Processing the Latent Image
The Latent Image:
It is the invisible image formed as a result of exposure to radiation and
which may be visible by photographic development.
Film Processing:
Before the routine use of automatic film processing in radiography, x-ray
films were hand processed. In hand processing, the expose film is first
immersed in a tank containing developer and then immersed in a stop bath,
followed by immersed in a fixer. The film is washing in running water and
hung to drip dry.
Developer stop bath fixer waterfilm dryer
Sequence of events in a radiograph:
Films can be processed with manual dipping or with automatic film
processors.
Step
Purpose
Manual
Automatic
Wetting
Swells the emulsion for better chemical
15 s
---
5 min
22 s
30 s
---
15 min
22 s
penetration.
Developing Produces a visible image from the latent
image.
Stop bath Terminates development and removes
excess chemicals from emulsion.
Fixing
Removes remaining silver halide and
hardens gelatin.
Washing
Removes excess chemicals
20 min
20 s
Drying
Removes water and prepares film for
30 min
26 s
viewing
The Developer Solution:
It is a chemical solution converts the invisible latent image to a visible
manifest image.
Developer Components:
1- Solvent:
Water is the solvent used to wet the emulsion.
2- Developing agent:
Most developer is formed of phenidone and hydroquinone.
- Hydroquinone develops the dark areas of the film.
- Phenidone develops the grays.
3- Accelerator:
It is alkali provides an alkaline medium in which the developing agent can
operate.
4- Buffers:
This is a chemical compound that has the effect of maintaining the PH of a
solution within close limits.e.g (Sodium Carbonate)
5- Restrainers:
It can modify the behavior of developing agent to be more selective.
It reduces the tendency of developing agent for un-exposed silver halide
crystals.
6- Preservative:
It reduces the oxidation of developing agent e.g. (Sodium Sulfite).
7- Hardener:
Its main value is to toughen the gelatin e.g. gluteraldhyde.
8- Sequestrating agent:
It is a chemical substance that prevents precipitation of insoluble mineral
salts.
Fixer solution: It is composed of
1- Solvent
Water acts as a solvent and diluted at the same time.
2- Fixing agent:
It is a chemical that combines with the insoluble silver halides in the film
emulsion to from soluble compound that can be easily washed out of the
emulsion.
3- Acid:
A weak acid, acetic acid, is sufficient ( pH= 4 – 4.5)
- ensure that development ceases when the film enter the fixer
- provide a suitable environment for hardening agent
4- Buffers:
To ensure neutralization of developer
To maintain optimum hardener activity
5- Preservative:
It retards the decomposition of thio-sulphates e.g. sodium sulphate
6- Hardener:
To limit water uptake by the emulsion, reducing the drying time and
prevents physical damage.
Aluminum chloride and aluminum sulphate is appropriate
7- Anti-sludging agent:
It is a chemical substance that reduces the formation of sludge e.g. boric
acid.
Automatic Film Processor
Developerfixer waterfilm dryer
Components of an Automatic Processor
Component
Function
Transport system
Moves film
Temperature Control
Controls Developer temperature
Circulation
Agitates chemicals
Replenishment
Maintains concentration
Wash
Removes chemicals
Dry
Removes moisture vents exhaust
Electrical
Fused power
Transport System Functions:
Moves film through processor at the correct speed.
1- Entrance Rollers
Activates replenishment of developer and fixer.
2- Crossover Racks
Move film from on tank to the next tank and remove chemicals from
film.
3- Turn around
Also called master rollers, turn the film around at the bottom of the
tanks.
4- Motor drives gears that turn the rollers.
Replenishment System
- Each time a film passes through the chemicals; fresh chemicals are
pumped into the tank.
- This maintains the proper concentration and level of chemicals in
the tanks.
- Developer replenishment is 60 to 70 ml for each 14 x 17.
- Fixer replenishment is 100 to 110 ml for a 14 x 17.
Dryer System
- Dryer removes all of the moisture from the film.
- Consists of a heat coils, thermostat, ducts and blower. Heat should
be exhausted to the return air system of the dark room.
- Some processors used Infrared Heater to dry the film.
Importance of Proper Development
- Development is a chemical reaction governed by:
1- Time
2- Temperature
3- Concentration of the developer
- Long time with low temperature or high temperature with short time
will work.
Fog Film:
- Any deviation from those parameters will result in a loss of image
quality, usually resulting in fog.
- Fog causes an increase in base fog and a drop in contrast. A fogged
image is gray with poor contrast.
- There are three ways to fog film:
1- Chemical fog: contaminated developer, high temperature, slow
transport.
2- Radiation fog: unintentional exposure to radiation.
3- Improper storage: wrong safelight or storage in high heat and
humidity, expired or out of date film.
Factors affecting processing
Factors affecting Developing
1- Constitution of developing solutions:
- Choice of developing agent
- Concentration of developing agent
- The pH of developer solution
- Concentration of restrainer
2- Developer Temperature:
If high ↑ image density & ↑chemical fog but ↓image contrast
If low ↓ image density with possibility of loss of contrast
3- Developing Time:
It is determined by developer activity and type of film emulsion.
Factors affecting fixation:
1- Constitution of fixer solutions:
- Choice of fixing agent
- Concentration of hardeners
- Presence of hardeners
- The pH of fixing solution
2- Fixer Temperature
3- Fixing Time
Factor affecting washing efficiency
1- Film emulsion
2- Condition of wash water
3- Temperature of wash water
4- Washing time
Factor affecting drying time
- Wetness of emulsion (thickness and hardener)
- Drying condition (air humidity and air temperature)
Processor Maintenance
1- Daily
1- Remove cross-over assemblies and clean rollers under warm
(38˚C) and wipe dry.
2- Wipe down the entry rollers.
3- Wipe off chemical deposited.
4- Wipe all top rollers above solution level.
2- Weekly
1- Repeat the daily cleaning.
2- Put splashguard between developers. And fixer tank and remove
deep racks and clean it.
3- Operate each rack manually
4- Re-install racks and replace cross-over assemblies
5- Check water supply filters.
6- Clean drier section air tubes and rollers
3- Monthly
1- Repeat daily and weekly maintenance
2- Drain main tanks and clean
3- Close drain valve and fill tanks with water and switch on.
4- Turn off processor and drain tanks and re-fill with fresh chemical
The processing area
This area is known as the dark room which is very important in any radiology
department
Location of darkroom
It should be centrally placed relative to the x-ray room, which it serves
Dark room size and shape
A long dark room is more convenient than a square one because:
o More wall space is available for equipment
o No wasted floor space in the center of the room
Construction of dark room
1- Wall should have a hard smooth finish
2- Ceilings should be finished in oil-smooth finish
3- Floor should be water proof, resistant to photographic chemical and
staining
4- The entrance should allow easy access, yet prevent passage of light
5- Cassette hatch should be installed in one wall of the dark room
6- Water service in the form of adequate plumbing and drainage together
with hot and cold water supply
7- Electricity service in the dark room
8- Ventilation and heating
Dark room accessories
o Film hangers
o Stainless steel clips
o A ringing timer
o A thermometer
Beam-Restricting Devices
Three factors contribute to an increase in scatter radiation:
• Increased kVp
• Increased Field Size
• Increased Patient or Body Part Size.
X-ray Interactions
• a – some interact with the patient and are scattered away from the patient.
• b – some are absorbed
• c - some pass through without interaction
• d – some are scattered in the patient
• c & d are image forming x-rays.
There are two principal means to reduce scatter radiation:
• Beam Restricting Devices limit the field size to reduce scatter and primary
radiation.
• Grids to absorb scatter before it reached the image receptor.
There are three principal types of beam restricting devices:
1- Aperture Diaphragm
Aperture diaphragms are basically lead or lead lines metal devices placed in
the beam to restrict the x-rays emitted from the tube.
2- Cones & Cylinders
Cones and cylinders are modifications to the aperture.
Cones are typically used in dental radiography.
3- Collimators
The light localizing variable aperture collimator is the most common beam
restricting device in diagnostic radiography.
• Not all of the x-rays are emitted precisely from the focal spot.
• These rays are called off-focus radiation and they increase image blur.
• First stage shutters protrude into the tube housing to control the off-focus
radiation.
• Adjustable second stage shutter pairs are used to restrict the beam.
• Light localization is accomplished by a small projector lamp and mirror to
project the setting of the shutters on the patient.
The Grid
• Collimation reduces scatter radiation but that alone is not sufficient for
larger body parts.
• In 1913, Gustave Bucky demonstrated that strips of lead interspaced with
radiolucent material is an effective means to reduce scatter radiation
reaching the film.
• Only rays that travel in a relatively straight line from the source are allowed
to reach the film.
• The others are absorbed by the lead.
There are three important dimensions on a grid.
1- Width of the grid strip (T)
2- Width of the interspace material (D)
3- Height of the grid (h)
• There are three important aspects of grid construction;
1- Grid Ratio
2- Grid Frequency
3- Grid material
Grid Ratio
• High ratio grids are more effective in cleaning up scatter radiation because
the angle of scatter allowed by the high ratio is less than permitted to pass by
low ratio grids.
• Ratios range from 5:1 to 16:1
• 8:1 and 10:1 grids are the most popular ratios in general radiography.
Grid Frequency
• The number of grid lines per inch or centimeter is called the Grid Frequency.
• Grid frequency range from 25 to 45 lines per centimeter or 60 to 145 lines
per inch.
• The advantage of high frequency grids is there are no objectionable grid
lines on the image.
Grid Material
• The most common grid material is lead because of its cost and ease of
forming the strips.
• The interspace material is used to maintain a precise separation of the lead
strips.
• Plastic fiber or aluminum is used as the interspace material.
• Aluminum is used as the cover for the grid to protect it from damage and
moisture.
Three types of grids
1- Parallel Linear Grids
2- Crossed Grids
3- Focused Linear Grids
Parallel Grid
• Cheap and easy to manufacture.
• Problem: Grid cutoff at the outer edge of the 14”X17” film.
• Cut off is most pronounced at short SID.
Crossed Grid
• Two parallel grids can be sandwiched together with the lines running across
the long axis and short axis of the film.
• More efficient than parallel grid.
• Grid cut off is the primary disadvantage of a crossed grid.
• Tube can not be angled.
Focused Grids
• Focused grids are designed to minimize grid cut off.
• The grid lines are angled to match the divergence of the beam.
• Focused grids are marked with an intended focal range and the side that
should be towards the tube.
• If the tube is improperly aligned or the SID is under the focal range, grid cut
off will occur.
The Bucky Grid
• If the grid moves during the exposure, the grid lines can be blurred out. This
was discovered by Hollis Potter in 1920.
• There are two types used today, reciprocating and oscillating.
• The reciprocating design is moved by a motor during the exposure.
• The oscillating design is moved by an electromagnet in a circular pattern.
• The mechanism adds space between the patient and the film.
• The motion can move the film resulting in image blur.
• When they fail, the lines appear.
Grid Problems
Radiographic image defects
1- Un-sharpness:
Un-sharp image means blurred image and this may be due to:
- Geometric un sharpness(UG)
- Motional un-sharpness(UM)
- Photographic un-sharpness(UP)
2- Over-under penetration:
The abnormal density of radiographic image is caused by
abc-
Radiographic errors such as
Poor choice of exposure factors
Failure to match exposure factors to the film-screen system
Use of non-standard focus film distance
abc-
Equipment errors such as
Reduced x-ray output
Premature termination of exposure
Inadequate mains electrical supply
Processing errors such as abnormal developer temperature
3- Poor contrast
Contrast: is the difference in density between two adjacent area of the image
Factors affecting contrast:
- Subject contrast
- Screen contrast
- Fogging
4567-
Graininess
Double image
Image artifacts
Distortion
exposure factors :
Definition:
These factors that affect the quantity and distribution of radiation energy to which
the image receptor is exposed
1- Kilo-voltage
Raising the tube kilo-voltage increase the density of x-rays produced
and consequently the image density
2- Milli-ampere-second
As general rule in radiography, we almost prefer to use a maximum
mA and a minimum time combination
3- Focal spot
4- Filtration
5- Focus Film Distance
6- Collimation
7- Table-top attenuation
8- Grid