History

By the time x-rays were discovered, photography was already an art.

Photographic film with a nitrocellulose base was already being marketed by George Eastman.

The first x-rays were recorded on glass plates.

These were coated with emulsion on one side only.

The exposure dose was quite high.

Glass plates were used until World war I.

During WWI, nitrocellulose based film

was found to be a more feasible choice for recording x-rays.

This film was single-emulsion.

It was later discovered that double-emulsion responded to x-rays faster.

The flaw with nitrocellulose based film was its easy flammability.

In 1924, cellulose acetate replaced the nitrocellulose based film.

The Radiographic Film

A Radiographic film is similar in construction and characteristics to a photographic film.

Its spectral response is different from photographic film but its mechanism of operation is the same.

The film is sandwiched between the radiographic intensifying screens in a protective cassette.

The intensifying screens change the x-rays into visible light. The visible light exposes the radiographic film.

Film Structure

Radiographic Film has two basic parts.

Base EmulsionMost films have two

layers of emulsion so these are referred as Double Emulsion Film.

An adhesive layer attaches the emulsion to the base.

The emulsion is enclosed in a protective layer of gelatin called the overcoat.

Transparent Base

It is the foundation of the film. 150 to 300 µm thick.

Transparent Base

Provides a surface and support for the emulsion.

It must have strength, but it should be flexible.

Most film bases are composed of polyester.

Polyester can withstand higher temperatures and is more fireproof.

The base is usually tinted blue to reduce light glare.

Amount of tint varies per

specifications of different manufacturers.

Emulsion

The emulsion is the heart of the film.

The x-rays or light from the intensifying screens interact with the emulsion and transfer information to the film.

A homogeneous mixture of gelatin and silver halide crystals and is about 3 to 5 µm thick.

Gelatin

The gelatin is clear so it transmits the light to the silver halide crystals.

It is porous so the processing chemicals can easily penetrate to the silver halide crystals.

The primary function of the gelatin is to provide a support medium for the silver halide crystals by holding them in place.

Silver Halide Crystals

98% Silver Bromide 2% Silver Iodide May be

tabular,cubic,octahedral,polygonal in shape.

Tabular shape used most commonly for general radiography.

About 0.1µm thick and 1µm in diameter.

Silver Halide Crystals

The differences in speed, contrast and resolution depends upon the process by which the silver halide crystals are manufactured.

From the time the emulsion ingredients are brought together until the film is packaged, the whole process occurs in complete darkness.

The Latent Image

Invisible image produced on the film after exposure prior to development.

The latent image is the invisible change in the silver halide crystals.

The interaction between the photons and the silver halide crystals produces the latent image.

The Latent Image Formation This interaction is sometimes referred

to as the photographic effect.

This process is not well understood and is still under research.

The Gurney-Mott theory is presently an acceptable explanation of the photographic effect.

Ionic structure of silver halide crystal

Bromide and iodide ions are mostly concentrated on the surface of crystal giving it a negative charge.

The silver ions are inside known as interstitial silver ions,so inside is positively charged.

The Latent Image Formation

A Radiation interaction releases electrons.

B Electrons migrate to the sensitivity center(contaminant in the silver halide crystal,usually silver sulfide).

C At the sensitivity centre, atomic silver is formed by attracting an interstitial silver ion---latent image centre.

D The process is repeated many times resulting in the build up of silver atoms.

E The remaining silver halide is converted to silver during processing.

F The resulting silver grain is formed.

Silver halide that is not irradiated remain inactive. The irradiated and non-irradiated silver halide produces the latent image.

Types of Films

Screen films most commonly used. Screen film used with intensifying

screens. Single emulsion- emulsion on one side of

base. Double emulsion used with two screens.

Direct exposure film or non-screen film.

Special purpose films

Standard screen-film sizes

English Units SI Units

7 x 7 in 18 x 18 cm

8 x 10 in 20 x 25 cm

10 x 12 in 24 x 30 cm

14 x 14 in 35 x 35 cm

14 x 17 in 35 x 43 cm

Screen Film Factors

Main factors to be considered when selecting filma) Contrast & Speedb) Crossoverc) Spectral matchingd) Reciprocity Lawe) Safelights

Contrast

Contrast of a film depends on its latitude.

Latitude is the range of exposure techniques that produce an acceptable image.

Latitude is inversely proportional to contrast.

High contrast film has low latitude Medium contrast film has medium

latitude Low contrast film has high latitude

High contrast has small uniform grains Low contrast has larger grains and wide

range in size.

Speed It is the sensitivity of film to x-rays and light.

The size and shape of the silver halide crystals are the main factors that determine speed.

Faster speed films are almost always double emulsion.

Light spectrum from screens must match to achieve optimum speed.

Crossover

Crossover is the exposure of an emulsion by light from the opposite radiographic intensifying screen.

Reducing crossover by adding a dye to the base

Crossover causes blurring of the image. Can be reduced by Tabular grains---flat, large surface

area to vol ratio Addition of a light absorbing dye in

crossover control layer.

Spectral Matching

The most important consideration in selecting screen film is spectral absorption matching.

The material in the screen will determine the color of light emitted by the screen.

Special dyes in the film are used to match the screen to the film.

Spectral Matching

Calcium Tungstate screens emit blue and blue violet light.

Replaced by Rare earth screens. Rare earth screens emit ultraviolet,

blue, green and red light.

Spectral Matching

If the light spectrum does not match, there will be a significant loss of speed alongwith increased patient dose.

Reciprocity Law

In radiography, it is generally assumed that the total exposure of a film depends only on the total quantity of radiation (mAs) and not on the exposure time. This is known as the reciprocity law.

Reciprocity law Exposure=intensity x time =Constant Optical

Density

Reciprocity Law

The reciprocity law is true for film exposed directly to x-rays.

It fails when film is exposed to light from radiographic intensifying screens.

Reciprocity law failure is important when the exposure times are very long (as in mammography)or very short (angiography).

The result is a loss of speed.

.

Safelights

Working with film in the darkroom requires special lighting to avoid exposure of the film.

Filters are used to avoid exposure of the film.

Safelights

An amber filter can be used for blue sensitive film only.

A red filter is used for blue-green sensitive film.

The color is not the only concern, the wattage of the bulb and distance between the lamp and work surface is also very important.

Special Film Types

Direct exposure film: used without intensifying screen.

were used for small body parts. Requires 10 to 100 times more

exposure. The emulsion is thicker than screen film.

Renders excellent detail. No longer used.

Single emulsion film: once used for extremities but now most extremity cassettes are double screen type.

Again required more exposure.

Mammography Film: Only single emulsion film currently used in modern radiography.

Laser Film: Used in with a laser printer for digital radiography, CT and MRI.

Subtraction Film: used in angiography to do subtraction where the bone is removed for better visualization of the arteries.

Spot film: Special roll film of 70 to 105 mm width used in fluoroscopy.

Can be processed in x-ray film processor.

Cine film: 35 mm black & white film supplied in

rolls of 100 and 500 ft used in coronary angiography. Requires motion picture film processor.

Handling and Storage of Radiographic Film X-ray film is a sensitive radiation

detector and it must be handled in an area free of radiation.

Film storage must be shielded.

The darkroom adjacent to the x-ray room must be shielded.

Improper handling of the film will result in poor image quality due to artifacts.

Avoid bending, creasing or rough handling of the film.

Avoid sharp objects contacting the film.

Hands must be clean and dry.

Avoid hand creams, lotions or water free hand cleaners.

Static electricity or a dirty processor can cause artifacts.

Film is sensitive to heat and humidity.

Heat and humidity causes fog or a loss of contrast.

Film should be stored at less than 20º C (68ºF)

Humidity should be between 40% and 60%.

Film must be handled and stored in the dark.

Low level diffuse light causes fog. Bright light causes gross exposure. Luminous watches, cell phone and

darkroom light leaks should be avoided.

Films should be used no longer than the stated Shelf life.

The oldest film in stock should always be used first.

Expired film results in loss of speed and contrast and an increase in fog.