ROC study of four LCD displays under typical medical center lighting conditions

Abstract

Nine observers reviewed a previously assembled library of 320 chest computed radiography (CR) images. Observers participated in four sessions, reading a different 1/4 of the sample on each of four liquid crystal displays: a 2-megapixel (MP) consumer color display, a 2-MP business color display, a 2-MP medical-grade gray display, and a 3-MP gray display. Each display was calibrated according to the DICOM Part 14 standard. The viewing application required observer log-in, then randomized the order of the subsample seen on the display, and timed the responses of the observer to render a 1-5 judgment on the absence or presence of ILD on chest CRs. Selections of 1-2 were considered negative, 3 was indeterminate, and 4-5 were positive. The order of viewing sessions was also randomized for each observer. The experiment was conducted under controlled lighting, temperature, and sound conditions to mimic conditions typically found in a patient examination room. Lighting was indirect, and illuminance at the display face was 195 +/- 8% lux and was monitored over the course of the experiment. The average observer sensitivity for the 2 MP color consumer, 2 MP business color, 2 MP gray, and 3 MP gray displays were 83.7%, 84.1%, 85.5%, and 86.7%, respectively. The only pairwise significant difference was between the 2-MP consumer color and the 2-MP gray (P = 0.05). Effect of order within a session was not significant (P = 0.21): period 1 (84.3%), period 2 (86.2%), period 3 (85.4%), period 4 (84.1%). Observer specificity for the various displays was not statistically significant (P = 0.21). Finally, a timing analysis showed no significant difference between the displays for the user group (P = 0.13), ranging from 5.3 s (2 MP color business) to 5.9 s (3 MP Gray). There was, however, a reduction in time over the study that was significant (P < 0.001) for all users; the group average decreased from 6.5 to 4.7 s per image. Physical measurements of the resolution, contrast, and noise properties of the displays were acquired. Most notably, the noise of the displays varied by 3.5x between the lowest and highest noise displays. Differences in display noise were indicative of observer performance. However, the large difference in the magnitude of the noise was not predictive of the small difference (3%) in the observer sensitivity for various displays. This is likely because detection of interstitial lung disease is limited by "anatomical noise" rather than display or x-ray image noise.

Fig 1

Normalized noise power spectra measured from the consumer color display.

Fig 2

Normalized noise power spectra measured from the professional color display.

Fig 3

Normalized noise power spectra measured from the 2-MP gray display.

Fig 4

Normalized noise power spectra measured from the 3-MP gray display.

Fig 5

Average observer sensitivity plotted versus noise variance. The noise variance is the sum of the display and x-ray image noise. The value in parenthesis represents the multiplicative factor by which the variance is greater than that of the 2-MP gray display.