Digital Radiography Image Quality: Equipment Specifications, Existing Standards, Observations, and Challenges

Image quality is affected by several factors, beginning with the acquisition process and device and including the manner in which images are displayed. In digital systems, the functions of acquisition and display are clearly separable, so that the evaluation and optimization of image quality can take place at both ends of this imaging continuum. The analysis of image quality also depends on the imaging task. Digital radiography is used in a wide variety of imaging tasks (eg, chest, musculoskeletal, genitourinary), but there are basic image-quality parameters that can be defined that are applicable to all these tasks.

Specifications for equipment used in digital image data management will vary depending on the application and the individual facility’s needs, but in all cases, they should provide image quality and availability appropriate to the clinical needs, whether those needs be official interpretation or secondary review. Compliance with the current Digital Imaging and Communications in Medicine (DICOM) standard of the ACR and the National Electrical Manufacturers Association (especially part 14, on gray-scale displays) is strongly recommended for all new equipment acquisitions, and consideration of periodic upgrades incorporating the expanding features of that standard should be part of ongoing quality control programs. Compliance with the Radiological Society of North America and the Healthcare Information and Management Society’s Integrating the Healthcare Enterprise initiative, as embodied in the available technical frameworks, also is strongly recommended for all new equipment acquisitions.

Relevant standards for the soft-copy display of images have been summarized in the literature and include Society of Motion Picture and Television Engineers (SMPTE) Recommended Practice 133-1991, the National Electrical Manufacturers Association–DICOM standard (PS 3), Deutsches Institut für Normung V 6868-57, International Organization for Standardization 9241 and 13406 series, the Video Electronics Standards Association Flat Panel Display Measurements standard, American Association of Physicists in Medicine (AAPM) Task Group 18 Recommendations and Standards, and Integrating the Healthcare Enterprise Consistent Presentation of Images. Each provides guidance and tools for the acceptance and quality testing of medical display devices and should be consulted if further detailed information is desired.

Digital radiology encompasses several modalities and image interpretation tasks. The images are acquired at different resolutions, bit depths, and matrix sizes. The task of a radiologist can vary depending on the nature of the imaging request. This makes the development of a set of guidelines for the whole practice of digital radiology both a necessity and a challenge. The minimum set of guidelines outlined in this document address image quality from a technical perspective. These technical measurements are generally easy to make and comply with most of the digital radiology reading rooms. Numerous studies have demonstrated clear connections between these types of technical measurements and clinical interpretation performance. The optimized reading environment improves diagnostic accuracy and may improve the efficiency with which a radiologist interprets images.

One of the main challenges that radiology is currently facing in terms of image quality of displays for interpreting medical images is the widespread availability and relatively low cost of off-the-shelf displays. There are several nonmedical commercial displays that are starting to rival the performance of dedicated, high-performance medical displays in terms of resolution and luminance. They are, however, color displays that are not optimized for grayscale images. Color displays typically have lower contrast ratios (the black levels are not as black as with monochrome displays) and higher noise levels than high-performance medical-grade displays. Both parameters have the potential to degrade image quality and hence interpretation accuracy. Further studies are needed in this area.

The appeal of color displays goes beyond low cost. In addition to color Doppler sonography, many radiologic imaging applications are starting to incorporate color. Three-dimensional color renderings of CT and magnetic resonance imaging data are becoming more useful and more popular not only with surgeons but with radiologists. Few studies, however, have been done to demonstrate the influence of these displays on reader accuracy or reader efficiency (workflow). The 3-D renderings also have given rise to the possibility of true stereo color displays being used in radiology interpretation. Radiologists are very efficient at interpreting 3-D information from 2-D images, but these new displays (which do not require shuttered eyeglasses, unlike older stereo displays) may change dramatically the way that information is presented to radiologists. Again, whether these new displays and display techniques can improve reader performance has yet to be studied.

Digital radiography will continue to go through several changes in the future, as it has in the past. The ways to display these digital images will continue to change as well. The key point of this document is to raise awareness about the need to maintain high standards of image quality. Even as display technologies change, the need to follow the guidelines outlined in this document will continue. The parameters that we identified and provided minimum standards for are likely to remain the same basic set of parameters that will be important with any type of display. As new display technologies are developed, however, it will be necessary to evaluate them and determine how well they comply with these guidelines.