Units of Instruction
Unit 1 - Radiographic Screens
Unit 2 - Radiographic Film and Processing
Unit 3 – Digital Imaging Plates and Processing
Unit 4 – Sensitometry
Unit 1 Objectives - Intensifying Screens
On completion of this unit, the student will be able to:
- Explain the purpose of radiographic intensifying screens.
- Describe the function of each layer of an intensifying screen.
- Evaluate the desirability of phosphor materials according to atomic number, conversion efficiency, spectral emission and fluorescence.
- Describe luminescence.
- Analyze the effect of phosphor crystal size, layer thickness and concentration of intensifying screen resolution.
- Explain the effect of film/screen contact on resolution.
- Describe how to remedy quantum mottle.
- Classify intensifying screens according to intensification factor, descriptive rating and relative speed number.
- Describe the effect of K-shell absorption edges on intensifying screen efficiency.
- Describe the components of a radiographic cassette.
- Describe the proper cleaning and care of radiographic cassettes and screens.
- Evaluate film/screen combinations for specific clinical uses.
- Relate the emission spectra of various intensifying screens to specific types of radiographic film.
- Explain radiographic film/screen combination relative speed numbering systems.
- Calculate relative speed conversions from one film/screen combination to another.
- Describe various methods of measuring resolution, including a basic description of linepairs per millimeter, line spread function and modulation transfer function.
- Relate film/screen contrast to latitude.
- Determine appropriate film/screen combinations for various clinical situations.
Unit Objectives - RADS.103
Radiographic Film/Processing
Radiographic Film
At the completion of this unit, the student will be able to:
1. Define the following terms:
- polyester
- sensitivity speck
- latent image
- emulsion
- base
- orthocromatic film
- panchromatic film
- spectral matching
- radiation fog
- light fog
- artifact
- shelf life
2. Given a cross-sectional diagram, be able to list the various components of a piece of film.
3. Identify the purpose and characteristics of each film component.
4. List the principle ingredients of radiographic emulsion and the associated atomic numbers of each ingredient.
5. Recognize the chemical equation for the precipitation of silver bromide from silver nitrate and potassium bromide.
6. Describe the Gurney Mott Theory.
7. Define the following terms:
- exposure latitude
- film speed/sensitivity
- spectral response
- contrast
8. Distinguish the difference between the following terms:
- low contrast versus high contrast film
- long scale versus short scale contrast
- green sensitive versus blue sensitive film
9. Identify the types of phosphor screen crystals associated with blue
and green film types.
10. Identify the appropriate types of safelight filters.
11. Identify the effect of a film’s speed and radiation dose to the patient.
12. Contrast the differences between screen and direct exposure film.
13. Identify the distinguishing characteristics of the following:
- video film
- duplicating film
- cine film
- dental film
14. Define the term artifact and give examples of each.
15. Identify the appropriate temperature and humidity levels for appropriate film storage.
16. Identify the limit of storage time for radiographic film.
Radiographic Processing
1. Briefly describe the evolution of radiographic film processing.
2. List the six steps involved, in order, in the processing of a radiograph.
3. Explain the process of film development.
4. List the name , and describe the function, of each chemical component
found in the developer and the fixer chemistries.
5. Describe the synergistic properties of the automatic processor
reducing agents.
6. Understand what is meant by the term archival quality, as it relates
to radiographic films.
7. List the approximate times for each phase of automatic processing.
8. List the approximate temperatures for each automatic processing
area/chamber.
9. Describe what is meant by the term “REDOX Reaction”.
10. Understand the effect on a film processed in solution containing
depleted glutaraldehyde.
11. Understand the effects of hypo retention on a processed radiograph.
12. Describe and understand the basic principles of Alternative Processing Methods:
Rapid Processing, Extended Processing, Daylight Processing, and Dry Processing.
Radiographic Film
Exit Beam - A pattern in which different areas have different numbers of photons
corresponding to the pattern of tissue thickness, atomic numbers and densities through which the beam has passed.
- Aerial Image - can be "captured" by various types of image
receptors
- i.e. fluoroscopic screens
film screen systems
film
- x-ray film is similar to photographic film
- 2 essential components
1. base
2. emulsion
x-ray film is manufactured with stricter QA measures than is photographic film
- see diagram in text of a piece of film in cross-section
Components
Supercoating
- protective covering of gelatin
- protects emulsion from scratching, pressure and contamination
- allows for relatively rough handling of film before and after exposure
Adhesive Layer
- between the emulsion and base
- assures adhesion of the emulsion to the base
- maintains proper contact of base/emulsion during use and processing
Base
- consists of a transparent sheet of polyester plastic providing uniform
lucency
- flexible yet rigid construction material
- maintains size and shape during processing
- does not contribute to image distortion - termed dimensional stability
- usually tinted blue to reduce radiologist eyestrain/fatigue
- conducive to more accurate diagnoses
History of Radiographic Film
Original Radiographic Film - 1. Glass Plates covered with emulsion
↓
2. Cellulose Nitrate Base (Flammable)
↓
3. Cellulose Triacetate Base (Non-Flammable)
- Safety Film
↓
4. Polyester Base - introduced in the 1960's
- resists warping
- strong
- flexibility permits easy transport
through automatic processors
- base is thinner than triacetate, yet is as
strong
Film Emulsion
Components and Characteristics
Emulsion - material which is spread over the film base and interacts with remnant
x-rays and light photons generated via the aerial image
Emulsion = Gelatin (suspension) + silver halide crystals (active ingredients)
Gelatin - suspending agent for the silver halide crystals
- transparent
- obtained from cattle skins/ ground bones treated with mustard oil
- porous - allows for processing chemicals to penetrate the silver
halide crystals
- holds silver halide crystals uniformly in place
silver halide = silver bromide + silver iodide
- 95% - 5%
Atomic Numbers
Emulsion Elements Base Atoms
Z# of Iodine = 53 Z# of Gelatin = 7
Z# of Bromine = 35 Z# of Base = 7
Z# of Silver = 47
X-Rays/Light Photons + High Z # Atoms = Radiographic Image
Silver Halide Crystals - Flat/Triangular
- arranged in a cubic lattice
Formation of the Silver Halide Crystal
Silver Nitrate + Potassium Bromide = Silver Bromide ↓+ Potassium Nitrate
+ Silver Iodide (small quantity)
Silver Halide Crystal
* the potassium nitrate is soluble and is washed away
* the process of forming the silver halide crystal is done in total darkness
Silver Halide Crystals are imperfect.
- imperfections result in the imaging properties
of the film
Imperfection = Silver Sulfide
- chemical contaminant which affixes itself to the crystal
lattice surfaces
Silver Sulfide is termed the sensitivity speck.
During processing, silver atoms are attracted to the sensitivity speck.
Radiographic Film Characteristic Differences (speed, contrast, resolution)
Determined by 1. Silver halide manufacturing process
2. crystal/gelatin mixture ratio
3. # of crystal sensitivity specks
4. crystal size and distribution
- the concentration of silver halide crystals is the principle determinant of film
speed
each film manufacturer has its own secret regarding the film emulsion composition
Formation of the Latent Image
- silver halide crystals when struck by light or x-rays undergo an electrochemical change
- increased susceptibility to the action of certain chemicals (developers)
- exposed silver halide crystals make-up the latent image
Latent Image - defined as the invisible image, produced in the film
emulsion by light or x-rays, which is changed to a visible or
manifest image during development
Manifest Image - formed after processing
The formation of the latent image is not well understood.
- theorized by Gurney-Mott
Silver, bromine and iodine are arranged in a crystal lattice ion form.
Ion - an atom having to many or to few electrons, it therefore is not electrically
neutral.
Ionic Bonds
- An atom consists of protons, electrons and neutrons
- Protons contained within the nucleus of the atom have a positive charge
- Neutrons contained within the nucleus of the atom have no charge
- Electrons revolve around the nucleus at specific distances which are termed
shells
- Shells are identified by letters of the alphabet starting with the innermost
shell termed the K shell- the next is the L shell and so forth
Neutral atoms have an equal number of electrons as protons.
There is a specific number (maximum #) of electrons that can occupy a particular shell.
K = 2 electrons
L = 8 electrons
M = 18 electrons
N = 32 electrons
A shell may contain less than the maximum number of electrons starting with the M shell before electrons begin appearing in the next shell.
- atoms are chemically stable, however, when eight electrons occupy the
outermost shell
- this is termed the octet
An attraction between two ions forming a chemical bond is called an ionic bond
In the formation of the silver halide crystal, silver atoms release an outer shell electron thereby becoming a positive ion.
Electrons released by silver are picked up by bromine and iodine atoms thereby becoming negative ions.
The silver Halide crystal is said to be non-rigid and under certain conditions, atoms and electrons migrate with in the crystal.
Bromine and iodine are concentrated on the crystal's surface.
Positive interstitial silver matches the negative outer charge.
The sensitivity speck is located on the outside of the crystal.
Photon Interaction With the Silver Halide Crystal
- radiation interacts with silver and halide atoms to form the latent image.
Radiation photons have the ability to dislodge loosely bound electrons thus creating
secondary electrons.
Loosened electrons may travel large distances in the crystal and have the ability to dislodge additional electrons from other ions.
Light photons from screens also have the ability to dislodge electrons in the crystal.
Some migrating electrons are attracted to the positively charged silver ions contained in the sensitivity speck.
Neutral bromine and iodine atoms are now free to migrate, as ionic bonds are broken. These atoms
travel into the gelatin portion of the emulsion.
The latent image is formed when atomic silver is produced at the sensitivity speck.
A group of silver atoms is called a latent image center.
Crystals with silver deposited at the sensitivity speck will be developed into
black grains.
Non-irradiated crystals remain crystalline and inactive.
Processing via chemical reactions, transform a latent image into a manifest image.
Tyes of Film
- an increase in the number of modern imaging modalities has led to a large number of film types being developed.
The most commonly employed film type today is screen film.
Other film types include: nonscreen or direct exposure
video recording film
mammography
duplication
subtraction
cine
dental
Screen Film
- used chiefly in cassettes with image intensifying screens
- combination provides for the high speed (lower patient dose)
and the excellent quality of today's radiographic image
There are three characteristics to consider when choosing a screen film.
1. Contrast - the ability of an emulsion to display a radiographic image
via a certain tonal range (white - gray - black)
- high contrast = short scale (black/white)
- low contrast = long scale (black/gray/white)
A film's contrast is inversely proportional to its exposure latitude.
Exposure Latitude - the range of exposure factors that will produce
an acceptable radiograph
- an emulsion should have sufficient latitude to
allow a reasonable degree of error in exposure without
serious impairment of radiographic quality
- excessive film latitude may impair image
visibility - the ability to see fine recorded
detail
Screen films are available in two or more latitudes.
- medium, high or higher contrast films
Difference: High Contrast Emulsion - small silver halide grains
- uniform grain size
Low Contrast Emulsion - large silver halide grains
- wide range of grain sizes
2. Film Sensitivity/Speed
Definition: the relative ability of an emulsion to respond to radiation such as light and x-rays
- An emulsion is said to be fast or have speed if a small exposure produces a radiograph of
adequate density (darkening)
Thick Emulsion = sensitive film = high speed film
Lg. Grain Emulsions are more sensitive than small grain emulsions.
In today's market, silver is costly. In an effort to hold down the cost of film, emulsions are being manufactured using less silver, yet are able to produce the same optical density per unit exposure. This is referred to as the covering power of the emulsion.
To increase a film's speed, screen film is almost always double emulsion.
3. Spectral Absorption Characteristics
- certain films respond to certain colors in the spectrum
spectrum - a series of colored bands defracted and arranged in order
of their respective wavelengths by the passage of white light through a
prism.
ROY G BIV = red, orange, yellow, green, blue, indigo, violet
A film's sensitivity to a certain color of light is termed the spectral response.
Film types, therefore, must be properly matched to the spectrum of light emitted from the screen.
Standard silver halide film is sensitive to blue/violet light. Calcium tungstate screen emit this color
making it the best combination. Films responding to blue light are called blue-sensitive film.
Rare-earth screens emit blue/green light. This is termed orthochromatic film.
- green sensitive film
Panchromatic film is used in photography and is sensitive to the entire color spectrum.
If there is an improper match of screen/film types there will be decreased image receptor speed.
Films require specific darkroom precautions.
Safelights - incandescent lamps with colored filters provide minimum darkroom
Illumination
Blue-sensitive film - amber colored filter should be used
- amber filters emit colors above the spectral response of
blue sensitive film
- an amber filter would fog green-sensitive film, therefore a red
filter must be used. Red filters may also be used for
blue sensitive film.
Handling and Storage of Films
Radiographic film is more sensitive to radiation than are humans.
Film is also sensitive to other factors:
- Mishandling
- Humidity
- Heat/Static
- Light
- Dirt
Improper handling or processing can cause artifacts.
Heat and Humidity - film is sensitive to the effects of increased temperature and
humidity
Heat - decreases contrast and increases fog
- film should not be stored at temps. Greater than 20 degrees C.
(68 degrees F)
- Ideally film should be stored in refrigerated areas
- film may be stored 1 year or longer if maintained at 10 degrees C.
(50 degrees F)
- film should never be stored near a source of heat
Humidity - ideally films should be stored at a humidity of 30% - 60%
- the lower the humidity, the greater the chance of static
electricity causing an artifact on the film.
Light - low level, diffuse light exposes a film and increases fog
- bright light exposes a film and creates gross, blackened artifacts
- A darkroom must be light tight.
- the film storage bin should be electrically coupled to the darkroom
door to prevent it from opening when the darkroom door is opened
Radiation - Film bins should be lead lined
- the fog level for unprocessed film is .2 mR (a very minimal amount
of radiation)
- lead lining should provide protection from this amount of radiation
Shelf Life of Film
- film is supplied in boxes of 100 sheets
- film should be used ASAP
- film should never be used past the expiration date on the box
- aging of film results in decreased speed/contrast and increased fog
- film should be stored "on edge" rather than flat
- this should be done to decrease pressure marks, prevent film from
sticking together or prevent warping of the film
- concept of First In First Out (FIFO) - film should be thought of as being
perishable and therefore should be
rotated
-30 days is a reasonable maximum storage time for radiographic film