XRAY FILM

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• X-RAY FILM INTENSIFYING SCREENS
• Introduction
• Film is the simplest image receptor, but it is
  usually coupled with intensifying screens, which
  help reduce the dose to the patient.
• X-ray film responds to a range of wavelengths and
  is in fact far more sensitive to light than it is
  to x-rays.
• After exposure to x-rays an invisible image known
  as the latent image is formed.
• During processing, the chemicals react with the
  film in order to produce a visible image.
• Film Construction
• Radiographic film generally consists of a
• Transparent base
• Adhesive layer
• Radiation sensitive emulsion
• Protective coating
• Most x-ray film has emulsion coated on both sides
  and is known as double emulsion film (Fig. 1).
• The Film Base
• This is a transparent material usually polyester.
• It provides a rigid structure onto which the
  emulsion can be coated.
• It has strength and flexibility for handling.
• Usually between 150 200 mm thick.

2

• Adhesive Layer
• Used to attach the emulsion layer to the base.
• Consists of a mixture of gelatin and polyester.
• Emulsion Layer
• This is the radiation sensitive layer of the
  film.
• It consists of silver halide crystals (1 mm in
  diameter) suspended in gelatin (Fig. 2).
• About 90 of the halides is bromide and about 10
  is iodide.
• The gelatin helps the crystals to form a more
  uniform distribution and prevents the crystals
  from clumping together.
• The layer is about 5-10 mm thick.
• It is a delicate layer and can be easily damaged
  by chemical, mechanical or thermal means.
• Double emulsion film has increased sensitivity to
  radiation as there are more crystals for the
  radiation to interact with.
• The shape, size and distribution of the crystals
  all affect the properties of the film. These
  include
• Sensitivity or speed
• Latitude
• Graininess
• Sharpness

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• Protective Coating
• Protects the emulsion from scratches, pressure
  and contamination during handling.
• Consists of hardened pure gelatin to allow film
  transportation through the processor.
• It is sufficiently permeable for chemicals to
  diffuse in during processing.
• Formation of Latent Image
• X-rays leaving the patient and hitting the
  radiographic film deposits energy in the atoms of
  the silver halide crystals.
• This energy is deposited in a pattern
  representative of the part of the body being
  radiographed.
• No image can be seen at this stage, however, the
  invisible latent image has been formed.
• When a silver halide crystal forms, each silver
  atom loses an outer-shell electron, which
  attaches itself to either a bromine or iodine
  atom.
• The silver (Ag) is therefore a positive ion,
  while the bromine (Br_) and iodine (I_) are
  negative ions.
• The silver, bromine and iodine ions are fixed in
  a cubic crystal lattice (Fig. 3).
• The crystal structure is not perfect, however,
  and some silver ions leave their normal lattice
  sites and settle into random, interstitial
  positions (Fig. 4).
• Although each crystal is electrically neutral
  overall, bromide ions tend to concentrate on the
  surface giving the crystal a negative surface
  charge (Fig. 5).

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• A silver halide crystal also contains silver
  sulfide impurities (trace amounts of sulfur are
  added to the gelatin) which settle as sensitivity
  specks on the crystal surface (Fig. 5).
• A sensitivity speck acts as an electron trap.
• On exposure to radiation, the intensifying screen
  emits photons which interact with the silver
  halide crystals of the x-ray film via the
  photoelectric effect or Compton effect.
• In either case an electron is liberated from a
  Br_ ion.
• Br_ photon ? Br e_ (Fig. 6A).
• The neutral Br atom diffuses out of the crystal.
• The released electron has enough energy to
  liberate more electrons.
• Some of these electrons pass near or through the
  sensitivity speck and are trapped there making
  the speck negatively charged (Fig. 6B).
• The sensitivity speck now attracts positively
  charged, mobile interstitial silver ions to it
  (Fig. 6C).
• On reaching a sensitivity speck, a silver ion is
  neutralised and deposited there (Fig. 6D).
• Ag e_ ? Ag.
• The speck is electrically neutral again and can
  trap another electron.
• Ten or so silver atoms deposited this way, create
  a latent image centre (Fig. 6E).
• Crystals with silver deposited at the sensitivity
  speck turn into black grains during the
  developing process.

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• Intensifying Screen
• Device which converts the energy of an x-ray beam
  into visible light.
• Usually the x-ray film is sandwiched between two
  screens which are permanently placed within the
  radiographic cassette.
• Structure
• A screen consists of four layers (Fig. 7).
• Base
• Reflective layer
• Fluorescent or phosphor layer
• Protective coating
• Base
• Usually made of polyester.
• Has strength, flexibility and is chemically
  inert.
• About 1 mm thick.
• Reflective Layer
• Is highly reflective.
• Made of magnesium oxide or titanium dioxide.
• Ensures that any light produced in the phosphor
  that moves towards the base is reflected back
  towards the film (Fig. 8).
• This reduces the amount of radiation needed for a
  particular examination.
• It does, however, increase blurring, ie, reduce
  detail.

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• Phosphor Layer
• This is the active layer which converts x-ray
  energy into light.
• Consists of fluorescent particles suspended in a
  binding substance.
• Compounds of the rare earth elements gadolinium
  (Gd), lanthanum (La) and yttrium (Y) are widely
  used as phosphors.
• Usually about 150 300 mm thick.
• Protective Layer
• Thin layer usually made of cellulose acetate.
• Protects phosphor layer from abrasions and
  damage.
• It is transparent to allow the light produced in
  the phosphor to reach the film.
• Luminescence
• Is the ability of a substance to emit light in
  response to excitation (usually by increasing the
  energy of outer shell electrons).
• X-ray irradiation can produce luminescence.
• Two types
• Fluorescence or instantaneous light emission
  (within 10_ 8 s).
• Phosphorescence or delayed light emission.
• Radiographic intensifying screens fluoresce only
  while being struck by x-rays.

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• Phosphors
• Need to have high atomic number, high conversion
  efficiency, appropriate spectral emission and
  minimal phosphorescence or afterglow.
• Atomic number needs to be high to increase the
  probability of an x-ray interacting with the
  phosphor.
• High conversion efficiency means that many light
  photons should be produced per x-ray photon.
• Typical conversion efficiency is 1000 for 50
  keV x-rays.
• Spectral emission refers to the wavelengths of
  light emitted by the phosphor.
• Spectral emission must match the spectral
  sensitivity of the film.
• The rare earth phosphors exhibit all of the above
  characteristics.
• Screen Characteristics
• Three primary characteristics
• Screen speed
• Image noise
• Spatial resolution
• Screen Speed
• Is a relative number that describes how
  efficiently x-rays are converted into usable
  light (Table 1).
• Speeds range from 100 (slow, detail) to 1200
  (very fast).
• A measure of screen speed is the intensification
  factor.

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• It is defined as the ratio of the exposure
  required to produce the an optical density with a
  screen to the exposure required to produce the
  same optical density without a screen.
• The intensification factor thus measures the
  magnitude of dose reduction to the patient (Table
  1).
• Image Noise
• Appears on radiograph as a speckled background.
• Occurs more often when fast screens and high kVp
  are used.
• Reduces image contrast.
• Spatial Resolution
• Refers to how small an object can be imaged.
• Screens reduce spatial resolution.