Display device guidelines are currently divided according to two basic categories of digital image data set size when used for rendering an official interpretation: small matrix size (eg, computed tomography, magnetic resonance imaging, ultrasound, nuclear medicine, digital fluorography, and digital angiography) and large matrix size (eg, digital radiography, computed tomography, digitized radiographic films, and digital mammography). This guideline covers only the use of non-mammography large-matrix images. Specific guidelines for digital mammography can be found in the ACR’s Practice Guideline for Determinants of Image Quality in Digital Mammography (proposed for 2007). The present guidelines also apply to primary displays or those used for diagnostic interpretation. Secondary displays (eg, those used by clinicians or technologists) for radiographic images do not need to adhere to these guidelines as long as the images are not used for primary interpretation purposes. A number of authors have reviewed the factors that contribute to image quality in soft-copy reading of radiographic images. The minimum quality specifications are summarized here.
Matrix Size and Display Size
Soft-copy displays should render images with sufficient pixel density to allow viewing of the whole image with sufficient spatial detail at a normal viewing distance of approximately 30 to 60 cm (with eyeglasses specifically selected for this distance when required). Matrix size should be as close to the for-processing image data as possible, or attainable with magnification. A 5-megapixel (MP) (2,048 × 2,560 pixels) monitor (usually in portrait mode with a diagonal dimension of 54 cm [21 in]) exceeds the matrix size stipulated by the ACR’s standard of a resolution of at least 2.5 lp/mm at the detector plane when acquiring a 35 × 43 cm image (equivalent to 14 × 17 in), and thus is sufficient for viewing all types of computed radiographic and digital radiographic images in a single view. Note that the US Food and Drug Administration recommends that only monitors that have been approved for digital mammography be used for interpreting digital mammography images.
A 1-MP (1,024 × 1,280 pixels), 2-MP (1,200 × 1,600 pixels), or 3-MP (1,536 × 2,048 pixels) monitor will not permit full simultaneous viewing of 35 × 43 cm images at a detector plane resolution of 2.5 lp/mm. For those images, zooming and roaming display functions are required to achieve a correspondence between the detector element matrix and the display pixel matrix so that the resolution of the display monitor does not limit the resolution of the partially displayed image. This is true for any size image for which the detector element matrix size exceeds the display pixel matrix size.
Luminance and Contrast
The luminance of a display can affect image quality significantly, so the appropriate range of luminance should be maintained. The ratio of maximum luminance to minimum luminance of a display device for images (other than for mammography) should be at least 100. The maximum luminance of grayscale monitors used for viewing digital conventional radiographs should be at least 200 cd/㎡. Smaller ranges could lead to inadequate levels of contrast in displayed images, and larger values could lead to poor visualization of details at the extremes of the luminance range because of the limited range of the contrast sensitivity of the human eye. The contribution of ambient light reflected from the display surface should be included in luminance measurement considerations, because some level of ambient light is always present. Luminance should be as uniform as possible across the entire display.
The contrast response of a display should comply with the AAPM Task Group 18 recommendations. A high display contrast ratio with a low minimum luminance level (0.5 cd/㎡) is most desirable. Contrast response should not deviate from the DICOM Grayscale Standard Display Function (GSDF) contrast values by more than 10%.
Bit Depth
It is necessary for a soft-copy display device to render image details with sufficient luminance quantification to prevent the loss of contrast details or the appearance of contour artifacts. Thus, a minimum of 8-bit luminance resolution (bit depth) is required. Nine-bit resolution or higher is recommended if the for-processing image data are greater than 8-bit. In general, the higher the luminance ratio of the display, the larger the bit-depth resolution that is recommended.
Display Calibration
All monitors and corresponding video graphics cards used for primary diagnosis or for image adjustment and evaluation (eg, a technologist review monitor) must provide a means to be calibrated to and conform to the current DICOM GSDF perceptual linearization methods. The intent of the DICOM GSDF is to allow images transferred using the DICOM standard to be displayed on any DICOM-compatible display device with a consistent grayscale appearance.
Additional factors to consider when characterizing a soft-copy display for interpreting medical images include the modulation transfer function and noise. The modulation transfer function at the Nyquist frequency of the display should be greater than 35%, as recommended by the AAPM Task Group 18 documents. A display device also should not add more than a third of the noise of a typical image, limiting the display relative noise to 0.6% to 0.8%.
Desirable display calibration features include remote performance monitoring, calibration, and quality control. Monitor set matching of contrast ratio, brightness, and color are generally accomplished with the DICOM GSDF, although color does not have a standard calibration method to date.
Glare and Reflections
Veiling glare or the spread of light within the display can reduce contrast, so the glare ratio should be greater than 400 for primary displays. Reflections from ambient light sources should be kept at a minimum. Indirect and backlight incandescent lights with dimmer switches rather than fluorescent lights are recommended. Light-colored clothing and laboratory coats can increase reflections and glare. The intrinsic minimum luminance of a device should not be smaller than the ambient luminance (minimum luminance should be at least 2.5 times ambient light). Cathode ray tube (CRT) displays typically have antiglare coatings that can help reduce these effects, but not eliminate them. Protective shields on liquid crystal displays (LCDs) add to reflections and should not be used if possible.
Color Tint and Color Displays
Both monochrome and color displays have a color tint that is a function of where the manufacturer sets the white point. The tint of the display can affect the comfort of the user. The color tint of the display (blue, gray, yellow, etc) is based on user preference but should be uniform across the display area, and monitor pairs should be matched from the same manufacturing batch.
Currently, most color displays have lower luminance and thus lower contrast ratios than monochrome displays and are generally not recommended for viewing certain radiographic modalities (chest, bone, mammography). There are currently no accepted standards or guidelines available for calibrating color displays when viewing grayscale radiographic images, so care should be taken. The DICOM GSDF can be applied to color displays but does not fully address this issue of calibration of color displays.
Technology-Specific Considerations
Both CRT displays and LCDs can be used as primary display devices. Both require about 30 minutes of warm-up time to reach maximum performance. Flat-surface displays (all LCDs and some CRT displays have flat surfaces) are preferred over those with curved surfaces (most CRT surfaces are curved). On-axis viewing is comparable for CRT displays and LCDs, but off-axis degradations in contrast are still possible with many LCDs and should be considered when viewing images on LCDs from nonorthogonal angles. If two displays are placed side by side for viewing images, it is recommended that they be tilted inward toward the viewer to minimize the impact of angular response variation. Angular performance should not lead to a deviation of the contrast response from the DICOM GSDF by more than 30% within the operating ranges of the viewing angles (usually <30°).
Secondary Displays
When the display systems are not used for the official interpretation, they need not meet all the characteristics listed above. If they are being used by a technologist to judge image quality during acquisition, consideration should be given to using a display as similar as possible to the diagnostic one in terms of maximum and minimum luminance, contrast ratio, and conformance to the DICOM GSDF. The display resolution need not be the same as long as zoom and pan (roaming) are easily available to the user so that the full intrinsic resolution of the image can be viewed.