Processor, Sensitometry,

Processor, Sensitometry,
Artifacts Rad Science Review Prof. Stelmark

Rad Science review Prof. Stelmark
FILM PROCESSING The latent image is invisible because only a few silver ions have been changed to metallic silver and deposited at the sensitivity center. Processing the film magnifies this action many times until all the silver ions in an exposed crystal are converted to atomic silver, thus converting the latent image into a visible radiographic image. Rad Science review Prof. Stelmark

Before the introduction of automatic film processing, x-ray films were processed manually. It took approximately 1 hour to prepare a completely dry and ready-to-read radiograph. Rad Science review Prof. Stelmark

Automatic processing revolutionized busy departments. Finished radiographs became available in 6 minutes, and the variability in results caused by the human element was eliminated. Departmental efficiency, work flow, and radiographic quality all improved. Rad Science review Prof. Stelmark

Processing Steps – Automatic Processor
Developing Fixing Washing Drying Rad Science review Prof. Stelmark

Developing The principal action of developing is to change the silver ions of exposed crystals into metallic silver. The developer is the chemical that performs this task. The developer provides electrons to the sensitivity center of the crystal to change the silver ions to metallic silver. The optical density of a processed radiograph results from the development of crystals that contain a latent image. Rad Science review Prof. Stelmark

The developer contains alkali compounds
Rad Science review Prof. Stelmark

With proper development, all exposed crystals that contain a latent image are reduced to metallic silver, and unexposed crystals are unaffected. The development process, however, is not perfect: Some crystals that contain a latent image remain undeveloped (unreduced), but other crystals that are unexposed may be developed. Both of these actions reduce the quality of the radiograph. Rad Science review Prof. Stelmark

Manufacturers of x-ray film and of developing chemicals have very carefully determined the optimal conditions of time, temperature, and concentration for proper development. Optimal conditions of contrast, speed, and fog can be expected if the manufacturer's recommendations for development are followed. Rad Science review Prof. Stelmark

The Fixer contains acidic compounds
Fixing Once development is complete, the film must be treated so that the image will not fade but will remain permanently. This stage of processing is fixing. The image is said to be fixed on the film, and this produces film of archival quality. Fixing agents remove unexposed and undeveloped silver halide crystals from the emulsion. The Fixer contains acidic compounds Rad Science review Prof. Stelmark

Developing is the stage of processing during which the latent image is converted to a visible image. Rad Science review Prof. Stelmark

Hypo retention is the term used to describe the undesirable retention of the fixer in the emulsion. Excess hypo slowly oxidizes and causes the image to discolor to brown over a long time. Fixing agents retained in the emulsion combine with silver to form silver sulfide, which appears yellow-brown. Rad Science review Prof. Stelmark

Washing The next stage in processing is to wash away any residual chemicals remaining in the emulsion. Water is used as the wash agent. In automatic processing, the temperature of the wash water should be maintained at approximately 3° C (5° F) below the developer temperature. Rad Science review Prof. Stelmark

Drying For the final step in processing, drying the radiograph, warm dry air is blown over both surfaces of the film as it is transported through the drying chamber. The total sequence of events involved in manual processing takes longer than 1 hour to be completed. Most automatic processors are 90-second processors and require a total time from start to finish—the dry-to-drop time—of just that, 90 seconds. Rad Science review Prof. Stelmark

The process of converting the latent image to a visible image can be summarized as a three-step process within the emulsion. First, the latent image is formed by exposure of silver halide grains. Next, the exposed grains and only the exposed grains are made visible by development. Finally, fixing removes the unexposed grains from the emulsion and makes the image permanent. Rad Science review Prof. Stelmark

System Subsystem Purpose Transport Transports film through various stages at precise intervals Roller Supports film movement Transport rack Moves and changes direction of film via rollers and guide shoes Drive Provides power to turn rollers at a precise rate Temperature Monitors and adjusts temperature at each stage Rad Science review Prof. Stelmark

Circulation Agitates fluids Developer Continuously mixes, filters Fixer Continuously mixes Wash Single-pass water flows at constant rate Replenishment Meters and replaces Dryer Removes moisture, vents exhaust Rad Science review Prof. Stelmark

Cutaway view of an automatic processor Rad Science review Prof. Stelmark

Transport System The transport system begins at the feed tray, where the film to be processed is inserted into the automatic processor in the darkroom. There, entrance rollers grip the film to begin its trip through the processor. A microswitch is engaged to control the replenishment rate of the processing chemicals. Rad Science review Prof. Stelmark

Always feed the film evenly, using the side rails of the feed tray, and alternate sides from film to film. This ensures even wear of the transport system components. From the entrance rollers, the film is transported by rollers and racks through the wet chemistry tanks and the drying chamber, and is finally deposited in the receiving bin. Rad Science review Prof. Stelmark

Roller Subassembly Three types of rollers are used in the transport system. Transport rollers, with a diameter of 1 inch, convey the film along its path. They are positioned opposite one another in pairs or are offset from one another . A master roller (or solar roller), with a diameter of 3 inches, is used when the film makes a turn in the processor. A number of planetary rollers and metal or plastic guide shoes are usually positioned around the master roller. Rad Science review Prof. Stelmark

When the film is transported in one direction along the rack assembly, only 1-inch (25-mm) rollers are required to guide and propel it. At each bend, however, a curved metal lip with smooth grooves guides the film around the bend. These are called guide shoes. Rad Science review Prof. Stelmark

Such a system consisting of a master roller, planetary rollers, and guide shoes is called a turnaround assembly. The turnaround assembly is located at the bottom of the transport rack assembly. For each chemistry cycle, a transport rack assembly is positioned in the tank. Rad Science review Prof. Stelmark

When the film exits the top of the rack assembly, it is guided to the adjacent rack assembly through a crossover rack. The crossover rack is a smaller rack assembly that is composed of rollers and guide shoes. Rad Science review Prof. Stelmark

Drive Subsystem Power for the transport system is provided by a fractional horsepower drive motor Rad Science review Prof. Stelmark

Temperature Control System The developer, fixer, and wash require precise temperature control. The developer temperature is most critical, and it is usually maintained at 35° C (95° F). Wash water is maintained at 3° C (5° F) lower. Temperature is monitored at each stage by a thermocouple or thermistor and is controlled thermostatically by a controlled heating element in each tank. Rad Science review Prof. Stelmark

Circulation System Agitation is necessary to continually mix the processing chemicals, to maintain a constant temperature throughout the processing tank, and to aid exposure of the emulsion to the chemicals. In automatic processing, a circulation system continuously pumps the developer and the fixer, thus maintaining constant agitation within each tank. Rad Science review Prof. Stelmark

Replenishment System Each time a film makes its way through the processor, it uses some of the processing chemicals. Some developer is absorbed into the emulsion and then is neutralized during fixing. The fixer, likewise, is absorbed during that stage of processing, and some is carried over into the wash tank. The replenishment system meters the proper quantities of chemicals into each tank to maintain volume and chemical activity. Although replenishment of the developer is more important, the fixer also has to be replenished. Wash water is not recirculated and therefore is continuously and completely replenished. Rad Science review Prof. Stelmark

Dryer System A wet or damp finished radiograph easily picks up dust particles that can result in artifacts. Furthermore, a wet or damp film is difficult to handle in a viewbox. When stored, it can become sticky and may be destroyed. The dryer system consists of a blower, ventilation ducts, drying tubes, and an exhaust system. The dryer system extracts all residual moisture from the processed radiograph, so it drops into the receiving bin dry. Rad Science review Prof. Stelmark

Silver Recovery Because fixer solution is used to remove unexposed silver halide from the film, used fixer solution contains a high concentration of accumulated silver. Some type of silver recovery must be used when radiographic processing accumulates high concentrations of silver. Silver recovery refers to the removal of silver from used fixer solution. For some facilities that regularly process large volumes of radiographs, the financial rewards of silver recovery may be an added incentive. Silver-recovery units are available for on-site silver recovery and generally require servicing by an outside contractor familiar with the equipment and its method of removing silver. These silver-recovery units are connected directly to the drain system of the fixer tank to remove silver as used fixer solution passes through the unit. After the silver has been recovered, the used fixer is drained. Rad Science review Prof. Stelmark

Daylight Processing Rad Science review Prof. Stelmark

Daylight systems are being adopted. When a daylight system is used, the radiologic technologist needs only position a cassette with an exposed film into the appropriate slot of this system. The film is automatically extracted from the cassette and is sent to the processor. The processor may be an integral part of the daylight system, or it may be a separate unit docked to the daylight system. The cassette is reloaded with unexposed film of proper size before it is released by the system for the next exposure. Speed is the quality that makes the daylight system attractive. It takes only about 15 seconds for the radiologic technologist to insert the exposed cassette into the daylight loader and retrieve a fresh cassette. Total load, unload, and processing time is approximately 2 minutes. Multiple film sizes are automatically accommodated. Rad Science review Prof. Stelmark

Sensitometry Rad Science review Prof. Stelmark

The study of the relationship between the intensity of exposure of the film and the blackness after processing is called sensitometry. Knowledge of the sensitometric aspects of radiographic film is essential for maintaining adequate quality control. Rad Science review Prof. Stelmark

Characteristic Curve The two principal measurements involved in sensitometry are the exposure to the film and the percentage of light transmitted through the processed film. Such measurements are used to describe the relationship between OD and radiation exposure. This relationship is called a characteristic curve, or sometimes the H & D curve after Hurter and Driffield, who first described this relationship. Rad Science review Prof. Stelmark

Two pieces of apparatus are needed to construct a characteristic curve: an optical step wedge, sometimes called a sensitometer, and a densitometer, a device that measures OD. An aluminum step wedge, or penetrometer, can also be used as an alternative to the sensitometer. Rad Science review Prof. Stelmark

Radiographic film is sensitive over a wide range of exposures. Film-screen, for example, responds to radiation intensities from less than 1 to greater than 1000 mR. Consequently, the exposure values for a characteristic curve are presented in logarithmic fashion. Furthermore, it is not the absolute exposure that is of interest but rather the change in OD over each exposure interval. Therefore, log relative exposure (LRE) is used as the scale along the x-axis. Rad Science review Prof. Stelmark

The LRE scale usually is presented in increments of 0.3 because the log of 2, doubling the exposure, is 0.3. Doubling the exposure can be achieved by doubling the mAs. Rad Science review Prof. Stelmark

The useful range of OD is approximately 0.25 to 2.5. Most radiographs, however, show image patterns in the range of 0.5 to 1.25 OD. Attention to this part of the characteristic curve is essential. However, very low OD may be too light to contain an image, whereas very high OD requires a hot light to view the image. Rad Science review Prof. Stelmark

ODs of unexposed film are due to base density and fog density. Base density is the OD that is inherent in the base of the film. It is due to the composition of the base and the tint added to the base to make the radiograph more pleasing to the eye. Base density has a value of approximately 0.1. Rad Science review Prof. Stelmark

Film contrast is related to the slope of the straight-line portion of the characteristic curve. The characteristic curve of an image receptor allows one to judge at a glance the relative degree of contrast. If the slope or steepness of the straight-line portion of the characteristic curve had a value of 1, then it would be angled at 45 degrees. An increase of 1 unit along the LRE axis would result in an increase of 1 unit along the OD axis. The contrast would be 1. Rad Science review Prof. Stelmark

An image receptor that has a contrast of 1 has very low contrast. Image receptors with a contrast higher than 1 amplify the subject contrast during x-ray examination. An image receptor with a contrast of 3, for instance, would show large OD differences over a small range of x-ray exposure. Rad Science review Prof. Stelmark

Speed The ability of an image receptor to respond to a low x-ray exposure is a measure of its sensitivity or, more commonly, its speed. An exposure of less than 1 mR can be detected with a film-screen combination, whereas several mR are necessary to produce a measurable exposure with direct-exposure film. Rad Science review Prof. Stelmark

Speed Point The speed of radiographic film typically is determined by locating the point on a sensitometric curve that corresponds to the optical density of 1.0 plus B+F. This point is called the speed point. This optical density point is used because it is within the straight-line portion of the sensitometric curve. The speed point serves as a standard method of indicating film speed. Rad Science review Prof. Stelmark

The characteristic curve of a fast image receptor is positioned to the left—closer to the y-axis—of that of a slow image receptor. Radiographic image receptors are identified as fast or slow according to their sensitivity to x-ray exposure. Rad Science review Prof. Stelmark

Usually, identification of a given image receptor as so many times faster than another is sufficient for the radiologic technologist. If A were twice as fast as B, image receptor A would require only half the mAs required by B to produce a given OD. Moreover, the image on image receptor A might be of poor quality because of increased radiographic noise. Rad Science review Prof. Stelmark

Latitude An additional image receptor feature easily obtained from the characteristic curve is latitude. Latitude refers to the range of exposures over which the image receptor responds with ODs in the diagnostically useful range. Rad Science review Prof. Stelmark

Latitude also can be thought of as the margin of error in technical factors. With wider latitude, mAs can vary more and still produce a diagnostic image. Image receptor B responds to a much wider range of exposures than A and is said to have a wider latitude than A. Rad Science review Prof. Stelmark

As development time or temperature increases, changes occur in the shape and relative position of the characteristic curve. Rad Science review Prof. Stelmark

Digital Imaging The response of a digital image receptor to the intensity of radiation exposure is different when compared with that of radiographic film. The digital image receptor is more responsive to the wide range of x-ray intensities exiting the anatomic part. In addition, a digital imaging system can retain significantly more information than radiographic film. The information received from the digital image receptor and processed in the computer represents the dynamic range capabilities of the digital system. Dynamic range refers to the range of exposure intensities an image receptor can accurately detect. The greater the number of x-ray photon intensities recorded and available to create an image, the wider the dynamic range of the imaging system. Digital imaging systems have the ability to visually display a wider range of densities than film radiography. Rad Science review Prof. Stelmark

As evidenced by the sensitometric curve for film, x-ray intensities must fall within a smaller range to display radiographic densities that can be visible. The linear response of a digital image receptor results in a greater range of densities available for display within the digital image. The digital image can display a shade of gray that represents low x-ray intensity, as well as medium and high x-ray intensities. Rad Science review Prof. Stelmark

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