Ralph Schaetzing, PhD

Flat-panel liquid crytsal displays (LCDs) are getting better and more affordable all the time. Certainly in computer systems, but also in medical image reproduction, theyt are displacing the venerable cathode-ray tube (CRT), which has been the mainstay of soft-copy display for decades. Will LCDs overtake CRTs in digital mammography as well?

Traditionally, mammograms have been interpreted from film, using a viewbox. This viewing combination meets (and defines) the stringent application requirements of mammography, such as high spatial resolution, high luminance, and a large contrast ratio. As digital image acquisition systems and picture archiving and communications systems (PACS) became more common in general radiography, CRTs became an interesting alternative to traditional film display. Even though their resolution, luminance, and contrast-ratio capabilities are inferior to those of film (even film produced by digital imagers), CRTs provided other perceived benefits to users, such as reductions in film and chemistry costs. Despite their shortcomings, no evidence of a reduction in diagnostic performance using high quality CRTs was found in general radiography. They still were not widely accepted in mammography, however.

In parallel with developments in full-field digital mammographic acquisition systems over the past few years, CRT manufacturers were able to produce devices that did address the main quality requirements of this application. Monochrome CRTs with 5 megapixel (MP) display matrices, luminance levels of 400 to 600 candelas (cd)/m² (about one-fifth that of a viewbox), and contrast ratios of more than 400:1 became available. These were cleared for mammography use by the US Food and Drug Administration (FDA). Soft-copy reading of mammograms became a reality, albeit at a substantial cost, since these special CRTs are not inexpensive.

Until the past two years or so, LCDs had difficulty displaying diagnostic general radiography images, much less mammographic images; however, that has changed. Today, monochrome LCDs are available with matrix sizes of 5 MP (and even a 9-MP device has come on the market). They have luminance levels exceeding 600 cd/m² and contrast ratios of up to 900:1. This makes LCDs attractive candidates for viewing diagnostic mammograms. Currently, a few LCD manufacturers have received FDA clearance to market 5-MP monochrome displays for digital mammography.

Are they better than CRTs? This depends on the buyer's priorities. The purchase price of LCDs for mammography applications is still substantially higher than that of CRTs, a fact that is currently slowing their dissemination for this application. On the other hand, the expected lifetime of LCDs is longer than that of CRTs, and their power consumption is significantly lower, so considering the total cost of ownership narrows the gap considerably. As the demand for (and acceptance of) flat panel LCDs expands across many different markets, production costs are also decreasing. Still, the price difference between CRTs and LCDs will probably exist for sometime.

Technically, LCDs have come a long way. More powerful and uniform backlit illumination, faster liquid crystal materials, and better electronic and mechanical control of those materials have eliminated many of the previous drawbacks of LCDs. For example, the problem of narrow viewing angle, familiar to users of laptop LCDs, has been addressed using clever techniques for liquid-crystal alignment and better illumination and switching schemes. While their light output still peaks in the forward direction, LCDs have edged closer to the CRT's more uniform emission characteristics. On the other hand, with their near perfect pixel arrays, LCDs do not suffer from the spatial nonuniformities common in CRTs (such as sharpness changes and geometrical variations from the display's center to its corners).

The superior luminance capabilities of LCDs are mainly a function of their backlighting systems. Brighter backlighting produces higher display luminance. This also makes it more difficult to produce a good black, however, because the black level is determined by the amount of light that leaks through the liquid crystal in its nontransmissive state. CRTs have the opposite problem. It is relative easy to produce a good black by cutting off the electron beam, but more of a problem to produce high luminance. CRTs are capable of producing several thousand cd/m², comparable to the output of a film viewbox, but at the expense of a much shorter tube life. While it is relatively easy to change the backlight in a burned out LCD, replacing the light source of a CRT is not possible: the entire tube must be replaced.

Operationally, CRTs and LCDs both provide a viable way to perform soft-copy reading. The form and reduced glare and reflection of flat-panel LCDs allow more flexibility with respect to room and work space design. In addition, their lower power consumption reduces the need for air conditioning, compared with CRTs. There is also some evidence that the somewhat higher working luminance and low flicker of LCDs may lead to less reader fatigue than is seen with CRT use. Quallity control is a critical element in digital radiography, but it becomes especially important when users may view the same image on one of many soft-copy output devices distributed across a facility. LCDs may have an advantage here. For example, some modern LCDs are self-calibrating. They contain sensors that measure certain display characteristics (such as luminance) periodically, and are capable of generating the appropriate internal lookup tables. They can even compensate for ambient light in the reading room, so that the user sees consistent imaging performance across devices and over time. Some of this capability can also be found in newer CRTs, but testing, calibration, and adjustment are not covered quite as broad a scale.

In the end, the choice of CRT or LCD for diagnostic mammography is based on the user's preferences. Both technologies are FDA-cleared for use in this application. Both output technologies are capable of displaying high-quality images, and both operate efficiently in a PACS environment. CRTs have the lead in viewing angle, black level, and display speed. LCDs have an edge in working luminance, resolution, contrast ratio, glare/reflection properties, power consumption, and form factor. Today, cost-sensitive users will probably still favor the CRT, particularly in mammography environments with many soft-copy devices. The cost of high-end LCD ownership is coming down, however, and it is probably safe to say that the flat-panel LCD is the future of digital mammography.

Ralph Schaetzing, PhD, is technical director, digital imaging, HealthCare, marketing, Americas, Agfa Corporation, Greenville, SC.