A few years ago, there was a move in medical imaging to use off-the-shelf consumer-grade displays, especially for those clinicians in a web-based and remote reading environment. These displays were inexpensive and widely available. Lately, though, there’s been a shift away from these commodity displays. Analysts and clinical users have found that their quality doesn’t hold up over time. The backlight stabilization is weak, for example, and tends to fade within a few years. Original equipment manufacturers, as well, have begun to take a pass on commodity displays, finding them unreliable and costly to replace in the long run.
“For a reliable and correct diagnosis, the image quality of the monitors is indispensable,” says Jochen Hansmann, MD, assistant medical director for diagnostic radiology at the University of Heidelberg Clinical Center in Heidelberg, Germany. “The best radiograph may become useless, if the image is not displayed on the monitor correctly and faithfully in detail.”
Heidelberg’s radiology department consists of 25 radiologists and 50 medical technologists. Earlier in the decade, Hansmann recognized the advantages and potential of making diagnoses with digital data. He was significantly involved in the decision in 2003 to install a RIS/PACS in the department. With the further installation of a hospital information system (HIS), the entire facility became filmless by 2005. Today, every RIS/PACS station consists of three monitors: two for diagnosis (a 2 megapixel [MP] monochrome and a 2 MP color) and one for showing RIS data (1 MP color).
Hansmann and colleagues chose the RadiForce monitors from Eizo. “The best feature was the grayscale reproduction of these color monitors. Up to now, the quality with color monitors was lower than with grayscale monitors. But with these monitors, there is no difference,” Hansmann says.
Choosing a medical display
Standard computer displays offer limited resolution that is not optimized for diagnostic imaging. Medical-grade displays, on the other hand, offer resolutions up to 2048 x 2560 (5 MP) in portrait or landscape that correspond better to the image format of DICOM medical images.
Consumer-grade displays typically offer a maximum luminance of 250 to 300 candela per square meter (cd/m 2). State-of-the-art medical displays, by contrast, achieve luminance levels of more than 1,000 cd/m 2, which is much closer to conventional film. According to DICOM 3.14, “a larger luminance range results in a broader spectrum of grayscales that can be discerned by the human eye [also known as Just Noticeable Differences or JNDs].” As a result, subtle lesions will be easier to detect on a medical display and radiologists can reach a diagnosis faster.
Luminance is not the only important parameter for diagnostic reading. For many applications, contrast is even more important than luminance. Medical displays offer a contrast (up to 1,000:1) that is substantially better than most consumer displays, which have on average contrast ratio of only 300:1. Displays with better contrast ratios are capable of rendering more DICOM JNDs than their low-contrast counterparts.
The number of available shades of gray on most consumer displays is limited to 256 (8 bit). Medical displays have a much wider grayscale range, enabling them to render every grayscale as defined by DICOM. The Coronis grayscale display family from Barco, for instance, offers up to 4,096 shades of gray (12 bit), while the Dome family of displays, part of the NDS Surgical Imaging portfolio of systems, can be optimized for up to 3,061 shades of grays.
Such an extensive range is necessary to comply with the guidelines set forth by American Association of Physicists in Medicine (AAPM) and the European Reference Organization for Quality Assured Breast Screening and Diagnostic Services (EUREF).
All LCD displays, however, suffer from luminance non-uniformity. This means that images will appear slightly differently in the corner of the display than in the center. This luminance non-uniformity can be as much as 25 to 30 percent. But the bar for diagnostic image quality is being continuously raised. AAPM and EUREF already have proposed to limit this non-uniformity to 10 percent. Medical display vendors such as NDS Surgical Imaging, Barco and Eizo typically offer a luminance uniformity correction function that provides