General-purpose radiographic systems are used to perform routine diagnostic x-ray procedures provided by most hospitals, freestanding clinics, physician offices, and urgent care centers. More than 60% of all radiographs taken for routine examinations of the skull, respiratory organs, and skeletal system are produced by general-purpose table systems.
From traditional film-based radiography, which was virtually unchanged in many ways for almost a century since its inception in 1895, to computed radiography (CR) that debuted in 1987, to present-day digital detectors, the industry has evolved into fully digital image workflows.
Digital radiographic (DR) systems use various methods to acquire electronic x-ray images, which are digitized for viewing, storage, or hard-copy printing. Digital images are available almost immediately for viewing on a monitor and can be manipulated electronically to enhance the region of interest and can be transmitted digitally to other locations. The patient positioning and imaging techniques are identical to those used in conventional radiography.
A radiographic room typically includes a radiographic table, an upright stand, the x-ray system (including the generator, tube, housing, suspension system, and collimator), and at least one digital detector.
The x-ray generator provides the x-ray tube with the power needed to produce an x-ray beam. In current systems, high-frequency transformers are employed to modify the input voltage. X-rays are produced in the x-ray tube when a stream of electrons, accelerated to high velocities by a high-voltage supply from the x-ray generator, collides with the tube’s target anode (positive electrode). The cathode (negative electrode) contains a tungsten wire filament that provides the source of electrons when heated.
Either a stationary or a rotating anode can be used in the x-ray tube. A stationary anode consists of a 2 to 3 mm thick plate of tungsten embedded in a copper block, which aids in heat dissipation. A rotating anode consists of a large tungsten or tungsten alloy disk connected to the anode assembly by a molybdenum stem. The stream of electrons from the cathode is directed against the beveled edge of the tungsten disk, which rotates at a speed of approximately 3,000 rpm (revolutions per minute) during an exposure. The focal spot, the small area of the target struck by electrons, remains fixed while the disk rotates; thus, the disk continuously presents a cooler surface to receive the bombarding electrons, and the heat is distributed around the disk in a broad ring. The focal-spot area for tubes with rotating anodes can be reduced to one-sixth that for tubes with stationary anodes under similar exposure conditions.
To protect the patient by confining the x-ray beam and decreasing scatter radiation, x-ray beam collimators (also called beam-limiting devices) are attached to the opening in the x-ray tube housing to regulate the size and shape of the x-ray beam. As in all diagnostic radiographic systems, the primary beam should be confined to cover only the region of interest.
Collimators consist of sets of lead plates (or shutters) that move as independent pairs to control beam dimensions. A lightbulb in the collimator produces a light beam that coincides with the x-ray field. In addition to showing the x-ray field configuration, the collimator’s light beam also identifies the center of the field. Both capabilities help ensure accurate localization of the x-ray beam on the patient.
Digital systems offer a number of key advantages over conventional radiographic setups. Chiefly, the dynamic range of digital detectors is much larger than that of x-ray film: x-ray film can record exposure differences of approximately 100:1, while digital imaging receptors can record differences of approximately 10,000:1. This large dynamic range allows a wider range of exposures, decreasing the need for additional exposures and reducing radiation to the patient. Additionally, since the detector converts x-rays to a digital image, there is no need for film handling or supplies.
DR differs from traditional film-based radiography from the point at which the x-rays reach the detector. The x-rays enter the patient’s body and some of them are attenuated—that is, either absorbed or scattered by the tissue they strike. The x-rays that are not attenuated pass through the patient and reach the digital detector. Several digital detector technologies exist, including phosphor plates (commonly called computed radiography [CR]) and DR detectors (also called flat panels).
There are two types of digital detectors: indirect DR and direct DR systems. In the former, x-rays are captured by a scintillator (fluorescent screen) and converted to visible light. The light is transformed by a photodiode array and read out by a charge-coupled device (CCD) or thin-film transistor (TFT) into an electronic signal that is digitized. Indirect DR detectors include a scintillator layer (e.g., of cesium iodide [CsI]) along with a photodiode layer (e.g., an array of amorphous silicon).
Direct DR detectors include an x-ray detector layer and a read-out layer. One of the most common structures in these detectors is a layer of amorphous selenium along with a TFT panel. Amorphous selenium detects x-rays through photoelectric interactions, in which electron-hole pairs are produced on exposure. These electron-hole pairs are attracted to electrodes and form a latent image that is read out from a TFT array, creating a digital signal. This process is called direct DR because no intermediate steps are required to convert x-ray photons to digital signals. Direct DR reduces the scatter that occurs while light is traversing the phosphor detectors in indirect DR, film radiography, and CR.
A DR detector might be fixed in the table, tethered to the table, or portable in a wireless configuration. In the latter, the DR detector is read out right after the exposure and the image data is wirelessly sent to an image processing station to display and further process. The wireless capability means the awkwardness of a tether is eliminated and allows easier positioning; portable detectors can be positioned like cassettes, permitting views that were impossible with conventional radiography with the added benefit of immediate image previews. Purchasing a wireless detector can be the least expensive way to attain DR benefits with film-based or CR equipment; most vendors offer digital upgrades or retrofits to their older, film-based systems. Numerous models fit into a standard cassette holder and therefore can be used with any standard radiographic table. Additionally, the same wireless detectors can be used with mobile x-ray units.
Image processing tools contribute to productivity by permitting technologists the ability to enhance contrast, zoom in/out, or stitch images together (e.g., long bone studies) within minutes after exposure. Radiologists can review images at bedside to determine whether additional studies are necessary. Features such as dual-energy subtraction, CAD, and orthopedic planning programs are common add-ons or standard features. The ability to integrate with digital picture archiving and communications systems (PACS) and radiology information systems (RIS) is a great advantage of DR, as these systems have become essential to the current healthcare environment.
Recent U.S. legislation aimed at incentivizing healthcare providers to upgrade to DR will reduce Medicare reimbursement rates for non-DR technologies. As a result, U.S.-based facilities predominantly using film and CR will need to plan for the transition to DR or choose to absorb the financial loss.
This article is adapted from ECRI Institute’s Healthcare Product Comparison System (HPCS), a searchable database of technology overviews and product specifications for capital medical equipment. The source article is available online to members of ECRI Institute’s HPCS; learn more at www.ecri.org/components/HPCS.
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