Processor, Sensitometry,
Artifacts
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-toread 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
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•
•
•
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
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
Transports film through
various stages at precise
intervals
Transport
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
Monitors and adjusts
temperature at each
stage
Temperature
Rad Science review Prof.
Stelmark
Circulation
Replenishment
Agitates fluids
Developer
Continuously mixes,
filters
Fixer
Continuously mixes
Wash
Single-pass water flows
at constant rate
Developer
Meters and replaces
Fixer
Meters and replaces
Removes moisture,
vents exhaust
Dryer
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
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
Rad Science review Prof.
Stelmark
When the film is transported in one direction along the rack assembly, only 1-inch (25mm) 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
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
Rad Science review Prof.
Stelmark
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
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.
Rad Science review Prof.
Base density has a value of approximately
0.1.
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
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
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
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