Transcript The radiograph

The radiograph
Photographic principles
Objectives
To examine how invisible x-ray image has
to be recorded and converted into a visible
form
 To examine in detail the photographic
effect of radiation on sensitive emulsion
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The photographic effect
This is the principle of traditional method of
recording the invisible x-ray image
 It uses photo-sensitive materials (some chemical
compounds) which undergo subtle structural
changes when exposed to electromagnetic
radiations such as visible light, ultraviolet
radiation or x-rays.
 These changes are not immediately visible, but,
 They are associated with an alteration in the
chemical behaviour of the substance.
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 The exposed materials respond differently in
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certain chemical reactions.
By careful chemical processing, it is possible to
differentiate between exposed and unexposed
materials, producing a visible difference between
them.
This creates a visible image.
The effect on chemical nature of material is
known as “Photographic effect” of radiation
The chemical processing necessary to make
invisible changes visible is called ‘photographic
development’
Photosensitive materials
In photography photosensitive materials
used are Silver halides.
 Radiography is a specialized application of
the photographic process
 Therefore in radiography too silver halides
are used as photosensitive material to
record the images
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Silver halides
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Are a group of chemical compounds
consisting of atoms of element silver
combined with atoms of halogen elements
– Silver bromide
– Silver iodide
– Silver chloride
 They are sensitive to light and x-rays
 Silver bromide is commonly used
Physical properties of silver halides
White or pale-yellow crystalline salts
similar in appearance to common salt
 The links between silver and halogen
atoms are ‘ionic bonds’.
 The electrical forces between positively
charged silver ions and negatively charged
halide ions fix the positions of the ions in a
regular three-dimensional crystal structure
or lattice
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Crystal /lattice of AgX
Chemical properties of Silver halide
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Pure silver halide crystals are relatively stable
and do not suffer chemical breakdown
But under certain conditions it is possible to
convert silver ions to atoms of metallic silver by
donating electrons
The agents that supply electrons for this process
are called reducing agents.
The action is chemical reduction of silver halide
to silver
Chemical reduction takes place rapidly on Silver
halide crystals that have been exposed to
radiation than those are not
This is the key feature of photographic
processing
Effects of exposure on silver halide
X-ray
photon
X-ray
photon
e
Sensitivity spec
Silver
ion
Bromine
ion
Silver halide lattice
Some x-ray photons
passes through
without action
Some photons give its
energy to a bromine ion
and emit an electron
Some X-ray photons pass through without
any interaction
 Some photons interact with bromine ions
and release an electron
 The electron moves around the crystal
 In a short time it loges in a low energy
electron trap near the surface of the
crystal
 This low energy electron trap is called a
‘sensitivity spec’
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Sensitivity specs are formed by deliberate
introduction of ‘impurities’ into the crystal
during its manufacture
 As it collects more and more electrons
sensitivity specs acquire a negative charge
 When this negative charge becomes
strong enough some positively charge
silver ions are drawn towards it.
 As they come the silver ions each gain an
electron to become a neutral silver atom
(metalic silver)
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Significance of formation of metallic silver
In normal circumstances only a few hundreds of
silver atoms are formed
 It is inadequate to produce a visible change in
the crystal
 But the tiny collection of silver atoms renders
the crystal much more vulnerable than
unexposed crystals to attack by the reducing
agent in photographic developer.
 The sensitivity spec now become a development
centre in exposed crystals enabling them to be
reduced completely to metallic silver during the
development process
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The latent image
The existence on a film of numbers of
silver halide crystals possessing
development centres is said to constitute a
latent image.
 A latent image is an invisible image
formed on a film as a result of exposure to
radiation and which may be made visible
by photographic development
(This explanation of latent image formation
is based on the Gerney-Mott theory)
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Differences between silver halide &
metallic silver
Silver halide
 Can be converted to
soluble compounds by
fixing agents
 Sensitive to light
 Image containing traces
of silver halide undergo
changes when exposed to
light terfore, Image
containing silver halide is
not permanet
Metallic silver
 Unaffected by fixing
agents
 Not sensitive to light
 Image containing only
metallic silver is
permanent
 Opaque to light and is
visible as darkened area
against a light
background
Photographic emulsion
Photosensitive layer containing silver
halide suspended in gelatin (on an x-ray
film) is called a photographic emulsion
 It is coated on a transparent base
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Properties of gelatin
Exists either as a liquid or as a solid jelly
 Can be transformed from one state to the
other
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Functions of gelatin as the emulsion binder
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Acts as the medium for formation of silver halide
crystals during chemical production
Maintains the uniform distribution of silver halide
in the liquid emulsion
Does not react chemically with silver halides
It allows the film base to be coated evenly with
warm liquid emulsion, which is then chilled and
allowed to set and dried
In solid state , it can be wetted and then allows
penetration by the chemical agents.
Holds firmly in position the metallic silver
particles
Provides a transparent medium, enabling
viewing of the image
Production of photographic
emulsion
Silver halide (bromide) is the product of
chemical reaction between silver nitrate and an
alkali halide (potassium bromide)
 The production of emulsion and manufacture of
x-ray film are highly complex process
 The entire procedure is carried out in conditions
of absolute cleanliness, with temperature and
humidity closely controlled, all light excluded in a
dust free environment.
 Rigorous quality control is maintained
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Steps in production
1.
2.
Solutions of silver nitrate and potassium
bromide (and other halides) are added at
controlled rates and in measured quantities to
liquid gelatin.
On mixing, it produces potassium nitrate (in
solution) and a precipitate of tiny insoluble
crystals or grains of silver bromide dispersed
uniformly through the gelatin
(the rate of mixing governs the film
characteristics)
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3.
(If mix rapidly all the grains produced will
be of roughly equal size – narrow grain
size distribution. It produces high contrast
characteristics.)
(if mixed slowly the grains produced early
will grow larger than those produced later.
This results in a wide grain size
distribution and lower contrast
characteristics.)
Unwanted potassium nitrate is removed
by allowing it to set, shredding it and
washing in water
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The gel is re-liquified and repeatedly
heated and cooled (ripening & digestion)
to grow in size and to allow sensitivity
specs to form. The size of grains
determines the speed of the emulsion.
Finally, prior to coating onto the film
base, other agents are added. E.g.;
sensitizers
antifrothing agents
plasticizers, hardeners, wetting agents,
antifoggants, bacterizides, fungicides
The coating process
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The liquid emulsion is coated onto a transparent
polyester base (film)
To aid adhesion the base is pre-coated with a
thin subbing layer (substratum)
Constant thickness emulsion should be
maintained
Once the liquid emulsion has been applied it is
allowed to set firmly
Then the thin protective supercoat of pure
gelatin is applied
Usually for x-ray films the emulsion is coated on
both sides of the base
Coating is a continuous process and the
completed product is wound onto large rolls
Finally it is cut into different sizes and packed
Light sensitive & x-ray sensitive
emulsions
There are two types of films according to
the exposure conditions
 Films exposed to light emitted from
intensifying screens or cathode ray tubes or
image intensifiers
 Films exposed solely to direct x-radiation
Light - sensitive films
(All films are sensitive to light. This term refers to the
films exposed with light). They are constructed to
obtain maximum absorption of light photons. This
is achieved by,
1. Close packing of silver halide grains
2. Increasing the size of the halide grains
3. Increasing the thickness of the emulsion layer
4. Modifying the shape of the halide grains
(Methods 2 & 3 has limitations – producing graininess
and reducing resolution)
Light - sensitive films
There are two types
 Duplitized films
 Single-coated (single sided) films
Duplitized films
Emulsion is coated on both sides of the base
 Has the similar benefit as gained by increasing
the emulsion thickness
 Used with two intensifying screens
 Advantages are:
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– increased sensitivity
– Increased image contrast
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Disadvantages are:
– loss of sharpness due to cross over effect
– significant parallax effect
– higher cost
Single-coated (single sided)
emulsion films
Emulsion is coated on one side of the base
 Used with one intensifying screen
 Small notch is cut on one edge to aid
identification of the emulsion side
 Disadvantages associated with duplitized
emulsion is eliminated by using single
emulsion
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Spectral sensitivity
The absorption of lights by the emulsion is
different for various colours in the spectrum.
This is referred to as spectral sensitivity
 Normal silver bromide emulsion is more sensitive
(absorb readily) to blue, violet & ultra violet
light.
 They are known as monochromatic emulsions
 The sensitivity can be changed and extended by
using sensitizers. – orthochromatic &
panchromatic
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Effect of x-rays on screen type film
The sensitivity of screen type film to x-rays is
minimal
 Can be seen by a test –
– cover one half of the film with a black paper
on either side.
– Load it into a cassette
– Give a suitable exposure
– Process the film
 Less than 5% of the image density is
attributable to the direct exposure to x-rays
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Other types of light sensitive film
Films to be exposed with the light emitted
(image) from a Cathode Ray Tube (CRT)
screen
 Fluorographic films recording image on
image intensifier output phosphor
 Laser imager films
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X-ray sensitive (direct exposure)
films
Image is produced direct exposure of the
film to x-rays only
 Sensitivity is increased by
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– Duplitized emulsion
– Increased emulsion thickness
– Using sensitizers
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Used in
– dental radiography
– Radiation monitoring –film badge
END
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