Dental Radiography Module 6

Question 1

what is direct action film?

Answer 1

Intra oral radiographs
• Periapical
• Occlusal
• 22 x 35mm - small
• 31 x 41mm - standard
• 57 x 76mm - occlusal
Plastic base covered in a layer of adhesive which fixes
emulsion to the base
• Consists of silver halide (Bromide) crystals
• X ray photons sensitize the silver halide crystals
• In the developer the sensitized silver crystals are
reduced to visible black metallic silver
• Metallic silver – exposed – latent image

Question 2

what is Indirect action film ?

Answer 2

Extraoral
• Oblique lateral radiographs
• Lateral skull radiographs
• Panoramic radiographs
Silver halide emulsion is sensitive to light rather than x- rays
• The correct combination of film and intensifying screens must be used
Different emulsions are sensitive to different light colours
• Standard silver halide sensitive to BLUE light
• Modified silver halide with ultraviolet sensitizers sensitive to
• ULTRAVIOLET light
• Orthochromatic emulsion sensitive to GREEN light
• Panchromatic emulsion sensitive to RED light`

Question 3

How does a intensifying screen work in indirect?

Answer 3

Two screens are used one in front of the film one at the
back
• The front screen absorbs the low energy x-ray photons
• The back screen absorbs the high energy photons
• The two screens between them stop the transmitted x- ray
beam and turn it into visible light by the photoelectric
effect
• One x-ray photon will produce may light photons which
will effect a large area of the film emulsion
• The amount of radiation used to expose the film is
therefore reduced but the resolution is decreased (the
fine detail)

Consist of fluorescent phosphors which emit light when
excited by x-rays which are imbedded in a plastic matrix
• Rare earth phosphors with gadolinium and lanthanum
• Yttrium (non rare earth phosphor
• Calcium tungstate (CaWO4)

Question 4

What are rare earth elements is considerations to xrays?

Answer 4

Material is difficult to separate from earth and each other
• 5 times faster than calcium tungstate screens therefore
less radiation used
• More expensive than calcium tungstate screens – which
should no longer be used
• Phosphors only fluoresce when they contain impurities of
other phosphors
• Typical screens
– Terbium activated oxysulphide emit GREEN light
– Thulium activated lathanum ocybromide emit BLUE
light
– Yttrium tantalate emits ULTRAVIOLET light

Question 5

what are the cassettes?

Answer 5

Cassettes
• Light tight aluminum or carbon container
• The film is sandwiched between the intensifying screens