Digital Radiography

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Role of DR hinges on efficiency and technology

By: Jane Lowers

Pure digital radiography can make imaging suites more efficient but is not yet available for portable or bedside applications.

As radiography becomes increasingly digital, it is stretching the capacity of PACS-based imaging departments. Larry Filipow, Ph.D., a radiation physicist and an assistant professor of radiology at the University of Alberta, also works as a full-time consultant on PACS implementation, radiology equipment installation, department layout and design, and operations review. He spoke with Diagnostic Imaging about the current and future roles of digital radiography.

DI: What advantages does direct digital radiography offer over conventional x-ray?

Filipow: Direct radiography is still the new kid on the block, and there are maybe 200 sites installed worldwide. The main advantages are better dynamic range and contrast detectability-the ability to see more gray levels. It has better throughput simply because it eliminates handling cassettes. If it's running properly, one DR room could process as many patients as three conventional film rooms. In that case, you would use one tech to perform the imaging and one to position the patients, whereas a conventional film room typically has one tech alone. It depends on your staffing levels, but it could be more efficient in human resource allocation if you've got two techs doing what was previously a patient load for three.

DI: What factors are departments considering in deciding whether to convert?

Filipow: You can't install new technology unless it's better, faster, and cheaper. You need to achieve at least two of those. Computed radiography systems that were installed in the early '90s were faster but not cheap. They didn't become cost-effective until PACS became more common and computing power and the price of memory dropped. If you're converting to DR, you still need to achieve two of the three. It's faster, yes, and the image quality is superior. It's not cheaper, but it might be fast enough to overcome the cost.

The hospitals I've seen going to DR are large teaching hospitals with large film loads and film handling problems. They also already have PACS. PACS is almost a prerequisite. It makes sense to do a lot of ultrasound and CT with a miniPACS, and many departments expand from there. Those modalities can be converted relatively inexpensively because their resolution, at 512 x 512 pixels and 256 gray scale, can be read on just about any monitor. The challenge is chest images. To read a chest image, you're dealing with 4000 x 3000 pixels and 4000 gray levels. Not many monitors can handle that. DR is a move to make once you've made film management on PACS a routine part of your department. A DR room can cost as much as a PACS.

DI: What are the limitations of DR?

Filipow: In many cases, the conversion takes place when there is new capital funding or a new building. One of the main clinical disadvantages of DR is that the equipment typically is fixed to a table or wall, so you can't do lateral, out-of-bucky exposures. Several vendors are starting to offer U-arms and multiposition detectors, so that problem may disappear. For now, a DR room will need to have CR as well, unless it is a dedicated chest room.

DR isn't portable, and there are some limits to what the detectors can do. You've also got 16 million transistors, so even when new, they won't all work, and you're not getting 100% of the true image. Some pixels will "drop out," like on a laptop screen, or an entire row or column won't respond. The systems are configured to compensate and are designed to work around this problem.

DI: What adjustments to workflow does DR require compared with CR or plain film?

Filipow: A lot of people go digital with PACS on all modalities before switching over general radiography. If you're going to look at chest images, many experts will say you need new monitors, though the jury is still out on this. A 1600 x 1200 monitor will take care of most of your needs, but a chest image is acquired at 4000 x 3000, so you need one that size to see the whole thing. At the moment, those can cost more than $20,000 each. Many radiologists want four monitors to look at chest images so they can see the PA and lateral views of both current and previous exams. That just adds to the expense. If you have one monitor, you can look at images one at a time and flip back and forth. With two, you can switch from current to previous, and that seems to be the industry standard.

The majority of PACS monitors are 1.5K or 2K, so either you aren't getting the full resolution in a shrunken image, or you can see only a portion at a time. Most radiologists want the full resolution, but it's just not cost-effective to buy the big monitors. So their behavior is going to have to change the most-away from the habit of full simultaneous viewing. Still, they get the images faster in an all-digital department, and the time savings might be 30% or more just by getting rid of film handling.

DI: How does the cost-efficiency work out?

Filipow: A department will save time on consumables, tech staff, and film handling, but it also will have to upgrade the infrastructure of

equipment and monitors, which can be necessary every time the warranty expires. Things change very rapidly. In an old radiography room, you had simple tables and light boxes and the electronics were very simple but built to last. Now the equipment is sophisticated but also complicated and designed for rapid evolution, and a department that has just spent millions will have to budget for upgrades. One colleague has compared PACS to jumping on a galloping horse: You just have to hang on and hope you eventually get control.

DI: How do CR and DR compare?

Filipow: All the CR vendors are planning new technology that will rival or supersede DR. Whereas DR plates' pixels are limited by the size of the detector element to the 200- or 140-micron level, a CR plate picks up images essentially on a molecular level, like film. The resolution of the image is governed by the resolution of the readout laser and photo pickup. Potentially, then, CR can offer higher resolution, without the loss of pixels found in DR. Still, CR cassettes have to be handled and fed into a reader, although even that might evolve in favor of a reader built into a bucky in a few years.

DR does appear to provide better dynamic range-more contrast range-than CR, and radiologists by and large are very pleased with DR images. New DR technology may also provide improved intrinsic spatial resolution, approaching that of conventional film performance, on the order of five line pairs per mm.

DI: What about dose?

Filipow: When CR became popular back in the '80s, techs discovered that you can overexpose the imaging plate-and hence the patient-and also underexpose it to a certain extent, and still get diagnostic-quality images, because you can make adjustments during digital processing. CR became popular for ICU and portable units because it required only one exposure to get something readable. However, the images weren't stored electronically, but printed out as film.

CR has about twice the dose of screen-film, and you have to take that into consideration compared with the range and manipulating ability you get. The added radiation isn't a big issue if, for a small potential risk, you get a considerable diagnostic benefit. DR seems to be faster, and the exposure levels may be more similar to plain film. If dose is a concern, DR may be a preference, but that has to be put into perspective. Diagnostic imaging doses are low, generally. CR rates may be the highest of the three, but they are not so high as to be a cause for concern.


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