Monitor Selection and Calibration in Veterinary Digital Radiography
by Matthew Wright, DVM, MS, DACVR

The monitor used to view digital images is an essential, but often overlooked, component of any digital radiography system. Poor monitor selection and/or calibration can render a $50,000 or $100,000 investment nearly worthless. It is essential that the monitor used to display all of the relevant information in your radiograph be properly calibrated to prevent you from missing lesions. An uncalibrated consumer-grade monitor may not fulfill this requirement. Unfortunately, there are no clear standards regarding displays used for medical imaging in either human or veterinary medicine, which makes deciding on a monitor difficult.

Ideally, a medical-grade grayscale monitor will be used to view digital radiographs. These monitors were designed for primary review of digital images and are several times brighter (measured in cd/m2 [candela]) than consumer-grade displays. Medical-grade grayscale monitors can display all of the necessary shades of gray (measured by the contrast ratio) in an image and can be calibrated to the DICOM standard. They will maintain their brightness over time when compared to consumergrade monitors.

If your digital radiography system comes with a medical-grade grayscale monitor, consider yourself lucky as these monitors cost at least $4,000. Recent research, however, has shown that a medical-grade grayscale monitor may not be necessary. Some consumer-grade color monitors, if properly calibrated, can be used for primary digital radiographic review. In a presentation given to the Radiological Society of North America (RSNA) in 2006, Dr. David Hirshorn reported that the differences in interpretation between a properly calibrated, high-end, consumer-grade display and a medical-grade grayscale display were not statistically significant.1 However, these top-quality color monitors are brighter than most consumer-grade monitors with a brightness greater than 400–500 cd/m2 and a contrast ratio of at least 800:1–1000:1.

Another way to get the most from your monitor is to set the ambient light in the room to a level that is almost equal to the brightness of your monitor and to position the monitor at eye level.2,3 Ambient light is important for digital radiography because the brightness of LCD screens is much less than a traditional light/view box. The ability of the eye to discriminate between gray levels is best when the ambient light level of the room is close to the amount of light coming from the screen. LCD monitors are also designed to be viewed “head on.” Off-axis viewing will result in significant image degradation with LCD displays.

Regardless of which type of monitor you choose, it will not be calibrated for viewing digital radiographic images when it is shipped from the manufacturer. You will have to calibrate the monitor and maintain its calibration every few months. There are numerous monitor calibration tools available to aid you in this correction. Ideally, monitor calibration is performed using a manual external calibration photometer that is attached to the front of the display or a built-in monitor calibration tool that automatically performs display quality control inspection. Unfortunately, because external photometers and auto-calibrating monitors are quite expensive, most veterinary practices are unable to purchase such tools. Following is a quick and painless alternative for approximating your monitor’s calibration.

To calibrate your monitor, you may use the American Association of Physicists in Medicine (AAPM) Test Target, can be downloaded for free at http://deckard.mc.duke.edu/~samei/tg18.

The AAPM’s TG18-QC Pattern test target is composed of a number of grey boxes. Look at the lower left and lower right boxes; in each of these boxes, you should see the words “QUALITY CONTROL” (one with a black background, then grey, then white). If you do not see the words “QUALITY CONTROL” in both the left and right sides of the image, your monitor needs to be calibrated. In most cases, the brightness, gain and contrast settings on your monitor will need to be adjusted so that you can see the words “QUALITY CONTROL” in the boxes on the left and right.

Maintaining the correct calibration of your monitor only takes a few minutes and ensures that your monitor will be set for optimal diagnostic image viewing.

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