Perhaps your department or facility is moving in the direction of PACS and soft-copy reading, or maybe it's still in the evaluation stage, or maybe you're even there and new, high-quality monitors are high on the "need" list. The hospital could be abolishing cathode ray tube (CRTs) and implementing liquid crystal display (LCD) systems. Whatever the scenario, one of the many items on the displays "must-do" list is quality control (QC) for all those new medical image display systems. Monitor QC may fall to the bottom of the list, but as hospitals forge ahead with LCDs, they need to establish a plan for regularly calibrating these systems.
Most LCD monitors are much smarter than their CRT counterparts; most products include internal backlight sensors that monitor and adjust the output of the backlight to maintain stable brightness. A backlight sensor, however, cannot run a QA check or calibrate the monitor to conform with the DICOM curve. Most LCD monitors include calibration software that enables a tech to verify calibration. A few monitors on the market are capable of intervention-free QA checks; the tests can be run without a person physically at the workstation to oversee the QA. Nevertheless, the healthcare facilities need to establish an ongoing calibration program to ensure that their LCD monitors are correctly calibrated and accurately displaying images.
DICOM Part 14 addresses the accurate display issue. The DICOM standard requires vendors to optimize grayscale to match human perception, so that users can see distinct steps along the DICOM gamma (or grayscale) curve. All new LCD monitors are shipped factory-calibrated to the DICOM standard; however, vendors' conformance to the DICOM curve is not consistent. Moreover, even shipping can affect the initial factory calibration of LCD monitors. Consequently, it is up to the end-user to verify whether or a not a new monitor complies with DICOM Part 14. The American Association of Physicists in Medicine Test Group 18 outlines techniques for measuring luminance, uniformity, resolution, noise, veiling and glare. (For details, see http://deckard.mc.duke.edu/~samei/tg18)
ENSURING A QUALITY INSTALLATION
While monitors do ship factory-calibrated to DICOM 14, the healthcare facility needs to verify that each new display meets DICOM standards. This task is a mere fine-tuning or tweaking as new monitors should be very close to DICOM conformance. There are a variety of personnel options for this task. A medical physicist or IT technician could take on this assignment, however, not all hospitals have the in-house IT expertise to undertake the initial installation and calibration. A third option is to purchase LCDs from a specialized reseller that can provide this service with the purchase.
The initial installation calibration relies on an external calibrator, typically a photometer (also referred to as a light or luminance meter, puck or photosensor), to measure the luminance level of the monitor. This test determines how the monitor conforms to the DICOM grayscale curve and whether or not the monitor requires any adjustments. Results from this test are compared to a look-up table (LUT), which is the memory used to modify the output of the display based in the input. The LUT contains up to 256 shades of gray with just noticeable differences (JNDs). The light meter is placed on the screen and the software drives the correct brightness for all of the shades of gray. At this point, it's also a good idea to look for degradation in output levels, check for lost pixels, verify that the monitor is at the correct viewing condition and measure contrast sensitivity with a phantom. After the new display is approved for use, the hospital can establish an ongoing QC program.
THE CALIBRATION PROGRAM
Hospitals have a veritable pool of employees who could be assigned monitor QC. Typically, LCD calibration falls under the purview of the PACS administrator, biomedical engineering staff or IT department. Occasionally, radiologists will assume responsibility for the calibration. No matter who takes on the task, vendors have made it fairly straightforward with calibration software programs.
With most monitors, a light meter is used to complete a hands-on calibration. Users can opt for anywhere from one to 256 test points corresponding to whites and blacks, which are plotted to the DICOM gamma curve. Dan Verbsky, field applications engineer with Richardson Electronics, says most people