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If anything can stop a PACS in its tracks, it is lack of diagnostic confidence. That is why quality assurance (QA) for PACS monitors is so critical. Users must be convinced that what they seeand do not seeon the screen is what is really there. This is no small task.
As hospitals transition to digital imaging, the number of displays they use for soft-copy viewing is growing exponentially. A full-blown PACS can include hundreds of displays spread over many departments and remote locations.Those displays can range from CRTs to a whole smorgasbord of flat-panel LCDs: 1,- 2-, 3-, and 5 mega-pixels (MP); grayscale or color; diagnostic, referral, and consumer grade.
And while most agree that making sure every one of those displays consistently meets established QA standards is a priority, an informal survey of PACS users reveals that standardized monitor quality programs are still mostly just an item on everyone’s to-do list.
“PACS is still a nascent industry,” says Shawn McKenzie, senior clinical PACS advisor at Sutter Health in Sacramento, Calif. “People are focused on implementation rather than maintenance, and the infrastructure of products and service providers has not matured enough to fully support monitor QA.”
McKenzie says that self-calibrating flat-panel LCD monitors have lulled some into thinking they do not need to be as concerned about QA. “Sure, they’re self-calibrating,” he says. “But to what level of specificity and sensitivity? That can make a big difference when you’re reviewing a chest x-ray with a 5 mm nodule.”
“We need to focus on monitor QA and we are definitely revamping our monitor QA program,” agrees Sandy Johnson, digital imaging quality assurance manager for UCLA Medical Center, which has a 100+ monitor PACS. “For us, it’s been a process of the early QA software being hard to use as well as dealing with shifts in technology. Until recently, monitor QA was mostly an analog process. Now it’s digital.”
A QUICK PRIMER ON MONITOR QA
Standards and Testing . At the heart of monitor QA is DICOM Part 14: Grayscale Standard Display Function.
The DICOM Standard facilitates communication and transfer of information within radiology and enables communication among devices made by various vendors. Part 14 seeks to standardize images from one display to another. Paraphrased it states:
A digital signal from an image can be measured, characterized, transmitted, and reproduced objectively and accurately. However, the visual interpretation of that signal is dependent on the varied characteristics of the systems displaying that image. Currently, images produced by the same signal may have completely different visual appearance, information, and characteristics on different display devices. 1
The DICOM curve provides an objective, quantitative mechanism for mapping digital values into a given range of luminance. Making sure that PACS monitors meet this standard is one of the most important and fundamental protocols of any monitor QA program.
Other routine monitor QA includes visual test patterns such as SMPTE (Society of Motion Picture and Television Engineers), Briggs, Grid, and Geometry as well as CIE (Commission Internationale d’Eclairage: International Commission on Illumination) viewing standards (an internationally agreed-on metric for color measurement) for color monitors.
Specific visual assessments also include:
- AAPM TG18 Test: Described in the draft report of the American Association of Physicists in Medicine (AAPM) Task Group 18. An extensive test performed with high-end spot meters.
- DIN (Deutsches Institut für Normung eV) Test: Supports a standard called Image Quality Assurance in X-Ray Diagnosis, Part 57: Acceptance Testing for Image Display Devices, prepared by the German Institute for Standardization. DIN compliance is mandatory for German medical facilities.
- JIS (Japanese Industrial Standards) Z 4752-2: Describes performance of constancy tests to determine the quality of image display devices. Applies to Japanese medical facilities and is similar to the DIN standard.
Monitor QA Protocols . Because CRTs cannot make a pixel grid of perfect squares and rectangles, over time their circuitry degrades and the squares blur into each other. Monitor QA for CRTs is a hands-on process in which a technician routinely checks each monitor (typically quarterly and more often is better) for that degradation, as well as for loss of luminance, horizontal tearing, and phosphor burn-in. The physicist or technician obtains objective measurements with a photometer and adjusts the monitor’s brightness and contrast to ensure grayscale luminance range. Further observation determines whether image geometry is square and whether images are stable. 2
Flat-Panel QA . QA for flat-panel LCDs is very different. In LCDs, contrast does not degrade because the pixels are etched in glass and stay square throughout the lifetime of the monitor. For these monitors, QA involves making sure that the LCD’s backlight maintains its target luminance, the monitor conforms to the DICOM curve (see story below), and no odd defects pop up such as stuck pixels (pixels that are either permanently switched on or off) or blots on the screen.
The DICOM CurveWhoever said “the eyes are fondest of glittering objects” really knew what they were talking about. The human eye is relatively less sensitive in perceiving the dark areas of an image than it is in the bright areas. This variation in human contrast sensitivity makes it much easier to see small relative changes in luminance in the bright areas of an image. This difference in sensitivity is the physiological basis of what is known as “the DICOM curve.” The DICOM curve allows you to standardize and map your display to the contrast sensitivity of the human eye. If you have an image made of pixels, on a digital level those can be represented in values from zero to 255, with zero being darkest and 255 being the lightest. This is known as the DICOM Look-up Table. If the human eye perceived contrast as a linear function, then the value 100 would be half as bright as 200. But, because our eyes see changes in brightness more easily, it is not linear. By mapping those values on a curve that corresponds to human visual perception, the DICOM curve accounts for that contrast sensitivity of our eyes and gives us an objective, quantitative mechanism for mapping digital values into a given range of luminance. |
Monitor QA Terminology
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Flat-panel LCDs designed for use in PACS are typically individually characterized and DICOM-calibrated right out of the box, and will offer automatic DICOM calibration and conformance testing with the addition of software localized to the machine. The flat panel’s backlight should maintain its target luminance throughout its lifespan. If it cannot do so, the monitor’s software typically provides an alert warning and logs the problem in an activity log.
A QA program for LCD monitors involves reviewing those alerts and data from each monitor’s localized software. For extra confidence, experts recommend that flat panels also be given routine visual checks and manual DICOM conformance testing, in which the tester measures gray level steps from black to white with an external photometer, and then compares the data to the DICOM curve and a luminance uniformity metric (LUM) defined by the Grayscale Standard Display Function (GSDF). The closer the LUM metric is to zero, the more perfect the display’s DICOM conformance.
MANAGING PACS-WIDE MONITOR QA
One way to manage monitor QA is to send a technician on foot to each monitor in the PACS for testing and data collection as well as to respond to any alerts that show on LCDs. That may be fine for a five-monitor PACS, but a 100-monitor PACS with several remote locations would put technicians through a lot of unnecessary exercise.
A number of OEM vendors now offer enterprise-management software that allows radiology and/or IT personnel to monitor and manage displays from one centralized location. Most manufacturers of flat panels for PACS offer this type of product, with varying degrees of flexibility in terms of which display types it can monitor. While some of these software “dashboards” are designed for both CRTs and LCDs, newer versions tend to focus primarily on flat panels, linking directly to each LCD’s localized self-calibration software.
Here is how enterprise software typically works:
- The enterprise management software is connected to each PACS display via a network and “stays in touch” with each display’s localized software, recording DICOM conformance and other data with date/time stamps.
- If something goes wrong with a particular monitor, the enterprise software alerts the PACS QA administrator via e-mail, page, or other notification and enters the information in a log. This log provides a record that can be used to prove DICOM conformance, as well as traceability as to who checked displays and when, should questions arise.
- In many cases, the enterprise software will allow the administrator to check and fix a display’s conformance problem from the central location, saving time, energy, and wear and tear on the QA technician’s feet.
EARLY SOFTWARE DISCOURAGING
Among PACS experts, there is skepticism over the validity and usefulness of enterprise management software for monitor QA.
David Hirschorn, MD, research fellow of radiology informatics at Massachusetts General Hospital/Harvard Medical School and director of radiology informatics at Staten Island University Hospital, says that as PACS displays become increasingly commoditized, manufacturers are putting out calibration packages and enterprise software to differentiate their products, not to meet market demand.
“How many places actually use this stuff?” Hirschorn asks. “If it’s more than a single digit percentage, I’d be surprised.”
Johnson at UCLA says that she would use the software if it meets her needs, but the early versions she tried did their job a little too well. “The original QA software we started out with was too sensitive. It put out a flood of alerts about monitor events’ we didn’t need to address. We were wasting time combing through error logs, so we stopped using it.”
She says the ideal monitor QA software would be extremely user-friendly and fully automated, catch even odd things such as a bulb defect, run on a server on someone’s desk, and provide early warning of significant monitor failure.
“We haven’t tested the newer software yet, but I’m hoping it can give me a very clear-cut picture of what’s going on with the monitors,” she says. “Don’t give me a ton of warning, just let me know when the monitor’s going to fail.”
McKenzie agrees: “If a monitor burps, the system alerts. It’s too sensitive.” He believes new generations of software will get it right.
MONITOR QA: THE FUTURE
Hirschorn sees monitor QA becoming easier and easier, with automatic calibration becoming standard in alleven consumer-gradeLCD monitors. “Thanks to the high-graphics demands of the huge computer games market, even consumer-grade displays are getting good enough to use for PACS, especially by clinicians and technologists, and this whole concept of monitor calibration is becoming more and more mainstream,” he says.
McKenzie sees plenty of as-yet-unmet industry opportunity in having vendors be responsible for monitor QA through either OEM or application service provider (ASP) agreements. “As of right now, monitor QA is a huge vertical in the PACS market that is mostly going unfilled,” he says. “I think we’ll eventually see vendors of monitor QA and other PACS servicing spring up and be accepted by PACS managers as a normal part of doing business.”
For her part, Johnson hopes to see more PACS and QA people educated on monitor QA. “Our physicists are really into QA,” she says. “But that’s not enough. We need classes in it. This is our archive medium.”
Johnson believes that as the installed base of flat-panel LCDs ages, QA will become more and more of an issue. “So far, our 2-year-old monitors look very stable, but how do we know for sure?” she says.
Tamara Greenleaf is a contributing writer for Decisions in Axis Imaging News.
References:
- Digital Imaging and Communications in Medicine (DICOM) Supplement 28: Grayscale Standard Display Function. Available as a PDF at: medical.nema.org .
- Fetterly KA, Hangiandreou NJ, Langer SG. Monitor QC. Decisions in Axis Imaging News. 2003;16(4):46-52.
