Getting the most out of digital image viewing - Veterinary Medicine
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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.


VETERINARY MEDICINE SUPPLEMENT


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


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Source: VETERINARY MEDICINE SUPPLEMENT,
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