How sharp is your image?

Popularity of soft-copy reading magnifies the importance of image quality

By: Deborah R. Dakins

The shift to filmless operations is opening new windows on soft-copy interpretation-literally. Soft-copy reading allows radiologists to be information-centric rather than technology-focused. With a mouse click, they can manipulate and evaluate a complete imaging data set, and even apply new image processing algorithms that highlight specific pathologies.

And that's not all. The ubiquity of Web-based viewers tied to today's PACS and teleradiology systems allows images to be read anywhere, anytime. But how can image quality be maintained from palmtop to home desktop to radiology reading rooms?

As more departments shift to filmless, soft-copy reading environments, ensuring that image display is of the highest quality has become a hot issue among standards organizations and vendors. Associations such as the National Electrical Manufacturers Association and the American Association of Physicists in Medicine, among others, are drafting image quality guidelines for soft-copy images. PACS and teleradiology vendors and the OEMs that specialize in display components have introduced a new generation of calibration and quality control products for soft-copy display. Standards are also in development specifically for digital mammography image quality and interpretation (see related article, p. 52).

From image processing tools to graphics controllers, a variety of software and hardware components control soft-copy image quality. The most influential component is the display monitor. Whether a traditional cathode ray tube (CRT) display or the increasingly popular liquid crystal display (LCD) flat-panel monitor is used, performance is measured by luminance, contrast, resolution, sharpness, and uniformity across the display. Another quality factor is dependability. An image should look the same on any monitor connected to the same network.

The only existing standard that can be applied to soft-copy images was developed by the American College of Radiology for digital image data management. Revised in June 2001, the standard addresses a host of technical and operational issues, of which soft-copy display is just a part.

Display workstations should be able to perform the following functions:

  • selection of image sequence;
  • window and level adjustment;
  • pan and zoom capability to meet guidelines for display of all acquired data;
  • calculation and display of linear measurements and pixel value determinations; and
  • display of all images acquired in a study.

The standard no longer maintains that 2K x 2.5K monitors are a necessity for soft-copy interpretation, but display systems do need to adhere to a minimum luminance standard of 50 foot-lamberts. That's appropriate, given real-world applications of soft-copy interpretation, as many departments routinely rely on 1K x 1K monitors for interpretation. Studies comparing spatial resolution of CRTs have found that the accuracy of 1K x 1K monitors equals that of 2K monitors.

Moreover, not all types of diagnostic images demand viewing using high-end 2K x 2K monitors, according to Joseph Marion, an executive director with Superior Consultants in Ann Arbor, MI. The interpretation of computed radiography or digital mammography images, for example, may benefit from a 2K x 2.5 monitor. But for cross-sectional images such as CT, where the viewing protocol involves bringing up a stack of images, a lower resolution monitor will work just as well, he said.

FROM CRTs to LCDs

From the beginning, CRTs have suffered by comparison with film, but the CRTs used today have improved greatly over first-generation devices. While the average light box luminance ranges from 600 to 1000 foot-lamberts, current CRTs in use for medical imaging range from 100 to 400 foot-lamberts, which is well above the ACR standard. But just as CRTs reached a performance high, LCD flat panels arrived on the scene.

At last year's RSNA meeting, BarcoView and Dome, the two technology leaders in diagnostic flat-panel displays for imaging and PACS vendors, introduced next-generation flat panels. BarcoView's Coronis 3MP 3-megapixel flat-panel monitor and digital imaging board provides 10 bits of gray scale. The company released 1-, 2-, and 5-megapixel flat-panel display systems in September, and now offers a complete range of digital gray-scale flat-panel systems in 1-, 2-, 3-, and 5-megapixel resolutions.

Spurred by the promise of improved brightness, life span, uniformity, and resolution, many departments are upgrading their diagnostic workstations from traditional CRT displays to LCD flat-panel monitors. At the Computer Assisted Radiology and Surgery (CARS) meeting last year, a panel of radiologists and consultants predicted that flat-panel displays would replace CRTs in medicine within the next five years. In addition to the potential to reduce distortion and eye fatigue, LCD flat panels may have other advantages, such as improved life-cycle costs and performance enhancements, compared with CRTs, Marion said.

"The benefits of a flat panel in terms of space and heat requirements and uniformity may potentially outweigh the advantages a CRT has," he said. "But it takes a high-end monochromatic solution to be effective in terms of a diagnostic application. People are starting to lean toward flat panels because of the performance advantage and shelf-life."

The two types of displays are comparable clinically. In a study presented at the Symposium for Computer Applications in Radiology in May, researchers at the Baltimore VA Medical Center found LCD monitors equal to CRTs in sensitivity, specificity, average interpretation time, and confidence in diagnosis.

Although flat panels are still expensive, they may be cheaper than CRTs over the long run because the average life span of a CRT is about two years. In another study presented at SCAR, researchers at Texas Children's Hospital reported that 25% of its CRT monitors required replacement after only one year of use. Expensive 2K x 2K-resolution monitors were most vulnerable: Two out of 10 that required replacement had been in use only 11 to 14 months. By contrast, the 11 low-resolution 1K x 1K monitors needing replacement boasted an average life span of nearly three years.

Display vendors marketing flat-panel monitors say the displays are designed to last 30,000 hours or three to five years. An automatic turn-off function could extend product life even more.

WHAT PRICE QUALITY?

LCD flat-panel monitors boast an inherent advantage over CRTs when it comes to luminance-and life span. But that same advantage over CRTs is also a potential disadvantage, said Dr. David Hirschorn, a fellow in radiology informatics and MRI at Harvard Medical School and Massachusetts General Hospital.

"CRTs shoot an electron beam at a phosphorus screen, which causes bursts of light to be emitted from the screen," Hirschorn said. "If you run the screen at too high a brightness, you can burn out the screen, and the image will be blurred. The brighter you run it, the shorter the life span."

On an LCD, the reverse is true. An LCD sports a bright light on a back panel. It is easy to make the display bright. The way an LCD monitor produces black is by filtering the light in a pixel. The challenge, according to Hirschorn, is making it black enough.

The bottom line when it comes to the quality of any display, he said, is the ratio between black and white.

"Is the problem how to make the display bright enough, or is it how to make it black enough? It's both. How many different shades of gray can you display to my eye? The answer is not so much how high you can go, or how low, but what the difference is between the two."

As part of his work as a research fellow in informatics at MGH, Hirschorn is also studying the potential viability of consumer-grade monitors for soft-copy reading. The issue could be controversial, as it flies in the face of the ACR's standard on luminance. But Hirschorn argues that while the standard makes a distinction between large-matrix and small-matrix images, the same luminance requirement is applied-and possibly unnecessarily.

They've already identified a few exceptions. Because of the subtle contrast inherent in computed radiography and digital mammography, high-end monitors will continue to be needed for soft-copy reading of these modalities, he said.

Consultant Marion added that radiologists also should take into account the inherent limitations of viewing gray-scale images on color monitors.

"There's definitely a degradation of the image when it is viewed on a color monitor versus a monochrome," he said. "You get what you pay for."

CONTROLLING QUALITY

Whether radiologists read their studies on a palmtop or a desktop, image quality remains a top concern. Both flat-panel monitors and CRTs degrade over time, requiring routine monitoring and recalibration that can be a headache for PACS administrators.

DICOM has defined a standard contrast curve, the gray-scale standard display function (DICOM part 14), against which different types of display devices can be calibrated. With it, soft-copy displays can be calibrated against a standard that will ensure that images look the same when viewed on different workstations at different times. With the new emphasis on soft-copy quality control, all major manufacturers are addressing quality-control solutions.

BarcoView, for example, has introduced a new functionality to its networked quality control system for flat-panel monitors, called PIN (product intelligence). The system enables a PACS administrator to check and monitor calibration remotely for displays that are linked via a local area network.

PIN relies on the company's I-Guard sensors, which automatically monitor calibration, eliminating the need to use a manual light meter to test the display's luminance. I-Guard is integrated with the display system to provide constant monitoring of LCD and backlight stabilization. Unlike a backlight sensor, I-Guard looks at the front, which is what the radiologist sees.

Data collected using the I-Guards are sent to a remote location, and the information is viewed by the PACS administrator. The data are also analyzed by MediCal, BarcoView's quality assurance and display management software.

PACS administrators can send commands to the monitor to recalibrate itself, if needed. The entire PIN process is invisible to those using the workstation, and applications continue to flow normally.

Such automated solutions may prove most helpful to departments and practices that lack a dedicated PACS administrator to provide quality control on a regular basis, Marion said. Automated QC also helps solve a recurring problem in medical imaging soft-copy display: end user manipulation.

"The human element can be a big factor in display quality. People like to fool with controls," he said. "Some like to adjust the brightness or contrast of the image to suit their own personal preference. But there is a lot of misunderstanding about how to correctly set up a monitor. The more you can automate it, the better."

Ms. Dakins is a freelance editor in Ben Lomond, CA.