Monitor quality assurance goes local

With more hospitals turning to digital imaging, the absence of quality assurance programs to measure the condition of PACS display monitors has become a problem, according to some industry experts.

"At present, there is no universally accepted QA program for PACS," said Catherine Kim Ly, a PACS analyst at Texas Children's Hospital in Houston.

Some professional groups, including DICOM Working Group 11 of the American College of Radiology-National Electrical Manufacturers Association and the American Association of Physicists in Medicine Task Group 18, are writing image quality guidelines. But so far there is no general agreement on the specifics of how to ensure optimal electronic display, Ly said.

"Radiologist workstations require constant calibration and QA," said Stephen Thompson, a medical physicist at the University of Texas M.D. Anderson Cancer Center. "They drift, lose lamps, get dirty, and sometimes are not set up properly to begin with."

Since Texas Children's depends on electronic image displays for 95% of its diagnostic image interpretation, the hospital decided to develop its own QA program.

Ly listed three cathode ray tube properties that complicate CRT use for displaying digital images:

?They produce about five times less light than a light box.
?The inherent display characteristic of a CRT is unlike the display of transilluminated radiograph.
?The CRT display function degrades over time.

Texas Children's adapted a QA program developed by the Society of Motion Picture and Test Engineers (SMPTE) to measure luminance from the display of a test pattern, already in hospital use for quality control of multiformat cameras and laser film printers.

The hospital is collecting data to determine the appropriate frequency of calibration, the useful life of monitors, appropriate manufacturers, model-dependent limits on maximum and minimum luminance (black level), symptoms of degradation, and monitor cleanliness.

Once a month, a Texas Children's biomedical engineer measures luminance by displaying the SMPTE pattern, then makes subjective evaluations of sharpness, geometric distortion, and artifacts. When a monitor's luminance falls outside of arbitrary 10% limits of maximum or minimum luminance, vendors are notified.

"We recommend a monthly calibration of all monitors used for primary soft-copy interpretation and a quarterly vendor calibration," Ly said.

Particular attention is paid to luminance drift, since this indicates the need to calibrate the monitors more frequently. If the device cannot be recalibrated to vendor specifications, this is a sign the monitor is coming to the end of its lifespan, Ly said.

"Our ongoing analysis of the lifetime of displays will be important when it's time to negotiate purchase contracts because gray-scale CRTs are expensive."

It is not known at this time whether the Texas Children's QA program applies equally well to monitoring flat-panel displays now being evaluated for clinical use.