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 (r 2 (quadratic) = 0.997). The results suggest that observer performance with LCD displays is superior to CRT viewing, at least for on-axis viewing.KEY WORDS: Vision model, observer performance, digital display A LTHOUGH PACS (picture archiving and communication systems) and teleradiology have changed the practice of radiology quite dramatically over the past number of years, there are still many changes taking place in the digital reading room. 1-3 One aspect that seems to evolve continuously is the display. 4,5 Although early investigations tended to focus on how many monitors were required in the digital reading environment and what resolution was required, 5 a critical issue today is whether flat-panel liquid crystal displays (LCDs) are more suitable for primary diagnostic reading than the more traditional CRT (cathode ray tube) display. 6,7 In many respects, the LCD has performance characteristics that exceed those of the CRT. 8,9 The MTF (modulation transfer function) of an LCD is essentially isotropic, whereas the MTF of a CRT is non-isotropic. At the highest digitally addressable spatial frequency along the CRT scanline in the horizontal direction, the value is usually only 10% to 20%. In the vertical direction the MTF at twice the line frequency is closer to 30% to 40%. 8,9 The result is that, for the CRT, more than half the contrast modulation is lost at the highest spatial frequencies, but this degradation does not occur with the LCD. Practically speaking, this could mean degradation of highfrequency lesions in radiographic images such as microcalcifications in mammograms, resulting in reduced detection performance.

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The Sarnoff JNDmetrix visual discrimination model (VDM) was applied to predict human psychophysical performance in the detection of simulated mammographic lesions. Contrast thresholds for the detection of synthetic Gaussian "masses" on mean backgrounds and simulated mammographic backgrounds were measured in two-alternative, forcedchoice (2AFC) trials. Experimental thresholds for 2-D Gaussian signal detection decreased with increasing signal size on mean backgrounds and on 1/f 3 filtered noise images presented with identical ("paired") backgrounds. For 2AFC presentations of different ("unpaired") filtered noise backgrounds, detection thresholds increased with increasing signal diameter, consistent with a decreasing signal-to-noise ratio. Thresholds for mean and paired filtered noise backgrounds were used to calibrate a new low-pass, spatial-frequency channel in the VDM. The calibrated VDM was able to predict accurate detection thresholds for Gaussian signals on mean and paired 1/f 3 filtered noise backgrounds. To simulate noise-limited detection thresholds for unpaired backgrounds, an approach is outlined for the development of a VDMbased model observer based on statistical decision theory.