An experimental investigation of the effect of light-box luminance on the detection of low contrast objects in mammography

Technology and Perception in the 21st-Century Reading Room

Radiology reading rooms have changed dramatically over the past 15 years, moving from analog-light-box-based environments to digital-display-based environments. Most of the focus in the early stages of this transition was on the technology, but it soon became obvious that it was not possible or even prudent to consider the technology without considering radiologists. The information being presented to radiologists in digital reading rooms is in many ways very different from that presented on traditional film. On one hand, the digital workstation display medium itself is very different from traditional film images hung on light boxes. On the other hand, without large-area light boxes, images such as those from computed tomography (CT) can no longer be displayed all at once in a series of film sheets. The digital world also introduces the possibility of manipulating image data in ways that were never possible with analog film. Not only can radiologists manipulate image data with various image-processing tools, but also, computers can analyze images and provide even more information to incorporate into the interpretation process. As a consequence of these differences, it has been necessary to focus attention on radiologists to discover ways to optimize the digital reading environment with respect to the human visual system and the way the eye-brain system processes information. This article reviews some of the important perceptual issues that have arisen in the digital reading rooms of the 21st century.

Recommendations for a mammography quality assurance program

In 1989 the ACPSEM published a position paper entitled "A Quality Assurance Programme for Mass Screening in Mammography". This paper described test parameters and performance specifications for the equipment related aspects of a mammography quality assurance program. Advice on test equipment selection was also provided. In the intervening period of time there have been considerable advances in mammography technology creating a need to review a number of the paper's recommendations. There have also been considerable developments in the mammography quality assurance (QA) field, not the least of which includes the American College of Radiology Mammography Accreditation Program (ACR-MAP) and the similarly structured Royal Australian and New Zealand College of Radiologists' Mammography Accreditation Program (RANZCR-MAP). In light of these developments it was decided by the Radiology Interest Group to review the ACPSEM position on those aspects of mammography QA that fall within the medical physicist's area of expertise. This document represents the outcome of those deliberations.

Factors affecting phantom scores at annual mammography facility inspections by the U.S. Food and Drug Administration

RATIONALE AND OBJECTIVES

The authors performed this study to evaluate the factors affecting phantom image score at the annual inspection of mammography facilities.

MATERIALS AND METHODS

In 1997, three U.S. Food and Drug Administration (FDA)-trained inspectors performed inspections of all mammography facilities in North Carolina. All federal and state inspection data were collected and evaluated by using linear regression analysis. Factors affecting the American College of Radiology phantom scores were assessed.

RESULTS

Phantom score was affected by inspector identity, view box luminance, and optical density. All of these factors had a statistically significant effect on mass score (P < .05). Inspector identity yielded a statistically significant effect on speck group score, fibril score, and total score. Luminance yielded a statistically significant effect on both speck group score and total score.

CONCLUSION

Phantom scoring should be automated to allow for more consistent interobserver scoring. In addition, radiology facilities can improve the likelihood of receiving a passing phantom score by reducing the ambient light and increasing the view box luminance in the location where the images are evaluated and the phantom is scored routinely. Radiologists should also consider increasing phantom and clinical image optical density to allow for improved phantom testing outcomes.

View box luminance measurements and their effect on reader performance

RATIONALE AND OBJECTIVES

The purpose of this study was to investigate the importance of view box luminance and viewing conditions on low-contrast detection by readers.

MATERIALS AND METHODS

Radiographs of a mammographic contrast-detail phantom were examined on 632 view box panels. The luminance of these panels was obtained by using a calibrated meter and ranged from 860 to 3,300 nit. Twelve radiologists reported the number of contrast-detail disks for each size (diameter, 0.3-7.0 mm) deemed to be visible on films with optical densities of 1.00-2.60. Radiologist performance in reading low-contrast phantom images was also studied as a function of room illuminance and image masking.

RESULTS

Median luminance was 1,700 nit, with 25- and 75-percentile values of 1,450 and 2,150 nit, respectively. Low-contrast visibility generally was independent of view box luminance, regardless of film density or disk diameter. Low-contrast visibility deteriorated when masking around the image was removed and at normal room illuminance. The greatest deterioration in performance occurred at the highest film densities and with the smallest size disks.

CONCLUSION

Detection of low-contrast features on radiographs is relatively independent of view box luminance, but it is degraded by the presence of stray light and by increased room illuminance.

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.

Film viewing conditions in mammography

A requirement for a minimum viewing box brightness of 3000 cd m-2 for reading mammograms has been widely advocated. Some recent work has challenged that opinion by reporting no significant variation in visibility of low contrast and fine detail objects over a wide range of brightness levels. This paper provides further experimental evidence to support the latter conclusion, at least over the range 1340-4190 cd m-2, and suggests that the currently recommended minimum viewing box brightness levels need to be revised. The importance of reducing room lighting levels is fully confirmed.