Display Calibration Made Easy

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Softcopy viewing of medical images require that the display tools perform day in, day out at an optimum level, and not just in radiology, but throughout the entire healthcare enterprise where primary reading stations are needed. Quality control fundamentally improves image quality and the consistency in which images are displayed. To be optimized, QC programs must be non-intrusive to radiologists, manageable for the QA administrator to carry out, consistent throughout the enterprise, and of course, cost-effective. Automatic and remote monitoring tools are making this possible, in addition to increasing productivity gains and streamlining QC programs.

The University of Pittsburgh Medical Center, which is on its second-generation PACS, has 400 primary reading workstations installed throughout the integrated healthcare system's multi-center campus. Prior to the availability of automatic and remote calibration tools, two IT personnel manually performed QA (quality assurance) on each display to determine if it was compliant with DICOM (digital imaging and communications in medicine) part 14 grayscale standard display function (GSDF). Traveling to more than 17 hospitals, evaluating each workstation—which could take up to 30 minutes—and coordinating a time that was non-intrusive to radiologist workflow proved to be almost impossible.

This list of problems is exactly why quality control (QC) programs are often neglected at many hospitals, especially in lieu of large PACS deployments that consume a lot of resources. But without QC, image quality of the display can degrade over time, opening up the possibility of improper diagnosis or interpretation of a softcopy image.

With advancements in calibration software, QC does not have to be as ominous as once perceived. A well-planned and executed QC program with the help of emerging calibration tools can alleviate the arduous nature of display calibration, lower administrative costs and ensure excellent image quality for interpreting physicians. UPMC realized this as it moved to the next wave in display calibration - remote monitoring.

The elements

As radiologists make life-critical decisions on medical imaging displays - be it LCD-based (liquid crystal display) or CRT (cathode rode tube) - QA checks and re-calibration is fundamental to ensure optimal image viewing.

Research attests that LCD-based displays are typically more stable and require less frequent calibration than their CRT counterparts, making them the standard of choice for new PACS rollouts and monitor replacements. Despite their advantages, an LCD's brightness will degrade over time. The most influencing parameter aside from lighting conditions is backlight degradation. Brightness decreases as the phosphors used in the fluorescent backlights of LCDs age. If the backlight is not monitored and re-calibrated, it could fail to be DICOM-compliant over time.

"DICOM calibration is a fundamental requirement for displays used in medical imaging," says Matt Harris, vice president, marketing and corporate development for Planar Systems Inc. "The reason why? The DICOM curve is really meant to represent how the human eye perceives shades of gray. The human eye can better perceive differences at low gray levels than at high gray levels. So the DICOM calibration function, used in LCD displays, is created to mimic the sensitivity of the human eye."

Harris explains that the typical response curve of an LCD looks nothing like that of a DICOM curve. "What DICOM calibration really does is change the response curve of an LCD into that of a DICOM curve, and it allows a much higher degree of gray level separation at low gray levels," he says. In a healthcare enterprise and distributed PACS environment, this ensures that softcopy images look the same when viewed on different workstations at different times.

LCD calibration

There are three modes of display calibration for flat panels, Harris explains. The first - factory calibration - ensures that the graphic board has been adjusted prior to shipment so that the display renders DICOM-calibrated images. An acceptance test upon arrival of the new monitor must take place to confirm the display is DICOM compliant.

The second way to calibrate uses an external photometer (sometimes referred to as a puck) to reset the display to the DICOM GSDF curve. "From time to time, the medical imaging display should be manually calibrated," says Harris. "The external photometer works with calibration software to update the calibration of the display, especially the backlight brightness."

The third uses a built-in calibrator that continuously corrects short-term and long- term instabilities, particularly the backlight. The internal sensor measures the output of the backlight and talks with the software on a continuous basis to make continuous calibration checks. Depending on the vendor, luminance sensors are placed in the center of the display or are otherwise corner-based. Some monitors also are packaged with integrated fast-feedback backlight sensors that control and adjust the luminance and whitepoint.

These systems automatically perform QA checks and DICOM calibrations on a regular basis without user interaction. QC of medical imaging displays is not eliminated, but is less frequent.  

Remote monitoring software allows administrators and service technicians to access the status of networked displays through a web interface. (See chart on previous page.) The software allows calibration to be managed remotely via a WAN or LAN. Features include scheduling for calibration as well as history and alert tracking. Administrators can monitor all the displays within the enterprise, launch calibration, and receive alerts if an LCD falls out of calibration or if calibration is unable to be completed.

A centralized management tool for QC helps PACS to achieve its potential in a hospital's distributed environment.

Putting standards to the test

In the Diagnostic Imaging Division at the University of Texas M.D. Anderson Cancer Center, all medical images acquired, with the exception of some mammograms, are transmitted and stored electronically. Physicians read the images at 50 PACS workstations dispersed throughout the facility.

QC starts from the very beginning. "The QC program needs to address initially the basic performance characteristics of the monitor," says Jeff Shepard, senior medical physicist, Imaging Physics Department. "You must perform an acceptance test when a monitor first comes in order to verify that it functions the way the manufacturer claims or the way you have required in your purchase specifications."

The displays are equipped with software for calibration, automatically monitoring their light output in 20-minute intervals. On a monthly basis, the imaging physics department will perform a visual assessment on each PACS display using a new test pattern purposed by the American Association of Physicists in Medicine Task Group 18 (TG 18).

Similar to a SMPTE (Society of Motion Pictures and Television Engineers) test pattern, Shepard says the guidelines are under review and not yet published.  "It has several different patches of gray in it that you can look at it, and there are lines through it to measure geometric uniformity," explains Shepard. "In each one of the gray steps, there are very small objects of low contrast, so that you can evaluate your ability to resolve subtle contrast in every brightness level in the image. If your display is calibrated properly with the right look up table (LUT), you can see those subtle contrast objects in all of those steps."

AAPM TG 18 developed a document that provides guidelines to medical physicists and engineers for the evaluation of electronic display devices intended for medical use, but Shepard says this was prior to the availability of calibration software. "At M.D. Anderson, we are finding that we don't need to check calibration on our brand of self-calibrating monitors more than once a year," he says.

Before selecting which displays and calibration software will be deployed for softcopy viewing, Shepard suggests a test drive of some sort. "If you know what the performance trade offs are, you can start looking at the real value of the product based on its price," he says.

"The differences are in the subtle details," opines Shepard. "When you are making a purchase decision, you really need to have an opportunity to spend some time with a monitor and evaluate how well it works. Look into different parameters, implement them, and try to see what sort of problems you run into."

The wave of the future

Image Systems Corp. announced at RSNA in November that UPMC is in the initial stages of replacing their grayscale CRTs with its grayscale LCDs and remote monitoring tools. The first hospital to implement the technology is UPMC's Northwest facility, which Jeff Roberts, UPMC's radiology informatics project manager, says is a completely digital facility.

"With Image Systems' CFS [calibration feedback system], we use a service called SNMP [simple network management protocol]," explains Roberts. "It's a network-driven software tool that communicates with CFS and allows IT to remotely monitor, control and schedule the calibration checks." SNMP is a commonly used protocol that makes integrating software into existing networks relatively easy.

CFS can be used to adjust the backlight, generate a new DICOM correction LUT, as well as provide reporting tools that monitor the lifetime of the display. This is beneficial not only for legal reasons but also budget allocation, indicates Roberts.

These tools will be used, but UPMC is still in the "getting-to-know" stages of the implementation. "One of the things that we are working on is developing standards for the CFS—how often do we have the CFS check the LCD, and what are the controls and settings that we are going to use for the check that we do," poses Roberts. "For example, you define the standards that you want checked, and for whenever they are scheduled to take place, the display will automatically perform the check. What is the luminance level? Is it out? If it is out, by what percentage? And if we don't want to be out by more than 2 percent, and it's out by 3 percent, can we set up the CFS to notify IT via a page or email."

"This is going to allow UPMC to effectively perform the QA/QC process remotely and automatically on its 400-plus PACS workstations with minimal man hours ... which will let me sleep better at night," continues Roberts. "It also is ensuring that radiologists are seeing the best images possible."

REMOTE MONITORING: Who's Got What

  • Planar Systems | Dome Dashboard
    Functions with Dome CXtra calibration software on Planar's Dome Cx, Qx and Px lines. The Cx line for primary reading (C5i, C3i and C2) has a built-in photometer in the center of the display, behind the glass.
  • National Display Systems | Integrity
    QA for the new AXIS series uses SNMP (simple network management protocol) to track networked displays for status, prompt for service by e-mail/pager and verification of proper function. Integrity runs in the background in combination with MD-Cal software on each workstation.
  • NEC-Mitsubishi Electronics Display of America | GammaComp MD Administrator
    Centralized control and management software available on NEC's MultiSync MD series, which in addition to GammaComp MD calibration software, features X-Light technology that controls and adjusts the luminance and whitepoint via an internal backlight sensor.
  • Siemens Display Technologies | SMfit ACT Remote
    Remote access management tool that allows access from one central workstation out to all systems on a network. Siemens' ACT Calibration software can be used on any Siemens CRT or LCD. Siemens has a front sensor on its high-end 5MP display but utilizes an internal backlight sensor to control panel stability on its other monitors.
  • Image Systems Corp. | Calibration Feedback System (CFS)
    Remote capability for 2MP, 3MP and 5MP grayscale displays. The displays operate with a backlight sensor to monitor and maintain brightness stability along with an in-bezel, retractable front sensor that measures and calibrates the display.
  • Barco | MediCal Administrator
    Remote QA checks and DICOM calibration on Coronis displays. A new feature allows management of these tasks via a PDA. Barco's Coronis line - including 1MP, 2MP, 3MP, 5MP, color Coronis and Coronis 5MP Mammo - features an I-Guard front sensor and Medical Pro calibration/QA software.
  • Quest | PM Medivisor
    Remote monitoring software for all Totoku monitors. Installed in each workstation, the software agents collect data such as luminance, operating hours, calibration accuracy, on each monitor and reports to a central server, permitting the health of every monitor on the network to be observed from one location.
  • Double Black Imaging | LumiCal Administrator
    Allows scheduling for calibration history and alert tracking. An administrator can monitor all the displays within the enterprise, launch calibration and receive alerts. In addition to LumiCal Agent software, DBI's LCDs feature 3 sensors; two in the front retractable module and one on the backlight.
  • Eizo Nanao | RadiNet Pro
    A network of up to 8,000 RadiForce monitors can be managed, ranging from monitors for diagnostic imaging in reading and modality rooms to HIS/RIS monitors handling reference images and electronic diagnostic chart monitors in clinics.