Quality Control & Medical Image Displays

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.


STARTING POINTS

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 take 33 samples, which takes about five minutes per workstation. Unlike CRTs, LCDs are extremely stable, and there are no hard and fast rules for calibration. Depending on the vendor, it's recommended that monitors be calibrated on a monthly or quarterly basis. Some sites start out with more frequent, even daily, calibration, and then reduce the frequency as the monitor demonstrates its stability.

Another option is a simple QA check. Like calibration, a QA check requires placing the sensor on the screen, but it does not have to interact with the LUT, so it can be completed in less than a minute. This assures the healthcare facility that the monitor is functioning correctly in a minimal amount of time. In the unlikely event that an LCD cannot meet the DICOM curve, the monitor must be sent back to the manufacturer for recalibration or a replacement sent as hospital lacks the tools and expertise for this task.

LCD vendors realize that end-users may not want to hassle with calibrating monitors. Planar and Barco, for example, have incorporated internal calibration mechanisms with their LCD monitors. Essentially, internal sensors monitor the white level, initiating conformance tests if white levels shift by more than 5 percent and calibrating the monitor as required. The process can be further streamlined with additional software that enables network-wide or remote calibration via a LAN or WAN.

Network-enabled calibration allows an administrator to review and manage the calibration status of all monitors, including remote LCDs from one desk.

An LCD monitor should last from 25,000 to 40,000 hours. The only component that wears out on LCDs is the backlight. Virtually all medical-grade LCD monitors include a backlight sensor that checks the output of the backlight and compensates by increasing the white level as the backlight degrades over time. Eventually the backlight wears out and the monitor cannot be calibrated and must be replaced.

A Look at Calibration Software

Many LCDs include native calibration software as part of the purchase. Some vendors tout additional software with extra features such as remote or network-wide monitoring. Chart Source: Vendor representatives

Ampronix: Dr. Kal
Optional software and photometer for use on multiple workstations. Allows user to select the target attributes and measures ambient lighting, sets the brightness and contrast ratios along a DICOM compliant gamma curve. Locks front panel controls to avoid arbitrary adjustment. Up to 4 panels can be connected and calibrated at a time.

Barco: Medical Pro
QA software and I-Guard sensor, included in the cost of the monitor, provide continuous LCD and backlight stabilization and fully automated QA. Tracks, maintains and logs display viewing performance, automates QA tasks, and initiates display system calibration. For an extra fee, MediCal Administrator WAN or LAN software allows the system administrator to remotely connect to and manage local workstations.

Data-Ray: Dr. Kal & Dr. Kal-net
Dr. Kal is included in cost of the monitor, guaranteeing DICOM compliance and assuring consistency over time and for up to four monitors in a single workstation. It sets the black levels and the white levels as well as gamma correction. DR. KAL-net is an optional network and remote Calibration Software that allows the administrator to schedule a time or to do a manual calibration without interfering with normal radiology tasks. Works as a QA Management System from a centralized location.

Eizo Nanao: Calibration Kit GX-1
Optional calibration pod includes photo sensor and software. Measures display values, communicates with circuitry and LUT built into the monitor and changes properties to ensure conformance with DICOM 14 and allow fine adjustment of grayscale tones. Video card independent system compatible with RealVision and Matrox video cards.

Image Systems Corp.: CFS (Calibration Feedback System)
Local and network-addressable display calibration system included in the cost of the display. Provides hands-free calibration; DICOM conformance; multi-monitor support; local and/or network support; system nonconformance alarms and reporting; background backlight stability verification, documentation and adjustment. Additional charges for network support and some hardware sensor pieces.

National Display Systems: Smart Controller
Included with display. Stores a DICOM calibrated LUT on-board. Intelligently compensates toward luminance output. Open architecture system can interface with any off-the-shelf video card with a linear LUT. Provides real-time user feedback via front panel LED.

NEC-Mitsubishi: MD Software
Allows the user to set custom display settings for gamma, white point and user-defined luminance response settings, as well as access self-diagnostic and calibration information about the monitor. MD Software included with the monitor. Optional Administrator version for WAN and LAN enables remote connection to local workstations.

Planar: CXtra
Provides consistent presentation of images through Dome Right-Light Sensor. Monitors flat panel's calibration state. Enables DICOM gamma correction. Automatically tunes luminance curve for individual panel's characteristics and coordinates it with the Right-Light-monitored luminance. Remote Calibration by using the SNMP technology built into most networks. (Free by using the MIB files supplied on the driver CD.)

Quest: PM Medivisor
For use with Totuku ME and CCL series of monitors. Optional network QA software continuously collects and controls operational status including luminance data. Facilitates unified data management.

Richardson Electronics: TekLink
24-hour online technical support offers Display Systems Group customers access to qualified technical specialists to support them in a vast array of installation challenges and problem solving, including first-time installation, product and compatibility support.

Sencore: accuGray
Additional software package calibrates soft-copy displays by comparing characteristic gamma to the DICOM model. LUT changes automatically calculated and updated. Remains resident on the system for calibration history tracking and reporting. Loads in the system tray at boot with appropriate modified LUT values. Can calibrate to a specified color temperature reference.

Siemens Display Technologies: SMfit ACT
Can be used to perform a complete "hands-on" calibration with a Siemens hand-held luminance meter and a "hands-off" luminance stability check by utilizing an internal sensing circuit within the monitor. Allows digital access to all monitor parameters. Able to perform automatic calibration and remote conformance testing of pre-set luminance settings. Sold in a calibration package with remote access software and luminance meter.