RADIOGRAPHIC

FILM
 INTRODUCTION
• Radiographic X-ray film performs many important functions in
diagnostic radiology, radiotherapy and radiation protection.
• It can serve as a radiation detector, a relative dosimeter, a display
device and an archival medium.
• 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.
 COMPONENTS OF RADIOGRAPHIC FILM

• Radiographic film has two basic

parts :
1. BASE
2. EMULSION
• Most films have two layers of
emulsion so these are referred as
“Double Emulsion Film”.
 1. BASE
• Function : to provide a support for the fragile photographic emulsion

• 150 𝜇𝑚 to 300 𝜇𝑚 thick

• Made up from polyester
• Characteristics :
1. It must not produce a visible pattern or absorb too much light
when the radiograph is viewed.
2. The flexibility, thickness, and strength of the base must allow for
ease of processing (developing) and produce a radiograph that
"feels right" when handled (a film too "floppy" to "snap" under
the hangers of a viewbox gets a cool reception).
3. The base must have dimensional stability; that is, the shape and
size of the base must not change during the developing process
or during the stored life of the film.
 2. EMULSION

• 3 𝜇𝑚 to 5 𝜇𝑚 thick (varies with film type)

• It is the heart of the film

• The x-ray 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
 Gelatin
• It is clear so transmits the light to the silver halide crystals
• It is porous so that processing chemicals can easily
penetrate to the silver halide crystals
• It is provide a support medium for the silver halide crystals
by holding them in place
 Silver Halide Crystal
• Made up of 98% Silver Bromide + 2% Silver Iodide
• The crystal is formed from ions of silver (Ag+), ions of bromine (Br-),
and ions of iodine (I-) arranged in a cubic lattice
• Shape : tabular, cubic, octahedral, polygonal etc..
• Tabular shape used for general radiography

• About 0.1 𝜇𝑚 𝑡ℎ𝑖𝑐𝑘 and 0.1 𝜇𝑚 in diameter

Tabular Shape

• 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, whole process occurs in complete
darkness
PHYSICAL PRINCIPLE OF IMAGE FORMATION
ON FILM
• An invisible image formed when a silver halide grain is exposed to X-
ray or light (from intensifying screen).
• Exposure to X-ray or light renders the film susceptible to the
chemical actions of the developer.
• This means that a silver halide grain that has been exposed to
radiation can’t be developed into visible image.
• Some indirect processes (the transformation of silver bromide to
atomic silver) indicate that the latent image is a formation of silver
atom aggregates inside the grain.
 Latent Image
• Metallic silver is black.
• It is silver that produces the dark areas seen on a developed
radiograph.
• The energy absorbed from a light photon gives an electron in the
bromine ion enough energy to escape.
• The electron can move in the crystal for relatively large distances as
long as it does not encounter a region of impurity or fault in the
crystal.
Br- + light photon -----> Br + electron
• A site of crystal imperfection, such as a dislocation defect, or an AgS
sensitivity speck, may act as an electron trap where the electron is
captured and temporarily fixed.
• The electron gives the sensitivity speck a negative charge, and this
attracts the mobile interstitial Ag+ ions in the crystal.
• At the speck, the silver ion is neutralized by the electron to form a
single silver atom:
Ag+ + electron -----> Ag
• This single atom of silver then acts as an electron trap for a second
electron.
• The negative charge causes a second silver ion to migrate to the trap
to form a two-atom silver nucleus.
• Growth of silver atoms at the site of the original sensitivity speck
continues by repeated trapping of electrons, followed by their
neutralization with interstitial silver ions.
• The negative bromine ions that have lost electrons are converted
into neutral bromine atoms, which leave the crystal and are taken up
by the gelatin of the emulsion.
• The development of a two-atom latent image according to the
Gurney-Mott hypothesis.
• A single silver halide crystal may have one or many of these centers
in which atomic silver atoms are concentrated.
• The presence of atomic silver is a direct result of the response of the
grain to light exposure, but no visible change has occurred in the
grain.
• These small clumps of silver can, however, be seen with electron
microscopy.
• These clumps of silver atoms are termed latent image centers, and
are the sites at which the developing process will cause visible
amounts of metallic silver to be deposited.
 TYPES OF FILM
• Double emulsion : Emulsion on both side of base
• Single emulsion : Emulsion on one side of base
 SENSITOMETERIC PARAMETERS OF FILM
1. DENSITY
2. SPEED
3. LATITUDE
4. CROSSOVER
5. SPECTRAL MATCHING
6. RECIPROCITY LAW
7. SAFE LIGHTS
 1. DENSITY
• Light transmission is a function of the film opacity and can be
measured in terms of optical density (OD) with devices called

• OD = log10(/I )
densitometer.

I = intensity transmitted through the film

Where, = initial light intensity

density (𝑂𝐷𝑓).
• Unexposed film would exhibit a background OD called the fog
• The density due to radiation exposure, called the net OD, can be
obtained from the measured density by subtracting the fog density.
• Ideally, the relationship between the dose and OD should be linear,
but not in every case.
• Some emulsions are linear, some are over a limited dose range and
others are non-linear.
• The dose versus OD curve, known as, “The sensitometric curve” or
“H&D curve”, in honour of Hunter and Driffield, who first
investigated the relationship.
• H&D curve :
• H&D curve has four regions:
– Region 1 : Fog, at low or zero exposures
– Region 2 : Toe, region of underexposure
– Region 3 : Linear Portion, intermediate exposure (optimum
measurement conditions)
– Region 4 : Shoulder & Saturation, high exposure (overexposure)
 2. 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 mostly double emulsion.
• Light spectrum from screens must match to achieve optimum speed.
 3. LATITUDE
• Latitude of a film depends on its contrast.
• Latitude is a Range of the exposures that will result in a density in
the useful range of optical densities.
• Latitude is inversely proportional to contrast.
• High latitude Low contrast larger grains
• Medium latitude Medium contrast
• Low latitude High contrast small uniform grains
 4. CROSSOVER
• It is the exposure of an emulsion by light from the opposite
radiographic intensifying screen.
• Causes blurring of image.
• Reduced by : tabular grains, large surface area to volume ratio and
addition of a light absorbing dye in crossover control layer.
 5. SPECTRAL MATCHING
• The most important consideration in selecting screen film is spectral
absorption matching.
• The material in the screen will determine the colour of light emitted
by the screen.
• Special dyes in the film are used to match the screen to the film.
• Calcium Tungstate screens emit blue and violet light.
• Replaced by Rare earth screens, which emits ultraviolet, blue, green
light.
• If the light spectrum doesn't match, there will be a significant loss of
speed along with increased patient dose.
 6. 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”.
• Reciprocity law
Exposure = Intensity ×Time = Constant Optical Density
• It 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 (mammography) or very short (angiography).
• The result is a loss of speed.
 7. SAFE LIGHTS
• Working with film in the darkroom requires special lighting to avoid
exposure of the film.
• Filters are used to avoid exposure of film.
• An amber filter can be used for blue sensitive film only.
• A red filter is used for blue-green sensitive film.
• The colour is not the only concern, the wattage of bulb & distance
between the lamp and work surface is also important.
 Use of Characteristic Curve
• To compare different types of films
• To compare different types of screens
• Useful tool to set up exposure devices
• To determine avg. gradient & therefore subject contrast
amplification
• To find film and exposure latitude
• To find absolute value of the speed of the films
• To monitor the performance of auto processor