Getting the most out of digital image viewing

Digital imaging systems are only as strong as their weakest link. Don't shortchange your high-quality radiographs with substandard digital viewing software or technology or an inadequate reviewing environment.

Since digital radiography displays radiographs on computer monitors rather than as hard copies (film), an integral component of any digital radiography unit is the image display. The quality of the image display greatly influences lesion identification and diagnostic accuracy. Soft-copy (computer monitor) review is only possible because of technological advances in both viewing software and monitor quality.

VIEWING SOFTWARE

To view radiographs effectively, radiography viewing software must be used. Many different versions of Windows- and Macintosh-based software are available, and they range in price from downloadable freeware to expensive programs sold on a per-license per-annum basis. Most of the medical viewing programs are Digital Imaging Communications in Media (DICOM) viewers, which can handle the various digital imaging modalities (see the article "An introduction to DICOM" ).


Recommendations for Best Digital Image Viewing
To fully optimize digital imaging, the software should include display tools. The American College of Radiology recommends a minimum standard for DICOM viewer software, which includes controls for window and level adjustment (analogous to contrast and brightness), pan and zoom functions, the ability to flip and rotate, and measuring tools. Almost all of the commonly used viewing programs exceed the minimum standard, offering many additional features to increase diagnostic utility.

MONITORS, DISPLAY PROTOCOLS, AND AMBIENT LIGHTING

Recommendations for workstation monitors, display protocols, and ambient lighting are based on experience from the human medical profession as well as digital radiology research involving studies of diagnostic accuracy and digital imaging in general.1-12 Unfortunately, no standardized rules exist for monitor quality, and advances in monitor and graphics card technology greatly exceed the pace of any regulatory body.

Monitor recommendations vary with the type of viewing workstation; for example, primary review stations where radiographs are first reviewed and a diagnosis is made differ from secondary or tertiary review stations (examination rooms, surgery suites). As a rule, primary review stations should have the best-quality monitor possible and be housed in a room with controlled ambient lighting.

Monitors

Active matrix liquid-crystal display (LCD) monitors have largely replaced cathode ray tube (CRT) monitors in diagnostic imaging. Liquid crystals are molecules that have variable physical properties in the presence of an external electrical field. LCD monitors have a backlight and operate by controlling the transparency of each pixel through modification of the external electrical field around the liquid-crystal component.

Historically, CRT and early LCD monitors were too dim, lost brightness over time, and had inadequate resolution to display large pixel matrices. But technical advances addressing these shortcomings have been made, particularly in LCD monitors, resulting in a wide variety of readily available medical-grade monitors. These technical advances have also resulted in a wide range in cost in both medical-grade and consumer-grade monitors .2

Medical-grade grayscale monitors offer several advantages including increased resolution, bit depth, brightness, and refresh rate. Consumer-grade monitors are generally color monitors with less brightness, smaller pixel matrices, and less sophisticated graphics cards, and they are not calibrated to the DICOM grayscale standard display function (GSDF), a quality-assurance measure for image display consistency among all calibrated monitors.

Compared with grayscale monitors, color monitors have disadvantages, which differ between CRT and LCD monitors. A single shade of gray on a CRT monitor comprises the three primary colors and uses three pixels, reducing the overall screen resolution. Active matrix LCD monitors display color through sub-pixel modification in which each displayed pixel is divided into three sub-pixels with each having an added color filter (red, green, and blue). The sub-pixel modification decreases light transmission from the backlight to the viewing surface. When not used for color, this sub-pixel modification can be used to increase the bit depth of grayscale display (i.e. increase the number of gray shades that each pixel can display). Some medical-grade monitors use sub-pixel modification so the displayed pixel gray shade is a combination of different gray shades at the sub-pixel level.