Active-matrix LCDs make colorful splash in radiology setting

Monochrome displays may be the gold standard in radiology, but new possibilities are opening up as commercial flat-panel color active-matrix liquid crystal displays achieve greater luminance and resolution.

Use of these displays in diagnostic imaging introduces new issues, however, including changes in luminance and contrast that occur from different viewing angles, or angular emission characteristics.

An FDA study published in August compared the effect of viewing angle on the luminance and contrast response of two high-pixel-density active-matrix LCDs - a 5-megapixel monochrome display and a 9-megapixel color display (J Digital Imaging, epub 03 August 2004).

"Our findings indicate small differences between the 5-megapixel and 9-megapixel monitors," said Dipesh H. Fifadara, an engineer at the FDA Center for Devices and Radiological Health.

Fifadara's study found that angular emission characteristics of the two systems are similar, with a difference of less than 10° for the angles at which the luminance ratio drops to 10%, 20%, and 50% of the maximum.

The comparison applies only to angular emission profiles, however, and was not designed to evaluate overall image quality of the display systems, Fifadara said.

The investigators chose to examine the viewing properties of 5- and 9-megapixel devices because they represent the highest available pixel density that has been considered or is being used in medical imaging workstations.

"The properties of these two devices can then be compared with previously published angular response of other devices, including lower resolution active-matrix LCDs," Fifadara said.

In radiology applications such as digital mammography, high-resolution LCDs provide a way to display full-field radiographs without panning or zooming.

Although the 9-megapixel display has an overall better angular contrast response, the angular luminance response is only better for small luminance values. For higher luminance values, the 5-megapixel display has a smaller deviation from the expected response.

The results also show that along the horizontal and vertical directions, the 5-megapixel display performs slightly better than the 9-megapixel display, by having more of its values within the 20% luminance value tolerance limits. The opposite is true in the diagonal directions, where the 9-megapixel display outperforms the 5-megapixel display.

"We cannot determine without further clinical trials whether these differences have a significant impact on radiologist performance in a clinical image featuring detection tasks, although the advantages in terms of economics and potential use of color merit the further investigation of new devices," Fifadara said.