by receiver operating characteristic (ROC) analysis was significantly better overall on the LCD display (P = 0.0120). The JNDmetrix model predicted the result (P = 0.0046) and correlation between human and computer observers was high (r2 (quadratic) = 0.997). The results suggest that observer performance with LCD displays is superior to CRT viewing, at least for on-axis viewing.

The Development Of A Digital Video Subtraction System For Intravenous Angiography

A system is under development for a relatively non-invasive technique for the assessment of atherosclerosis. The principle of this method is digital video x-ray subtraction for the visualization of arterial structures after the intravenous injection of contrast media. The prototype unit for the development of video subtraction techniques has been assembled and preliminary testing has started. Re-sults so far in dogs have shown good visualization of the heart, carotid arteries and renal arteries.

Characterization of high-resolution liquid crystal displays for medical images

The main subjects of the paper are the methodology of characterizing liquid crystal displays (LCDs) and the properties of a three-million-pixel monochrome display system. The system is characterized by display function and dynamic range as a function of viewing angle, spatial luminance uniformity, flicker, peak-to-peak temporal modulation transfer, spatial modulation transfer function (MTF), spatial noise power spectra, and single-pixel signal-to-noise ratios. The evaluated LCD has image quality that, in most respects, is superior to CRT monitors of comparable addressable pixel matrix. In particular, the LCD has perfect spatial modulation transfer. For the evaluated monochrome display system, the general limitation of liquid crystal display (LCD) drivers to 8-bits of grayscale precision is overcome by spatial as well as temporal modulation techniques. The architecture of the control electronics of the system is presented, and as part of it, the implementation of the modulation techniques. The spatial or a combination of spatial and temporal modulation techniques increases the precision with which luminance levels can be defined to 9.58 or 11.58 bits, respectively. The conformance of the calibrated display with the DICOM Standard Display Function is demonstrated without and with application of the modulation techniques. Excellent conformance is achieved for the combination of spatial and temporal modulation.

Performance tests and quality control of cathode ray tube displays

Spatial resolution, noise, characteristic curve, and absolute luminance are the essential parameters that describe physical image quality of a display. This paper presents simple procedures for assessing the performance of a cathode ray tube (CRT) in terms of these parameters as well as essy set up techniques. The procedures can be used in the environment where the CRT is used. The procedures are based on a digital representation of the Society of Motion Pictures and Television Engineers pattern plus a few simple other digital patterns. Additionally, measurement techniques are discussed for estimating brightness uniformity, veiling glare, and distortion. Apart from the absolute luminance, all performance features can be assessed with an uncalibrated photodetector and the eyes of a human observer. The measurement techniques especially enable the user to perform comparisons of different display systems.

Comparison of the physical performance of high-resolution CRT displays and films recorded by laser image printers and displayed on light-boxes and the need for a display standard

Performance figures are reported on absolute luminance, luminance uniformity, characteristic display function, internal scatter, dynamic range, distortion, modulation transfer and its spatial uniformity, and temporal and spatial noise of two commercial ORT display systems as well as films printed by laser image recorders and displayed on light-boxes. One of the ORT displays has a matrix of nominally 2000 x 2000 pixels, the other of 1000 x 1500 pixels. The laser image recorders cover a matrix of 3500 x 2200 pixels. When comparing equal pixel matrices of hard versus soft copy displays, typically hard copies facilitate greater information transfer than soft copies due to a presentation with higher absolute luminance, greater perceived dyiiainic range, and better spatial resolution. Perceived dynamic range and resolution are partially degraded in the ORT displays by internal scatter. Soft and hard copy displays are about equivalent in terms of luminance non-uniformity, noise, and geometrical distortion. All displays differ in their characteristic display functions and thus in perceived contrast resolution. A display function standard is proposed to the industry by which mismatches between hard and soft copy presentations can be nimized as well as means for maintaining standardized performance.