Digital (filmless) radiography systems are a relatively recent introduction in the private practice setting, but computed radiography has been used for over a decade in larger facilities and teaching institutions. With refinement and lower computer costs, the feasibility of digital systems can now be justified. As systems become available, clinicians have been seeking knowledge about them and attempting to determine how digital radiography may apply to their own practices.

Certainly, not all digital systems are created equal; many have several options available, and the clinician must determine which system is most practical for a particular need. To assist in sorting through the information, a series of articles will be presented on the subject of digital radiography, the goal of which is to introduce concepts and terminology of filmless systems. This article will introduce the various systems; subsequent articles will discuss how to evaluate system and image quality, picture archiving and communication systems (PACS), digital imaging in medicine and communication in medicine (DICOM), and finally financial feasibility of these systems.

Two Modalities
There are two recognized modalities in the filmless environment-computed radiography and digital radiography. All systems in the filmless category, whether computed, flat-panel, or charged coupled devices, differ distinctively from digitization of images by use of scanners or digital cameras. In exploring the systems available, there are recognized advantages with good-quality computed radiography and digital radiography systems. Not all systems offer the same number or level of the following advantages.

Digital images. The quality of digital images can be enhanced because the "dynamic range" is greater than that of film radiographs. In other words, the image can be adjusted and optimized anytime after x-ray exposure. Enhanced image quality improves the ability to evaluate and diagnose, which makes imaging a more valuable tool in the medical workup. Better images also transmit better electronically, which enhances information sharing (i.e., telemedicine).
Cost savings. Filmless systems eliminate the cost of x-ray film material. Additional savings occur with elimination of wet processors, chemicals, and maintenance. With proper technique, retakes are reduced as images are only redone for positioning errors and patient motion, which reduces waste and the cost of labor.
Storage and archiving. Gone are the days of misplaced films, staff time is no longer devoted to filing, and valuable space is no longer taken up for film storage.
• Elimination of the darkroom. This not only frees up valuable space within the practice, it also helps create an environmentally friendly workplace by eliminating the chemical smell and resolving the disposal issues associated with film processing.

Computed Radiography
Computed radiography is the first of the digital systems. With computed radiography systems, x-ray images are recorded and stored onto a reusable and highly sensitive imaging plate. The computed radiography imaging plate is protected and contained within a cassette, very similar to standard film. After exposure, the cassette is loaded into a computed radiography reader. The reader uses laser lights to scan the imaging plate to convert the latent image into an electronic file for storage. Upon completion of the scan, the imaging plate is "blanked," or erased, and available for reuse.

Advantages
Following are advantages of computed radiography systems in addition to the ones listed above:
• Cassettes can be used with multiple x-ray systems and do not require existing equipment to be modified.
• Cassettes allow for table-top, horizontal beam, and bucky tray utilization.
• Computed radiography is a proven, well-established system in the veterinary market, so refinement and performance of systems have been optimized for image quality in this market.
• Computed radiography systems range in cost from around $30,000 to more than $200,000, depending on the manufacturer, number of cassettes, and viewing stations. While this cost is greater than that of standard film processors, the advantages and material cost savings can justify the price of low- to mid-range systems in practices with one to two doctors.
Disadvantages
• The life span of computed radiography imaging plates ranges from approximately 10,000 to 15,000 exposures or more. The plates are sensitive to environmental exposures, such as dust, dirt, and oils from handling, and require cleaning to remove these materials. The cost to replace plates ranges from $300 to $2000.
• For veterinary practice, lower-cost computed radiography readers are usually able to accommodate only one cassette per plate at a time. (Higher-capacity "stack readers" do exist, but the price increase is significant.) As a result, processing time can approach and perhaps exceed that of standard film processing.

Digital Radiography
(Direct Capture Devices [Cassette-Less Systems])

Digital radiography comprises two types of systems: flat panel and charged coupled devices. Flat panel systems use a detector panel synchronized with the x-ray equipment. In the veterinary market, the (indirect) panel first converts the x-ray energy to light and then to a digital signal to be interpreted by computer to produce an image file. Charged coupled device systems rely on a fluorescent screen positioned directly under the x-ray table; the screen is linked to a charged coupled device via a lens and/or mirrors. These systems are analogous to a camera-they essentially take a picture of the fluorescent screen. There are various configurations and materials used in designing these systems, from a single charged coupled device with a simple cooling system, to a configuration of several devices and the requirement of a
liquid nitrogen cooling system.

An important point with digital radiography systems is the potential misuse of the term direct digital radiography by vendors. They may use this term to indicate a cassette-less system; however, to be technically correct, direct digital radiography converts x-ray directly to a digital signal rather than relying on an intermediary, such as light. Because of the higher price, no direct digital radiography systems are currently being offered to the veterinary market-this is not to diminish the quality of available systems but is a key point in assessing system claims.

As with computed radiography systems, not all digital radiography systems are the same. When systems are being compared, all components of the system need to be evaluated, from capture through viewing and image storage and distribution.

Advantages
• Flat panels in particular typically hold the largest dynamic range, resulting in greater image quality. (Image quality will be covered in further detail in the next article). With charged coupled device systems, the range can be more narrow.
• Speed, or throughput, is a key benefit with both types of digital radiography systems-images are available in fewer then 10 seconds.

Disadvantages
• Digital radiography systems must be synchronized to the x-ray equipment; as a result, they are not readily interchangeable with other x-ray machines and are typically fixed to one machine.
• Charged coupled device systems are not portable.
• Exposure ranges and technique charts vary-some systems may require an increased x-ray exposure greater than standard film techniques to achieve an optimal image.
• The price of basic charged coupled device systems begins at $50,000. Flat-panel systems range upward from $70,000, depending on system configuration, components, and panel size.

A great deal of information needs to be considered in the area of digital radiography. With the different modalities, the key features to consider are determined by the individual practice needs. Each type of digital radiography system has its place in the clinical setting and has proven to be a valuable addition to practice.