CRTs are on the way out, and LCDs are in. Grayscale dominates, but color technology is advancing fast. Calibration is easier, but not completely hands-free. Which features are here to stay? Medical Imaging chatted with a handful of vendors about the latest in medical-imaging monitors and where the market is headed.
LCDs Rule?But Not Completely
The market is moving toward LCD, with manufacturers focusing most of their efforts on this technology. But CRTs are not gone yet. “CRT will go away, but it’s still a niche,” says Todd Fender, product line manager of specialty products for NEC Display Solutions (Itasca, Ill). “It’s less expensive at some sizes, and a large refurbishment market exists for old CRTs.”
COLOR COMPARISONWith all of the new color monitors on the market, which one is right for you? Download a comparison chart here: Monitor Chart (PDF) |
For some institutions, the issue is one of cost. For others, the issue is one of comfort. “People are still comfortable with CRTs,” says Joey Sanchez, marketing coordinator with Eizo Nanao Technologies Inc (Cypress, Calif).
However, in some instances, the issue is one of quality. Early LCD technology could not rival CRT performance at high resolutions or large sizes. “At 30 inches or more, CRT still outperforms LCDs because of uniformity issues,” Sanchez says. Mammography’s high-resolution needs also create a demand for the less-expensive technology.
But LCD components, such as the glass, have improved, and prices have decreased. This year saw some of the first large-format LCDs, along with a number of new color displays, hit the market.
The Color Race
The increased number of color monitors in the market could be due in part to the increased use of color in medical imaging. Such modalities as ultrasound and PET use color, as do many software programs, particularly for 3-D imaging. Although color monitors are not overtaking the market yet, the day when they will dominate and be used for both color and grayscale imaging could be on the horizon.
“Color has great potential, but the market might not be ready,” says Ken Crocker, marketing manager with Siemens Display Technologies (Longmont, Colo). “There is still a need for monitors dedicated to color as well as to grayscale imaging.”
Older color systems typically support 8-bit color depth, which allows 256 colors. Newer systems use 24-bit, which provides 16,777,216 possible colors. The same number is obtained with 32-bit color modes.
The larger number of colors is intended to permit more visualization of variations. “With more colors, users are more able to notice slighter variations,” Sanchez says. Yet Crocker questions how many colors can really be differentiated. “The eye can take in only so many colors,” he says.
Shades of Gray
The eye can, however, see more variations in grayscale, according to Crocker. Systems today range from 8-bit, or 256 shades, to 10-bit, or 3,061 shades.
“Vision has been studied, and doctors can be confident that monitors have adequate shades of gray,” Crocker says, adding that, “More shades do not necessarily mean more information the eye can use.”
However, more shades can produce a more accurate rendition. Lynda Domogalla, marketing manager for Barco Medical Imaging Systems (Kortrijk, Belgium), notes that more is particularly helpful with certain procedures, such as mammography or viewing small bones.
“Studies have indicated that there are between 500 and 700 just-noticeable differences that the human eye can discern. So it’s more than 256; but beyond a certain number, more shades are not necessarily better,” she says.
How many shades of gray a monitor can render depends on a number of factors, including the capability of the graphics card, the operating system, and the display itself. “If your display controller will only output 8-bit, it doesn’t matter if the monitor is capable of displaying 13-bit,” Domogalla notes.
Similarly, if the card outputs 10 bits, but the operating system allows only 8 bits, then the image will be limited to 8 bits, says Stan Swiderski, senior product engineer on professional- and medical-series products for NEC Display Solutions.
Swiderski’s colleague, Fender, notes that newer operating systems, such as Microsoft’s Windows Vista (currently in development), will allow even greater variations, up to 12-bit, or 4,096 shades.
Gray Still Outshines Color
The number of colors and the number of shades are not necessarily the numbers on which to focus; rather, luminance is key. Color monitors tend to be less bright than their grayscale counterparts. “The color liquid crystals block some of the light,” Sanchez says.
To ensure brightness, the luminance can be set at its maximum, which, according to Swiderski, ranges from 0.50 candelas per square meter (cd/m2) to 500 cd/m2 for color monitors. Grayscale displays are significantly brighter, with a range from 0.50 cd/m2 to 1,000 cd/m2.
But using the display at its maximum brightness can both reduce the life of the monitor and tire the eyes. For color monitors, it’s a trade-off, Fender says?they are either brighter or shorter-lived. But he expects that will change over the next 2 years.
In fact, it’s already changing. This year, Barco released 2-megapixel (MP) and 3-MP color monitors that feature a calibrated luminance of 500 cd/m2. “The monitors are bright enough to diagnose grayscale as well,” Domogalla says.
Conservation’s Reward
“Certain modalities require from 180 cd/m2 to 500 cd/m2,” Fender says. “The American College of Radiology [ACR of Reston, Va] recommends at least 180 cd/m2 for grayscale monitors. But most physicians use the displays at a higher luminance?often between 300 and 500 cd/m2, with 400 being fairly typical.”
Higher luminance, therefore, does not necessarily mean the monitor will be brighter when calibrated, but it should indicate a longer life. “A higher possible maximum luminance means a longer period of time during which you can maintain the needed luminance,” says Siemens Display’s Crocker.
Adds Eizo’s Sanchez, “The public thinks that higher luminance generally means better, but greater brightness can tire the eyes more quickly. Most reading rooms are dark, and so doctors do not need lots of brightness.”
Many displays use cold-cathode fluorescent lamps (CCFL) for backlighting. But some companies, such as Siemens Display, are implementing alternate technology intended to increase the life of the backlight. A sure way to extend it is to operate the monitor at a lower brightness.
Calibration: Not Always Hands-Free
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| Barco’s new color monitor, the Coronis 3MP, calibrates to a brightness of 500 cd/m2. |
Most monitors are calibrated for use at a luminance less than the maximum. DICOM compliance requires that this level remain stable for the life of the monitor. Many lose brightness with age.
Most manufacturers have incorporated backlight sensors, located at the back of the computer, that monitor and stabilize the brightness of the backlight. But DICOM calibration requires additional measurements from the front of the screen.
To meet these standards, some monitors require a physical device that takes the measurements externally. How often this must be done depends on the manufacturer; one vendor suggests twice per year. LCD monitors tend to drift less than CRTs.
Crocker notes that the calibration devices themselves also should be checked. “They are measurement devices. All measurement devices drift,” he says, suggesting that facilities consult the manufacturer on the frequency for evaluation. “Some are done annually, others every two years.”
Calibration also can be handled electronically. Barco’s technology uses a corner sensor to take measurements from the front of the screen. The displays rely on a series of quality-assurance checks to determine if calibration has drifted, allowing automated calibration.
Remote calibration uses similar technology to reduce the cost of checking each monitor physically. The challenge with these systems, according to NEC’s Fender, is that they measure only an area of the screen, intended to be representative of the whole, but it isn’t always the case. He says, “A visual check is the most accurate.”
Visual checks also can indicate if the white point of a display has drifted, particularly in dual-monitor configurations where displays rest side by side. “White can be many different colors, and CCFL technology has a tendency to yellow with age,” Fender says. NEC sets all of its displays at the same white point and uses technology to monitor this point remotely. He adds, “This assures dual-configurations are at the same white point.”
Configuration Specs
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| Planar’s Dome E4c is a 4-MP color monitor with a 30-inch screen and a typical brightness of 330 cd/m2. |
Because monitors are so often set up in configurations alongside other monitors, matching white points are key. Three monitors seem to be the magic number. “Often, it’s two grayscale in portrait mode with a third color monitor to pull up images and reference information,” says Sanchez, who has also seen two color monitors with a third grayscale.
The modality affects the configuration and often is not altered throughout the day. Physicians are not frequently switching between portrait and landscape mode, though many models offer this feature.
As more large-format displays hit the market, this configuration could change. For example, new 30-inch monitors from Planar Systems (Beaverton, Ore) permit side-by-side comparison of images without the dividing bezel. More typical LCDs fall between 17 and 21 inches.
In terms of resolution, this 30-inch monitor from Planar features a resolution of 2,560 x 1,600, or 4 MP. This is high enough for all modalities with the exception of mammography, which requires 5-MP displays. CT and MR can be diagnosed on 2-MP displays, but the standard is typically 3 MP. “Three-megapixel displays are the most common because they’re mid-range,” Domogalla says. “And unless the monitor is dedicated to mammography, it’s good for all modalities.”
The Graphics Gods
RESOLVING RESOLUTIONResolution and megapixels are two ways of saying the same thing. Resolution reflects the number of pixels on-screen; more pixels or a higher resolution results in more detailed images. Megapixels are calculated from this number. For instance: 2,048 x 1,536 = 3,145,728, which is approximately equal to 3 million pixels or 3 megapixels (MP). |
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None of these factors mean anything without the graphics card or display controller, which enables the monitor to work. Each workstation requires its own card. Sometimes, the graphics card is packaged with the display; other times, it’s optional. Some monitors work only with proprietary technology; others will operate with a number of cards.
The controller affects performance. “How much data is processed and displayed to the screen? What is the performance, compatibility, number of shades of gray? How is it calibrated?” Domogalla asks.
Adds Crocker, “There has been a rapid development in controllers, primarily because of computer gaming.” Users want to be sure to get the right card.
And Sanchez advises, “On paper, most monitors look alike, generally because there are medical standards to meet. What differentiates each monitor? Do your homework and compare. Look at the monitors. Try to test them in the workplace.” In other words, monitor their performance before buying.
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Renee DiIulio is a contributing writer for Medical Imaging.
