In the race for the digital dollar, computed radiography (CR) is a stubborn contender that refuses to give quarter to its shiny new competitor, digital radiography (DR).

Rather than using film, CR uses conventional x-ray equipment to capture an image, then stores the image on a phosphor-based imaging plate. The plate, which is housed in a cassette or cassetteless system, contains photostimulable phosphors that store the image. When scanned with a laser beam, the stored image is released as visible light, then is captured and converted into a digital stream to create a digital image. The image is then subjected to postprocessing to increase contrast and other features for better quality. After processing, the images are available for viewing and distribution via a PACS.

Because it combines with existing x-ray detection systems, CR is seen by many as a cost-effective bridge from traditional analog to digital imaging. Typically less expensive than DR, CR systems also are small, flexible, and portable.

With CR’s advantages and a marketplace that is still waiting for DR prices to come down, manufacturers continue to introduce new CR innovations. And although most experts concede that DR will eventually win the race, the majority are hedging their bets with CR.

SMALLER AND MORE PORTABLE

Nearly everyone agrees that size and portability are key features of CR and a key area for new innovations.

“CRs are getting smaller and less expensive,” says Jonathan Tucker, PhD, a medical physicist for the Brooke Army Medical Center in San Antonio. “These new models are perfect for supporting a single room or maybe two rooms. They are incredibly valuable for emergency and bedside use. Our Army field units all have portable CR systems and they love them.”

Tucker says some manufacturers have achieved smaller size by going to a single-plate system as opposed to stacking up several plates. They also are designing the mechanics of reading and erasing plates to make systems smaller. One manufacturer, for example, recently came out with a system that uses a flexible plate. The plate is pulled in and wrapped in the shape of a cylinder with phosphor. The laser is on a spindle directly down the cylinder, reading the inside of the plate.

“That design made for a very small CR,” he says. “I can pick it up myself, pull it out of the box, put it on the table, start it up, and use it, all in 2 minutes. And it has almost the same quality and functionality as the larger CR systems. The throughput may be slightly slower, but even the single-plate readers read more than enough plates per hour to pay their way.”

Tucker believes that DR systems are still too big to be practical for widespread use. “It’s not practical to move DR detectors around,” he says. “Sometimes you can’t get them into position for certain types of exams. You still need a CR system in case there are some big views you can’t get with a DR system.”

He also mentions new wireless technology that is being incorporated into CR systems to make them even more portable.

DUAL-SIDE READING

When the storage phosphor of a CR plate is scanned with a laser, a typical CR reader captures the portion of light that comes out of the front. New CR dual-side reading technology has optics on both the front and back so the light coming out of the back side is captured as well. The system creates two separate images, one from the front side and one from the back side, then combines them to create a final image for the viewer.

Kenneth Fetterly, PhD, a medical physicist at the Mayo Clinic in Rochester, Minn, has conducted research to evaluate the benefits of dual-side reading on chest and mammography CR units, and says the dual-side technology shows clear evidence of being able to improve CR images.

In a recent paper, Fetterly and his associate, Beth A. Schueler, PhD, concluded that, compared to a standard CR system, a dual-side read produced results in which “the CR system noise was reduced for the dual-side read system compared to the standard system.” 1

“The technology effectively increases the quantum efficiency,” Fetterly says. “You’re detecting a larger fraction of light photons, which improves the statistical properties for a better signal-to-noise ratio.”

CR chest units with dual-side read are already out on the United States market, and units for other types of imaging are still under development. At least one manufacturer offers a dual-read CR mammography unit for use outside the United States. (CR has not yet received FDA approval for mammography.) Studies are currently under way to see whether dual-side reading’s higher detective quantum efficiency (DQE) can be used to reduce dose in pediatric imaging while preserving and even improving image quality.

DOSE CONTROL AND REDUCTION

While dual-side CR may offer possibilities for actual dose reduction, most of CR’s dose reduction benefits relate to CR’s ability to cut down on the number of retakes. The wide exposure latitude of the plates often allows a good-quality image with only one exposure, resulting in a lower overall dose to the patient.

Tucker says CR actually can encourage too much dose, and many of the more recent CR systems have incorporated safety mechanisms.

“CR, like any digital imaging, loves more dose,” he says. “The more dose, the better the image. With film, if you give too much dose, the film comes out black, but with CR you can expose 10 times or 100 times higher than with film screen, and if the exposure is short enough that you don’t get problems with patient motion, you get a better image. That’s fine for the image, but obviously bad for the patient.”

Tucker says that at least one manufacturer has incorporated a CR safety mechanism that requires the technician to keep the exposure within a safe range or the system will produce a bad image. “When we first saw that,” he says, “we worried that we’d have to be very careful how we made our exposures, but in fact it has turned out to be no more demanding than using a film-screen system. We’re using dozens of these systems in our Army field hospitals, and it’s been no problem whatsoever, not even in field conditions with casualties.”

FASTER, BETTER POSTPROCESSING

According to Katherine Andriole, PhD, a physicist at Brigham and Womens Hospital in Boston, some of the most useful CR innovations are being made in image postprocessing systems. Nearly every major manufacturer has come out with new postprocessing systems that improve final image quality.

Postprocessing is a series of steps in which the images are corrected for detector nonuniformities and defects, as well as edge enhancement, noise reduction, dynamic range compression, and other techniques to improve the visibility of specific diseases and organ systems. 2

CR and DR: Which Is Which?

As CR and DR continue to evolve, differentiating the two technologies may become an issue.

The differences used to be clear: CR was a cassette-based system that used storage phosphors. DR used a flat-panel, TFT-based (thin-film transistor) detector. But now, with several new storage media in development such as selenium-based DR that uses scanning heads instead of direct-couple TFTs, and cassetteless CR with selenium, keeping straight which technology is which will be an increasing challenge.

A quick survey of physicists suggests that while terminology differences make for interesting discussion, it is largely a moot question because they see the differences between CR and DR dissolving as the two technologies evolve toward each other.

“Right now, there’s still enough difference between the two,” says Jonathan Tucker, PhD, a medical physicist for the Brooke Army Medical Center in San Antonio. “But who knows what they’ll come out with next?”

—T. Greenleaf

“At this point in time, I think CR image processing is more advanced than DR image processing,” Andriole says. “Everyone’s trying to improve it.”

Tucker says the new postprocessing system he uses makes a big difference.

“It allows you to adjust contrast through the entire image,” he says. “In a chest study, for example, where there is a lot of material for the x-rays to pass through, the new postprocessing systems look at those areas that weren’t well penetrated and apply different processing there so when you get the final digital image captured by CR, it requires very little manipulation by the radiologist or reviewing physician in order to read it.”

Elizabeth Krupinski, PhD, an experimental psychologist in the Department of Radiology, University of Arizona, Tucson, studies human factors and workflow.

“Chest images are very different from bone in terms of inherent dynamic range,” she says. “The new postprocessing systems make a big difference because of their ability to tailor the processing automatically to the type of image.”

Krupinski conducted a research study of one manufacturer’s CR postprocessing system in a clinical setting and found that it required less manipulation (such as window leveling) and saved 13 seconds per case.

“The total viewing time was 52 seconds versus 65 seconds. That’s obviously a significant amount when you multiply it over the course of a day,” she says.

NEEDLE PHOSPHORS

One innovation on the immediate horizon is the use of needle phosphors instead of granular phosphor in the CR plate.

In conventional CR screens, the active layer consists of microscopic phosphor crystals held together by a binder. This granular phosphor arrangement is not optimal because when light hits the granular phosphor, it tends to scatter and blur.

Needle-phosphor technologies use phosphor in needle-like structures. The needles become guides for the light, allowing it to be channeled with little lateral spread. Image sharpness becomes much higher, and the needle structure allows for thicker screens and better absorption efficiency without decreasing spatial resolution.2

Andriole says that at least one manufacturer is experimenting with needle phosphors and what they have seen so far is improved spatial resolution and better signal-to-noise ratio. Most believe this technology will be available commercially in the near future.

COST STILL CRITICAL

In any discussion comparing film, CR, and DR, the issue of cost is always paramount. In most cases CR systems are less expensive than DR because they allow institutions to leverage their investment in current analog x-ray equipment as a detection mechanism.

Andriole says, “People are still asking, should I wait for DR costs to come down or just go with CR? Will there be life for CR after DR matures? In my opinion, CR will continue to have a place because they play different roles. CR is better for portability, for example.”

She recently did a cost study that compared film, CR, and DR for upright chest imaging. “The study clearly found that, bottom line, unless you have a high volume of patients where you can be pushing studies through all day long, CR is better in terms of cost. In very high-flow situations, DR’s greater cost is offset by volume,” she says.

Andriole does not see DR winning the cost race any time soon. “There are some issues with DR’s cost coming down still because it’s very difficult to make the detectors and there are very few manufacturers that vendors can go to make them,” she says. “I don’t see it coming down very significantly in the next couple of years, so CR will still be the better option for most situations from a cost point of view.”

NEXT LEG OF THE RACE

Ask most experts what is next for CR and the question of mammography immediately comes up. While film is still the gold standard and the FDA has not yet approved CR for mammography, most believe it is just a question of time and the first approvals will be coming in the near future.

“Here in our hospital, we’re totally filmless except for mammography,” Tucker says. “Film has some wonderful features, but except for capturing spatial resolution for very small objects, digital imaging is clearly the way to go.” He says that CR will offer a big cost savings to those who want to get to digital mammography, because everyone is heavily invested in high-quality film-screen systems for mammography, and CR offers a way to leverage that investment.

“I think we’ll see CR for mammography this year,” he predicts.

Fetterly sees opportunity for CR in other areas where film still excels over digital, such as thick patient anatomy. “Large patients with thick anatomy have always posed challenges for imaging,” he says. “Thick anatomy seems to be even more difficult for CR, and this is an area where CR can continue to improve.”

Although most industry experts agree that the future is filled with DR, few are willing to concede CR’s value now or in the immediate future. Given the amount of innovation still happening in CR, it would seem that manufacturers agree. Most are still working to improve CR in ways that take advantage of its particular strengths.

Many of those innovations are beginning to blur the line between CR and DR, and rather than it being a relay race in which the CR baton is handed off to DR, in the coming years we may find ourselves with a competitor that combines the best of both modalities.

Tamara Greenleaf is a contributing writer for Decisions in Axis Imaging News.

References:

  1. Fetterly KA, Schueler BA. Performance valuation of a “dual-side read” dedicated mammography computed radiography system. Med Phys. 2003;30:1843.
  2. Maidment A, Seibert JA, Flynn MJ. Technical advances in DR and CR: Part 1. Decisions in Axis Imaging News. 2004;17(3):45-51.