Display standardization meets image quality assurance

December 3, 2002

Using a simple digital CCD camera, radiologists can provide quality assurance for their display monitors, according to researchers at the University of Pittsburgh and the University of Arizona.Currently, interpretation may be difficult, especially for

Using a simple digital CCD camera, radiologists can provide quality assurance for their display monitors, according to researchers at the University of Pittsburgh and the University of Arizona.

Currently, interpretation may be difficult, especially for recently released active matrix liquid crystal displays (AMLCDs), according to Dev P. Chakraborty, Ph.D., chief of radiology physics at Pittsburgh.

Chakraborty unveiled a method for characterizing such display distortion values as a signal-to-noise ratio for various monitors during a scientific presentation on Monday.

Some traditional methods of measuring display monitor performance, such as modulation transfer function and noise power spectra, can be confounded by the scan lines and subpixel structure often found with AMLCDs.

Chakraborty and colleagues created digital test patterns on their monitors and then took a picture of that image with a CCD camera. All monitors tested provided monochrome high-resolution displays. Chakraborty subtracted the scan line information from the data because it provided a static noise on all images; therefore, its removal would not affect results.

"I found that doing this made the most sense," he said.

The researchers then used an algorithm that computed SNR to examine the digital file captured by the CCD camera. They reported that SNR dependence on the target contrast was nonlinear and increased with luminance and spacing. SNR decreased with background pixel value and was larger for horizontal lines than for vertical lines.

The actual quality assurance method could be viewed at an infoRAD exhibit presented by coauthor Hans Roehrig, Ph.D., a research professor of radiology and optical sciences at the University of Arizona.

"We want to be able to characterize all of the deficiencies of a display system so that we can eventually compensate for those deficiencies," Roehrig said.

Knowing the particular deficiencies of a certain monitor with a particular modality, for example, would allow radiologists to match the appropriate display with the appropriate modality for the best resolution possible, he said.

Setting up the system for measuring and characterizing display deficiencies can be time-consuming, and the researchers do not currently have a set number of tests for image quality.

"We should be able to come up with just a few tests to encompass the full quality of the monitor, but we are not at that point yet," Chakraborty said.