ecri institute - radiographic systems, film; digital

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Comparison ChartRadiographic Systems, Film; Digital

Radiographic Systems, Film; Digital

UMDNS information

This Product Comparison covers the following device terms and product codes as listed in ECRI’s Universal Medical DeviceNomenclature System™ (UMDNS™):

Radiographic Systems, Digital [18-430]Radiographic Systems, Film [17-174]Tables, Imaging, Radiographic [16-544]

Scope of this Product Comparison

This Product Comparison covers general-purpose radiographic systems, many of which offer linear tomography attachments. For a discussion of tomographic features and specifications, see the Product Comparison titled Radiographic/Tomographic Systems, Linear. For other related information, see the following Product Comparisons:

Digital Imaging Systems, Computed Radiography Radiographic Units, ChestRadiographic/Fluoroscopic Systems, General-Purpose; Cameras, Radiographic PhotospotRadiographic/Fluoroscopic Systems, Urological

The chart lists radiographic imaging table specifications, defines types of film and digital systems, and lists preferred x-ray tubes and generators, tube suspensions, and collimators. Some models in the chart do not include the entire system but may offer certain components without a table.

Purpose

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.

The most basic film systems produce individual still images that allow for the examination and differentiation of internal organs and tissue structures. They may also offer Bucky, cross-table, horizontal, off-table, and other techniques. For more specialized procedures, some units can be enhanced with optional modular components for fluoroscopy and linear tomography.

Digital radiographic 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. These images 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.

Principles of operation

The components of a general-purpose film or digital radiographic table system are the table unit; the Bucky film tray and grid system; the film or digital system; the x-ray generator; the x-ray tube, housing, and suspension system; and the collimator (see Figure 1).

Table unit

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The table unit consists of a rectangular steel or metal alloy pedestal base or an open frame that supports a rectangulartabletop. Tabletops are fabricated of carbon fiber, Formica, or other plastic or wood laminate. Regardless of the tabletopmaterial, units must have an x-ray absorption coefficient of <2 mm aluminum equivalent depending on mechanicalconstruction. Most tables listed in the chart have aluminum equivalents of ≤1 mm.

Tabletops vary in length from approximately 188 to 240 cm (74 to 94 in) and in width from approximately 61 to 86 cm (24 to 34 in). Fixed tables vary in height from approximately 69 to 89 cm (27 to 35 in). Narrower, lower table units permit x-ray department personnel to more easily manipulate a recumbent patient; however, they are less comfortable for the patient. Most tabletops are moved electromechanically (float-top) in two or four directions; some can tilt and move in as many as six or eight directions. Tableside and footplate controls raise and lower the tabletop to facilitate lifting patients onto its surface. Table systems with elevating tabletops have a range of motion between approximately 48 and 92 cm (19 and 36 in). Additional controls position the tabletop and patient relative to the x-ray tube unit. Many tables can support patients weighing up to 225 kg (approximately 500 lb), and some tables can support patients weighing up to 300 kg (approximately 660 lb). Tabletop movement may be power assisted by motors and gears housed within the table base. Power-assisted (motorized) movements are relatively quiet, reduce the risk of patient or staff injury, and when used with an overhead tube suspension system, facilitate off-table and chest Bucky radiography. Tables with tilt capabilities can be used for myelography and intravenous pyelography without moving the patient; those equipped with optional fluoroscopic or linear tomographic accessories can be set up quickly and safely for these procedures without moving the patient.

Tables can be equipped with handgrips, head clamps, compression bands, and footrests, depending on the desired x-ray technique.

Bucky film tray and grid system

A hollow, longitudinal bin is located directly under the tabletop unit and holds the film tray. To reduce scatter radiation and thereby improve image quality, moving or stationary radiographic grids are used with the film tray assembly. The grids consist of a series of lead foil strips interspaced with aluminum or plastic. Linear grids have parallel longitudinal strips and allow the x-ray tube to be angled along the length of the grid without loss of primary radiation. Crossed grids consist of two superimposed linear grids that have the same focusing distance; these grids are used in procedures with a large amount of scatter radiation, such as biplane cerebral angiography. Crossed grids cannot be used for techniques requiring an angled x-ray tube. Focused grids have angled lead strips and can be either linear or crossed. The focusing range indicates the distance from the x-ray tube within which the grid can be used without causing lines to be formed across the image. Parallel grids have lead strips that are parallel when viewed in cross-section. These grids are used only with very small x-ray fields, with long tube-to-grid distances, or in fluoroscopic spot-film devices.

Grid ratio, the ratio of the lead strips’ height to the distance between them, indicates the grid’s ability to remove scatterradiation. For the models listed, grid ratios vary from 6:1 to 14:1. The higher the ratio, the more radiation the grid absorbs;therefore, grids with higher ratios provide maximum image quality. Grids with an 8:1 ratio are adequate for <90 peakkilovoltage (kVp), and 12:1 grids are preferred for >90 kVp.

A moving grid (also called a Bucky or Potter-Bucky reciprocating grid) is used to eliminate the shadows cast by the lead strips.While the x-ray tube anode rotates, the grid continuously moves back and forth 1 to 3 cm during the exposure; the speed ofthe grid’s movement is automatically adjusted to avoid imaging the strips. Bucky systems are fully automatic, and mostaccommodate cassettes up to 35 × 43 cm (14 × 17 in) in size; some can use cassettes as large as 43 × 43 cm (17 × 17 in).Some systems also use automatic cassette-size sensing to speed examination time. At least one model in the chart offers aflexible Bucky that converts to a universal Bucky unit to optimize lateral exposures.






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Most Bucky systems also include automatic exposure control (AEC) devices, sometimes called phototimers, which automatically terminate radiographic exposures when sufficient x-ray intensity has reached the x-ray film cassette to provide acceptable film darkening. This feature ensures the production of satisfactory images, reducing the chance that human error will necessitate repeat examinations. AEC devices include one or more detector elements placed in front of or behind the film cassette; the two most common types of detectors are photomultiplier tubes and ionization chambers. AEC devices are calibrated so that a predetermined film density is reached regardless of patient size.

Film radiography

As the x-rays enter the patient’s body, some of them are attenuated—that is, either absorbed or scattered by the tissue theystrike. The degree of attenuation depends on tissue properties such as thickness and density. The x-rays that are notattenuated pass through the patient and reach the film cassette. An intensifying screen is generally placed on either side ofthe film to convert the x-rays to light, which exposes the film.

The x-ray film comprises a polyester or plastic base that provides support, a protective gelatin supercoating, and a silver-halide-based emulsion that forms the actual image. Chemical processing of the exposed film produces the image.

Digital radiography

Digital radiography (DR) is different from traditional film-based radiography only from the point at which the x-rays reach thedetector—the production of x-rays in DR identical to the method used in traditional radiography. Once the x-rays haveinteracted with the patient, they are captured by a charge-coupled device (CCD) or flat-panel detector (an array of thin-filmdiodes [TFDs], or thin-film transistors [TFTs]) instead of radiographic film. CCDs and flat-panel detectors are currently themost commonly used detectors (with the exception of computed radiography [CR] plates, which can replace film in anyradiographic imaging system; see the Product Comparison titled Digital Imaging Systems, Computed Radiography). Flat-panel detectors are composed of amorphous silicon or amorphous selenium. Amorphous silicon panels use cesium iodide (CsI) and an array of photodiodes to produce an image readout. Amorphous selenium panels detect x-rays by means of the photoelectric effect, in which electron-hole pairs are produced on exposure. These pairs are attracted to electrodes and form a latent image that is read out from a TFT array, creating an electronic signal that is digitized. This process is sometimes referred to as direct DR since no intermediate steps are required. CCDs and TFD flat-panel detectors utilize a process known as indirect detection. In this process, x-rays are captured by a fluorescent screen similar to the screens used in film radiography and are converted to visible light. The light is transformed by a photodiode array or optically coupled by CCD into an electronic signal that is digitized. Direct DR reduces the scatter that occurs while light is traversing the phosphor detectors in indirect DR, film radiography, and CR; the clinical implications of better images; however are not clear. CR technology is now also available in a DR configuration from at least one manufacturer. This allows better workflow and improved image quality compared to normal CR. The collection of these digital signals allows the radiographic image to be displayed on a computer monitor and stored and processed by computers.

From the user standpoint, the most important difference between these systems is the detector’s configuration. Some systemshave detectors built directly into the table or upright detectors, which are quick to align for most standard x-ray views.However, more elaborate views require a cassette-based detector, which manufacturers can mount on a C-arm, allowing morefreedom but requiring longer assembly time.






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X-ray system

The x-ray generator modifies incoming voltage and current (measured in milliamperes [mA]) to provide the x-ray tube with the power needed to produce an x-ray beam. Manufacturers offer single- or three-phase generators; several offer high-frequency generators.

X-rays are produced in the x-ray tube when a stream of electrons, accelerated to high velocities by a high-voltage supply fromthe x-ray generator, collides with the tube’s target anode (positive electrode). The cathode (negative electrode) contains atungsten wire filament that provides the source of electrons when heated.

Either a stationary or 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.

Rotating anodes can be operated continually at temperatures above 1,200°C because most of the heat from the target istransferred by conduction to the oil surrounding the tube and its housing. The molybdenum stem, which has a high melting point and is a poor heat conductor, serves as a partial heat barrier between the tungsten target and the anode bearings.

Overheating the x-ray tube promotes cracking and pitting and reduces tube life; therefore, rotating anodes are used more often than stationary anodes because they dissipate the heat produced during exposure over a large area of the anode, allowing the x-ray tube to withstand the heat associated with high x-ray output and large exposures.

To assist users with comparative information about tubes, manufacturers furnish charts defining a tube’s safe operating limitsand maximum power and the maximum safe duration of use for a single exposure. Tube cooling charts also show how rapidlyexposures may be repeated. X-ray tubes of either anode design should always be operated within their rated capacities.Damaged tubes are expensive to replace, and as they age, more filament current is needed to achieve consistent x-rayintensity.

The x-ray tube housing provides physical and mechanical protection for the x-ray tube. The housing should also provide high-voltage protection, offer means of heat dissipation, and contain radiation-shielding material. The metal housing, which is electrically grounded and well insulated, allows the tube to withstand up to 150 kV. A microswitch controls heat dissipation by activating a shutoff control circuit that turns off the high-voltage power (usually fed to the tube through high-voltage cables). Lead casing placed throughout the housing provides the required radiation shielding. Housing may also include fans to aid cooling.

Collimators

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 two 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 fieldconfiguration, the collimator’s light beam also identifies the center of the field. Both capabilities help ensure accuratelocalization of the x-ray beam on the patient. Collimators also have a backup system for showing the x-ray field size—acalibrated scale on the front of the collimator—if the light bulb burns out.

Reported problems

Problems reported with general-purpose radiographic systems involve their individual components. Mechanical problems, not electrical problems, are the major cause of breakdowns. For example, problems may occur in the mechanical servomechanisms used for positioning tabletops. Wires and cables may break, causing the tabletop to jam and collision






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sensors to fail. Over time, rotating anodes become pitted from misuse. Operator error can also result in serious patient or operator injury. Other reported problems involve x-ray tubes, collimators, generators, and light localizers that fail to comply with standards; electronic circuit failures in collimators and generators; and mechanical jamming of film trays.

ECRI has received reports of radiographic collimators coming loose from their x-ray tubes and falling; approximately 30 incidents have been reported, involving many brands and models. All users should be alert to the many signs of a failing collimator coupling that may be found during normal use.

Purchase considerations

ECRI recommendations

Included in the chart are ECRI’s recommendations for digital x-ray systems; recommended specifications have beencategorized into two groups based on the type of table—tilting and nontilting. The tilting table category specifies a system thatallows patients and x-ray tubes to be easily moved into position, regardless of exam type and patient responsiveness. Thesetables allow a wide range of tilting movements. Table tilt should range from -15° to +90° and have an average speed of6°/sec. Horizontal-to-vertical tilt should average 15 sec. The nontilting table category specifies a wide range of tabletopmotions but does not allow tilting angles.

DR systems generally perform upright examinations or table-based examinations, for which detector mounting is crucial. In table-based systems, the detector is fixed in the table system, precluding certain examinations due to patient positioning constraints. Some upright systems can be tilted to allow table based exams. Purchasers should assess all types of examinations being performed before deciding which type of system will best benefit their facility.

All Bucky systems for both tilting and nontilting tables should be motorized. A three-field AEC device is recommended to ensure acceptable film darkening. Grid ratios should be 10:1 or higher, because grids with higher ratios will provide higher image quality.

Another major consideration in acquiring a DR system is the system’s integration into picture archiving and communicationsystems (PACS) already in use in the facility. To integrate with the PACS, the DR system should provide at least the DigitalImaging and Communications in Medicine (DICOM) image object definition (IOD) for CR. The newest IOD for digital systems isthe DX IOD, which allows for increased productivity, as it enables greater functionality in DX-IOD-conformant PACS and DRsystems.

Other considerations

The number and types of procedures to be performed will influence the features selected for the system. Smaller focal-spotsizes can provide better spatial resolution on film for certain studies, and options such as tomography and table tilt canincrease the system’s overall procedural capabilities. Elevating tables may be better suited for departments handling traumaand emergency cases because the table height can be adjusted to facilitate patient transfer from a mobile stretcher orwheelchair. Generator options should also be considered; high-frequency generators require less space and often eliminate theneed for high-voltage cables.

DR reportedly offers many potential advantages over film-based radiography, including storage space reduction, enhanced image processing, and off-site diagnostic capabilities. Because of these reported advantages, facilities may wish to consider purchasing DR systems. At minimum, facilities should determine whether film-based systems under consideration can be converted (retrograde fit) into digital systems. If a digital system is being considered, compatibility with the DICOM 3.0 standard should be a requirement for all newly purchased equipment (including storage devices) to facilitate future additions to any network. Purchasers should require suppliers to provide DICOM conformance statements that explain in detail what information objects, service classes, and data encoding are supported by the system. The conformance statements should be requested in the same format and using the same vocabulary, thereby facilitating comparisons between suppliers, and should be inspected by a competent specialist.

Cost containment

General-purpose radiographic systems are available from many suppliers in more than one configuration; each can vary greatly in price from the basic configuration described in the chart. For a particular application, ECRI recommends that buyers consult specific suppliers about available systems and their prices.

A purchase decision should be based on issues such as life-cycle cost (LCC), local service support, discount rates and non-price-related benefits offered by the supplier, and standardization with existing equipment in the department or hospital (i.e., purchasing all systems from one supplier). The initial acquisition cost is only a fraction of the total cost of operation. Therefore, rather than making a purchase decision based solely on the acquisition cost of an x-ray generator, buyers should consider operating costs over the lifetime of the equipment. The following costs should be considered when purchasing a radiographic table system:

Special features (e.g., linear tomography, tilting)Service contractX-ray tube replacementUtilitiesAccessories (e.g., headrests, footrests, and cassette holders) that may be necessary for certain proceduresContributions to overhead

For further information on LCC, customized analyses, and purchase decision support, readers should contact ECRI’s SELECT™Group.

Hospitals can purchase service contracts or service on a time-and-materials basis from the supplier. Service may also be available from a third-party organization. The decision to purchase a service contract should be carefully considered. Purchasing a service contract ensures that preventive maintenance will be performed at regular intervals, thereby eliminating the possibility of unexpected maintenance costs. Also, many suppliers do not extend system performance and uptime






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guarantees beyond the length of the warranty unless the system is covered by a service contract. ECRI recommends that, to maximize bargaining leverage, hospitals negotiate pricing for service contracts before the system is purchased. Additional service contract discounts may be negotiable for multiple-year agreements or for service contracts that are bundled with contracts on other generators in the department or hospital. Purchasing multiple table systems from one supplier could result in a significant discount. Standardization of equipment can make staff training easier, simplify servicing and parts acquisition, and provide greater bargaining leverage when negotiating new equipment purchases and/or service contract costs.

Although the initial acquisition cost of a DR system is greater than that of a traditional film-based system, the elimination ofx-ray film and film processing can substantially lower costs, including processing costs resulting from hazardous wasteremoval and silver recapture. The cost of film duplication is also eliminated because images can be retrieved electronicallyfrom any interfaced workstation. Additionally, because DR systems store all their images directly on electronic media,personnel who would be dedicated to filing and retrieving old films can be assigned other tasks. However, current acquisitioncosts for digital systems far exceed (by approximately 3 to 3.5 times) those of film-based systems, partly because of limitedproduction. Adding to these costs is the need for high-speed networks and diagnostic-quality computer workstations to archiveand read digital images, as well as the need for PACS and film digitizers. Facilities considering digital systems should beginplanning the steps—including ancillary equipment purchases—that will be needed to implement a DR system.

Stage of development

DR is expected to replace traditional radiography in the future. The timetable for hospitals’ acquisition of DR systems isuncertain. Some manufacturers are developing more portable digital x-ray detectors in order to overcome the major obstacleof a fixed detector. Although DR is expected to supplant film-based radiography, the transition will likely take years.

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Chotas HG, Dobbins JT 3rd, Ravin CE. Principles of digital radiography with large-area, electronically readable detectors: a review of the basics. Radiology 1999 Mar;210(3):595-9.

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Nields M. DR: the final link to the digital department. Diagn Imaging 1998 Aug;20(8):53-4.

Piraino D. Flat-panel detectors usher in digital x-ray. Diagn Imaging 1997 Nov;19(11 Suppl):D13-4.

Strotzer M, Gmeinwieser J, Völk M, et al. Clinical application of a flat-panel x-ray detector based on amorphous silicontechnology: image quality and potential for radiation dose reduction in skeletal radiography. AJR Am J Roentgenol 1998 Jul;171(1):23-7.

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Last updated February 2006

Comparison ChartRadiographic Systems, Film; Digital





ecri institute - radiographic systems, film; digital

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