Number of megapixels affects display less than brightness of monitor

Monitor purchasers may be overbuying when they insist on monitors with 3 megapixels or more, according to research presented at the Society for Computer Applications in Radiology meeting held June 2 to 5 in Orlando. The key is matching the monitor to the application.

Research conducted at the University of Maryland School of Medicine found little or no difference between 1-MP and 5-MP medical-grade LCD monitors, when reading cervical spine images for fractures. Another study, this one done at Massachusetts General Hospital, indicated that consumer-grade LCDs may be good enough for reading CT images.

The Maryland study included 200 C-spine radiographs read by seven radiologists on 5-, 3-, 2-, and 1-MP medical-grade LCD monitors. Sensitivity and specificity were 81% and 77%, respectively, on the 5-MP monitor; 82% and 73% on the 3-MP; 87% and 76% on the 2-MP; and 80% and 75% on the 1-MP. Each monitor's identifying details were concealed during the study so the radiologists doing the interpretations could not tell which was which.

The likely key to the good results from the monitors with smaller matrix sizes was high luminance, said Dr. Eliot Siegel, chief of radiology and nuclear medicine at the VA Maryland Health Care System and coauthor of the Maryland study. Each was set to approximately 160 foot lamberts of luminance, more than three times the minimum 50 foot lamberts specified by the American College of Radiology.

The apparent effect of increased luminance on the quality of image display prompts the question of whether the ACR should raise the bar. Boosting the luminance requirement, however, would probably eliminate the use of most or all CRT monitors and all but a handful of consumer-grade monitors for visually demanding imaging studies such as C-spine radiographs, Siegel said.

Such a boost might be necessary only for very demanding tasks, however. Consumer-grade LCD monitors should be fine for CT images, according to Dr. David Hirschorn, lead researcher in a Massachusetts General study. His group compared a 21-inch 3-MP medical-grade gray-scale LCD monitor with a 19-inch 1.3-MP consumer-grade color LCD monitor to interpret 100 randomly selected CT scans.

Two attending radiologists searched for lesions and graded them for conspicuity. In all measures - number of lesions found, number of normals, and average conspicuity - there was no difference between the two monitors, Hirschorn said.

The next step will be to compare the two monitors in the interpretation of radiographs. Hirschorn said he expects that they will not be equivalent.