Radiographic Imaging Sciences

RADIOGRAPHIC IMAGING SCIENCES

Image formation and characteristics

• Film construction
• Types of Film
• Latent image formation
• The duplication film process
• Film storage and handling
• Film identification

X-ray Cassettes

FILM CONSTRUCTION

X-ray films for general radiography consist of an emulsion-gelatin containing radiation sensitive silver halide crystals, such as silver bromide or silver chloride, and a flexible, transparent, blue-tinted base.

The emulsion is different from those used in other types of photography films to account for the distinct characteristics of gamma rays and x-rays but X-ray films are sensitive to light. Usually, the emulsion is coated on both sides of the base in layers about 0.0005 inch thick

Duplitized film

Single sided – film

Base: A polyester material that provides support.-

Adhesive Layer/Subbing layer : Binds the emulsion to the base.- Emulsion Layer: Contains silver halide crystals suspended in gelatin; this is where the image is formed.- Protective Layer: A thin coating to protect the emulsion from physical damage

Radiographic Imaging Sciences

Putting emulsion on both sides of the base doubles the amount of radiation-sensitive silver halide, and thus increases the film speed .The emulsion layers are thin enough so developing, fixing, and drying can be accomplished in a reasonable time. A few of the films used for radiography only have emulsion on one side which produces the greatest detail in the image.

When x-rays gamma rays, or light strike the grains

of the sensitive silver halide in the emulsion, some of the Br- ions are liberated and captured by the Ag+ ions. This change is of such a small nature that it cannot be detected by ordinary physical methods and is called a "latent (hidden) image

Radiographic Imaging Sciences

" However, the exposed grains are now more sensitive to the reduction process when exposed to a chemical solution (developer), and the reaction results in the formation of black, metallic silver.

It is this silver, suspended in the gelatin on both sides of the base, that creates an image.

Exposure to X-rays

When the X-ray beam passes through the patient, it reaches the intensifying screen.- The screen converts X-rays into visible light (or sometimes direct X-rays hit the film in non-screen systems).

Interaction with Silver Halide Crystals

Interaction with Silver Halide Crystals:- The light (or X-rays) interacts with silver halide crystals (mainly silver bromide) in the emulsion layer of the film.- The energy from this interaction excites electrons within the crystals

Sensitivity Speck and Electron Capture

• The free electron migrates to a sensitivity speck (a defect or impurity in the crystal)

The speck becomes negatively charged and attracts positive silver ions (Ag⁺)

Formation of Metallic Silver

The silver ions gain electrons and are reduced to metallic silver (Ag).- This forms a small cluster of metallic silver atoms at the sensitivity speck — this is the latent image center.

Invisible Until Developed

These tiny metallic silver clusters are not yet visible.- During chemical processing, they are amplified to form a visible (manifest) image

Film storage and handling

Film seems to be derived from an old word for skin. Skin and film can be mistreated in similar ways:

• Skin can be scratched; so can film.
• Skin can be blistered by chemicals or heat; so can film.
• Skin can be damaged by prolonged exposure to the sun; so can film.

• Skin gets dry and brittle in low humidity and cold; so does film.

• Skin is susceptible to fungus in excessive heat and relative humidity; so is film.
• Skin can be burned; so can film.

Keeping your raw stock and exposed film safe

• Unprocessed film must be kept cool to preserve quality, ideally at 55°F / 13°C
• Keep film cool until just before shooting
• For storage of film for longer than 6 months, store at 0°F / -18°C
• Allow frozen film to warm up gradually before opening can, to avoid condensation
• Avoid prolonged exposure of film cans or camera magazines to sunlight
• Avoid storing film in hot vehicles

Exposed Unprocessed:

• Process film promptly after exposure
• Avoid “Latent Image Regression”
• Keep film cool after exposure
• If processing is delayed, put film in refrigerator
• Allow frozen film to warm up gradually before opening can for processing

Film storage and handling

X-ray film should always be handled carefully to avoid physical strains, such as pressure, creasing, buckling, friction, etc Marks resulting from contact with fingers that are moist or contaminated with processing chemicals, as well as crimp marks, are avoided if large films are always grasped by the edges and allowed to hang free

Use of envelope-packed films avoids many of these problems until the envelope is opened for processing.

Another important precaution is to avoid drawing film rapidly from cartons, exposure holders, or cassettes. Such care will help to eliminate circular or treelike black markings in the radiograph that sometimes result due to static electric discharges.

Film fog