Getting the most out of digital image viewing

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" ).

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.

Recommendations for Best Digital Image Viewing

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.

Brightness. Monitor brightness, or luminance, is measured in foot-lamberts (ft-L or the International System of Units candela/m2 [3.42 cd/m2 = 1 ft-L]) and should be a minimum of 50 ft-L (American College of Radiology and National Electrical Manufacturers Association technical standard). Grayscale monitors are twice as bright as color monitors; however, conventional view boxes are about 10 times as bright as high-end grayscale monitors.13 Lower brightness monitors decrease diagnostic accuracy and increase the time it takes to read the images and reach a conclusion.12,14 All monitors degrade over time, and quality-assurance steps are recommended to ensure maintenance of the GSDF.15,16

Resolution. Spatial resolution (pixel matrix) is most commonly described in megapixels (MP). Personal computer monitors range from 0.75 to 2 MP (1024 x 768 to 1600 x 1200 pixels). Medical-grade monitors range from 2 to 5 MP (1600 x 1200 to 2560 x 2048 pixels). Although 5 MP monitors are currently available, they are expensive, and their use in human medical imaging is generally restricted to mammography.

The diagnostic performance of low-resolution monitors, high-resolution monitors, and printed hard copies has been compared, with variable results. Older research indicated that hard-copy (film) radiography was more accurate than both types of soft-copy displays and that a significant difference did not exist between the two types of soft-copy display.17 Further studies have indicated that the diagnostic accuracy of high-resolution color monitors is comparable to that of high-resolution grayscale monitors if software tools such zoom and window and level adjustment are used.5-7 Evidence exists that higher resolution monitors are better than low-resolution monitors to differentiate low-contrast details.1,10

Contrast ratio. Contrast ratio (dynamic range) is the ratio of luminance between the whitest shade displayed and the blackest, and a higher ratio is preferable. Incorporated into this ratio is overall luminance of the monitor, and a brighter monitor will often have a better contrast ratio or a wider dynamic range. The contrast ratios for medical-grade grayscale (monochrome) monitors generally range from 600:1 to about 1000:1. Contrast ratio and, thus, monitor quality will be effectively reduced by high ambient lighting and, in the case of LCD monitors, will be effectively reduced by off-angle viewing.2,18

Display protocol

Soft-copy radiograph evaluation is also affected by how the radiographs are displayed on the monitor. Optimally, one radiograph should be displayed per 2 or 3 MP monitor so that the information displayed on the screen most accurately depicts the information stored in the digital matrix. Stated another way, squeezing a large image onto a small monitor will result in a loss of diagnostic information. To increase diagnostic accuracy, always use the image review software functions of window and level adjustment and zoom.1,6,17 In addition, to reduce backlighting, use the cropping tools or automatic collimation detection at the time of image production so the white surrounding the exposed portion of the radiographic detector is not visible to the viewer.

Ambient lighting

The lighting in which a radiograph is viewed is another important element in any radiography system and is even more important if soft copies are being reviewed. Compared with conventional view boxes, computer screens have lower luminance, decreased spatial resolution, and decreased dynamic range and are more affected by viewing angle and reflected light. Ambient lighting should be low, adjustable, and indirect. High ambient lighting effectively decreases the luminance of the monitor through reflection, which in turn also decreases the overall contrast ratio of the monitor13,19 and makes low-contrast details more difficult to see (e.g. small pulmonary nodules). Primary read workstations should be in rooms with shaded windows. It has even been suggested that clinicians avoid wearing white laboratory coats while interpreting radiographs to prevent glare.4,14

CONCLUSION

Digital imaging systems are often referred to as imaging chains in which the diagnostic utility of the system is limited by its weakest link. For this reason, it is important not to limit an imaging system with low-quality monitors. However, a good-quality monitor will not compensate for poor-quality radiographs or a low-quality digital radiography system.

Monitor purchasing decisions should depend on the requirements of the practice and the uses of the workstation. Ideally, all clinics using digital radiography would be equipped with high-quality, high-resolution grayscale monitors. At a minimum, a medical-grade monitor is recommended for the primary workstation, but a high-quality consumer-grade monitor can be considered for other applications.20

Sarah M. Puchalski, DVM, DACVR

Department of Surgical and Radiological Sciences

School of Veterinary Medicine

University of California

Davis, CA 95616

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