Display technology advances hold promise for radiology

May 1, 2008

As display technology evolves, radiology departments are presented with a growing variety of PACS displays with increased brightness, sharper contrast, and improved calibration capability. Two developments on their way to the consumer market are likely to offer significant potential benefits for radiologic image visualization.

As display technology evolves, radiology departments are presented with a growing variety of PACS displays with increased brightness, sharper contrast, and improved calibration capability. Two developments on their way to the consumer market are likely to offer significant potential benefits for radiologic image visualization.

The first is the DisplayPort digital display interface standard, already available on some new displays. The second is light-emitting diode (LED) backlighting of liquid crystal displays, which can be found today in a select variety of both laptop computers and desktop displays.

The DisplayPort standard specifies a digital video and audio interconnect that can be used to connect a computer to a display, including newer television sets. It is poised to replace older connectors such as VGA, DVI, and HDMI cables, as well as component video and S-video cables. As is usual with technological advances, DisplayPort is smaller and faster than its digital predecessor DVI, and the new standard offers much more.

First, it carries a digital audio signal in addition to the video, eliminating the need for a separate audio connection. Second, it incorporates an auxiliary channel to carry non-video-related data, such as that from universal serial bus devices and flash media cards. Until now, if you wanted to plug the USB cable of your keyboard or mouse into your monitor, you had to complete the chain to the computer by connecting a USB cable between your monitor and your computer. DisplayPort carries non-video-related data without the need for a separate USB cable to the computer.

As for resolution, DisplayPort can support the same number of pixels as the fastest type of DVI connection (dual-link DVI): 2560 x 1600, which is 4 megapixels.

The biggest potential benefit to radiologists is in the color depth that DisplayPort supports. It is the first nonproprietary digital standard that allows greater than 8 bits per component. Most displays use a video signal with red, green, and blue components. Color displays, which predominate in radiologic imaging today, typically mix these colors in equal proportions to produce 256 shades of gray. The DICOM Part 14 Grayscale Display Function defines exactly what brightness level these shades of gray should optimally achieve, based on the darkest black and brightest white of the display.

In order to "tune" the display closely to these desired values, it is best to have lots of values to choose from. This approach is analogous to having two digital FM radio tuners, one that allows you to move the dial in increments of 0.2 MHz and one with increments of 0.01 MHz. The second one allows you to pick dial settings much closer to your favorite radio stations than the first. If, over time, some drift of the tuner's internal components occurs such that the original dial settings no longer work well, the second tuner can be finely adjusted to compensate for the drift by rotating the dial one or two more increments in either direction, whereas the first dial can not be finely adjusted. Granted that most of the increments of the second tuner will not be used, because most of them are in between radio stations as opposed to the few that are right on the mark, but the increased number of increments gives the leeway needed for fine tuning.

So, too, with digital video signals. Although PACS software generally tries to display only 256 shades of gray at a time, having more than 256 shades to choose from allows for a much finer tuning of those shades. The DVI ceiling of 8 bits per component limits the available shades of gray to 256. DisplayPort supports up to 16 bits per component, yielding up to 65,000 shades of gray! DisplayPort thus has the potential to give a much tighter calibration of a color display to the DICOM curve.

It should be pointed out, however, that the interconnect between the computer and the display is only one of three components in the display chain, which also includes the graphics adapter (video card) and the display itself. All three of these need to support increased (i.e., greater than 8) bits per component in order to deliver on this promise, so DisplayPort is necessary but not sufficient by itself. Nonetheless, it is an important step in the right direction.

LED BACKLIGHTING


Color LCDs are illuminated by a backlight of white light that shines forward from the back of the display toward the user's face and is filtered and attenuated along the way to produce changes in color and brightness. For years, the predominant source of such backlights has been the cold cathode fluorescent lamp (CCFL), a variation on the common fluorescent lamp ubiquitous in office ceilings. While such lamps produce a lot of light from less energy and with less heat production than their incandescent counterparts, they do have some drawbacks.

Since the colors of an LCD are produced by filtering the backlight, an LCD can output only colors that were contained in the backlight to begin with. It turns out that the "white" light of CCFLs does not contain as broad a spectrum of colors as the white light of LEDs. Unlike CCFLs, individual LEDs actually produce light of different colors: red, green, and blue. These colors are then mixed to produce white light, offering much greater control over the color spectrum emitted by LED backlit displays. Less of the light produced goes to waste.

Another limitation of CCFLs is that they operate only together as a unit-the backlight lamps are either all on or all off. This makes it difficult to make the darker areas of the image, such as the air inside the lungs on a chest radiograph, really dark. LEDs, on the other hand, can slice the backlighting into 1-cm squares and selectively illuminate only the areas of the display where light is needed, thereby leaving the darker areas much darker. This produces contrast ratios simply not achievable with CCFL backlights.

The results can be stunning. I saw at RSNA 2006 a demonstration of such a display, which presented a shining star on a dark sky. The contrast between the star and the sky was so huge that it had almost a 3D effect. This capability may have a large impact on mammography in particular, where significant boosts in contrast can make all the difference in detecting cancer. It may also help in the detection of subtle findings on radiographs of conditions such as pneumothorax, pneumoperitoneum, subcutaneous gas collections, and vascular and parenchymal calcifications.

PENETRATING THE MARKET


So far, several laptop manufacturers have used LED backlights in their products, but the technology has yet to penetrate the mainstream display market. Some obstacles remain in keeping the light output of an array of LEDs uniform over time. Theoret¬ically, LEDs should consume less power than CCFLs, but this has not always been the case in actual products. They should also last longer than fluorescent lamps, but the cost of production of these displays still remains significantly higher. Perhaps as production ramps up, this will change.

Yet another benefit of this technology is the fact that it is more environmentally friendly. In February 2003, the

European Union launched its Restriction of Hazardous Substances (RoHS) directive to try to eliminate the use of mercury, lead, and other hazardous wastes from electronic devices. LEDs fare better in such measurements because they are mercury-free, unlike fluorescent lamps.

Both DisplayPort and LED backlighting bode well for radiology. DisplayPort should set the stage for improved monitor calibration and simplified connections. LED backlighting should provide unprecedented degrees of contrast for mammography and general radiography, where it is needed most. It should also ultimately result in displays that have better color capabilities, last longer, consume less power, and are less harmful to the environment. One might say that radiology has a bright future, indeed!

Dr. Hirschorn is a research fellow in radiology informatics at Massachusetts General Hospital/ Harvard Medical School and director of radiology informatics at Staten Island University Hospital. He can be reached at hirschorn.david@mgh.harvard.edu. Mr. Conway is director of radiology informatics at Massachusetts General Hospital.

He can be reached at kaconway@partners.org.