The radiographThe radiograph

Photographic principlesPhotographic principles

ObjectivesObjectives

To examine how invisible x-ray image has To examine how invisible x-ray image has to be recorded and converted into a visible to be recorded and converted into a visible formform

To examine in detail the photographic To examine in detail the photographic effect of radiation on sensitive emulsion effect of radiation on sensitive emulsion

The photographic effectThe photographic effect

This is the principle of traditional method of This is the principle of traditional method of recording the invisible x-ray imagerecording the invisible x-ray image

It uses It uses photo-sensitive materialsphoto-sensitive materials (some chemical (some chemical compounds) which undergo subtle structural compounds) which undergo subtle structural changes when exposed to electromagnetic changes when exposed to electromagnetic radiations such as visible light, ultraviolet radiations such as visible light, ultraviolet radiation or x-rays.radiation or x-rays.

These changes are not immediately visible, but, These changes are not immediately visible, but, They are associated with an alteration in the They are associated with an alteration in the

chemical behaviour of the substance.chemical behaviour of the substance.

The exposed materials respond differently in The exposed materials respond differently in certain chemical reactions.certain chemical reactions.

By careful chemical processing, it is possible to By careful chemical processing, it is possible to differentiate between exposed and unexposed differentiate between exposed and unexposed materials, producing a visible difference between materials, producing a visible difference between them.them.

This creates a visible image.This creates a visible image. The effect on chemical nature of material is The effect on chemical nature of material is

known as known as “Photographic effect” of radiation“Photographic effect” of radiation The chemical processing necessary to make The chemical processing necessary to make

invisible changes visible is called invisible changes visible is called ‘photographic ‘photographic development’development’

Photosensitive materialsPhotosensitive materials

In photography photosensitive materials In photography photosensitive materials used are Silver halides.used are Silver halides.

Radiography is a specialized application of Radiography is a specialized application of the photographic processthe photographic process

Therefore in radiography too silver halides Therefore in radiography too silver halides are used as photosensitive material to are used as photosensitive material to record the imagesrecord the images

Silver halidesSilver halides

Are a group of chemical compounds Are a group of chemical compounds consisting of atoms of element silver consisting of atoms of element silver combined with atoms of halogen elementscombined with atoms of halogen elements– Silver bromideSilver bromide– Silver iodideSilver iodide– Silver chlorideSilver chloride

They are sensitive to light and x-raysThey are sensitive to light and x-rays Silver bromide is commonly usedSilver bromide is commonly used

Physical properties of silver halidesPhysical properties of silver halides

White or pale-yellow crystalline salts White or pale-yellow crystalline salts similar in appearance to common saltsimilar in appearance to common salt

The links between silver and halogen The links between silver and halogen atoms are ‘atoms are ‘ionic bondsionic bonds’.’.

The electrical forces between positively The electrical forces between positively charged silver ions and negatively charged charged silver ions and negatively charged halide ions fix the positions of the ions in a halide ions fix the positions of the ions in a regular three-dimensional crystal structure regular three-dimensional crystal structure or latticeor lattice

Crystal /lattice of AgXCrystal /lattice of AgX

Chemical properties of Silver halideChemical properties of Silver halide Pure silver halide crystals are relatively stable Pure silver halide crystals are relatively stable

and do not suffer chemical breakdownand do not suffer chemical breakdown But under certain conditions it is possible to But under certain conditions it is possible to

convert silver ions to atoms of metallic silver by convert silver ions to atoms of metallic silver by donating electronsdonating electrons

The agents that supply electrons for this process The agents that supply electrons for this process are called reducing agents.are called reducing agents.

The action is chemical reduction of silver halide The action is chemical reduction of silver halide to silverto silver

Chemical reduction takes place rapidly on Silver Chemical reduction takes place rapidly on Silver halide crystals that have been exposed to halide crystals that have been exposed to radiation than those are notradiation than those are not

This is the key feature of photographic This is the key feature of photographic processingprocessing

Effects of exposure on silver halideEffects of exposure on silver halide

Silver ion

Bromine ion

e

Sensitivity spec

X-ray photon

X-ray photon

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 Some X-ray photons pass through without any interaction any interaction

Some photons interact with bromine ions Some photons interact with bromine ions and release an electronand release an electron

The electron moves around the crystalThe electron moves around the crystal In a short time it In a short time it logesloges in a low energy in a low energy

electron trapelectron trap near the surface of the near the surface of the crystalcrystal

This low energy electron trap is called a This low energy electron trap is called a ‘‘sensitivity specsensitivity spec’’

Sensitivity specs are formed by deliberate Sensitivity specs are formed by deliberate introduction of introduction of ‘impurities‘impurities’ into the crystal ’ into the crystal during its manufactureduring its manufacture

As it collects more and more electrons As it collects more and more electrons sensitivity specssensitivity specs acquire a acquire a negative chargenegative charge

When this negative charge becomes When this negative charge becomes strong enough some positively charge strong enough some positively charge silver ions are drawn towards it.silver ions are drawn towards it.

As they come the silver ions each gain an As they come the silver ions each gain an electron to become a neutral silver atom electron to become a neutral silver atom (metalic silver)(metalic silver)

Significance of formation of metallic silverSignificance of formation of metallic silver

In normal circumstances only a few hundreds of In normal circumstances only a few hundreds of silver atoms are formedsilver atoms are formed

It is inadequate to produce a visible change in It is inadequate to produce a visible change in the crystalthe crystal

But the tiny collection of silver atoms renders But the tiny collection of silver atoms renders the crystal much more vulnerable than the crystal much more vulnerable than unexposed crystals to attack by the reducing unexposed crystals to attack by the reducing agent in photographic developer.agent in photographic developer.

The sensitivity spec now become a development The sensitivity spec now become a development centre in exposed crystals enabling them to be centre in exposed crystals enabling them to be reduced completely to metallic silver during the reduced completely to metallic silver during the development processdevelopment process

The latent imageThe latent image The existence on a film of numbers of The existence on a film of numbers of

silver halide crystals possessing silver halide crystals possessing development centresdevelopment centres is said to constitute a is said to constitute a latent imagelatent image..

A latent imageA latent image is is an invisible imagean invisible image formed on a film as a result of exposure to formed on a film as a result of exposure to radiation and which may be made visible radiation and which may be made visible by photographic developmentby photographic development

(This explanation of latent image formation (This explanation of latent image formation is based on the Gerney-Mott theory)is based on the Gerney-Mott theory)

Differences between silver halide & Differences between silver halide & metallic silvermetallic silver

Silver halideSilver halide Can be converted to Can be converted to

soluble compounds by soluble compounds by fixing agentsfixing agents

Sensitive to lightSensitive to light Image containing traces Image containing traces

of silver halide undergo of silver halide undergo changes when exposed to changes when exposed to light terfore, Image light terfore, Image containing silver halide is containing silver halide is not permanetnot permanet

Metallic silverMetallic silver Unaffected by fixing Unaffected by fixing

agentsagents Not sensitive to lightNot sensitive to light Image containing only Image containing only

metallic silver is metallic silver is permanentpermanent

Opaque to light and is Opaque to light and is visible as darkened area visible as darkened area against a light against a light backgroundbackground

Photographic emulsionPhotographic emulsion

Photosensitive layer containing silver Photosensitive layer containing silver halide suspended in gelatin (on an x-ray halide suspended in gelatin (on an x-ray film) is called a photographic emulsionfilm) is called a photographic emulsion

It is coated on a transparent baseIt is coated on a transparent base

Properties of gelatinProperties of gelatin Exists either as a liquid or as a solid jellyExists either as a liquid or as a solid jelly Can be transformed from one state to the Can be transformed from one state to the

otherother

Functions of gelatin as the emulsion binderFunctions of gelatin as the emulsion binder Acts as the medium for formation of silver halide Acts as the medium for formation of silver halide

crystals during chemical productioncrystals during chemical production Maintains the uniform distribution of silver halide Maintains the uniform distribution of silver halide

in the liquid emulsionin the liquid emulsion Does not react chemically with silver halidesDoes not react chemically with silver halides It allows the film base to be coated evenly with It allows the film base to be coated evenly with

warm liquid emulsion, which is then chilled and warm liquid emulsion, which is then chilled and allowed to set and driedallowed to set and dried

In solid state , it can be wetted and then allows In solid state , it can be wetted and then allows penetration by the chemical agents.penetration by the chemical agents.

Holds firmly in position the metallic silver Holds firmly in position the metallic silver particlesparticles

Provides a transparent medium, enabling Provides a transparent medium, enabling viewing of the imageviewing of the image

Production of photographic Production of photographic emulsionemulsion

Silver halide (bromide) is the product of Silver halide (bromide) is the product of chemical reaction between silver nitrate and an chemical reaction between silver nitrate and an alkali halide (potassium bromide)alkali halide (potassium bromide)

The production of emulsion and manufacture of The production of emulsion and manufacture of x-ray film are highly complex process x-ray film are highly complex process

The entire procedure is carried out in conditions The entire procedure is carried out in conditions of absolute cleanliness, with temperature and of absolute cleanliness, with temperature and humidity closely controlled, all light excluded in a humidity closely controlled, all light excluded in a dust free environment.dust free environment.

Rigorous quality control is maintainedRigorous quality control is maintained

Steps in productionSteps in production

Solutions of silver nitrate and potassium Solutions of silver nitrate and potassium bromide (and other halides) are added at bromide (and other halides) are added at controlled rates and in measured quantities to controlled rates and in measured quantities to liquid gelatin.liquid gelatin.

On mixing, it produces potassium nitrate (in On mixing, it produces potassium nitrate (in solution) and a precipitate of tiny insoluble solution) and a precipitate of tiny insoluble crystals or crystals or grainsgrains of silver bromide dispersed of silver bromide dispersed uniformly through the gelatinuniformly through the gelatin(the rate of mixing governs the film (the rate of mixing governs the film characteristics)characteristics)

(If (If mix rapidlymix rapidly all the grains produced will all the grains produced will be of roughly equal size – be of roughly equal size – narrow grain narrow grain size distribution.size distribution. It It produces produces high contrast high contrast characteristics.)characteristics.)

(if (if mixed slowlymixed slowly the grains produced early the grains produced early will grow larger than those produced will grow larger than those produced later. This results in a later. This results in a wide grain sizewide grain size distributiondistribution and and lower contrast lower contrast characteristics.)characteristics.)

3.3. Unwanted potassium nitrate is removed Unwanted potassium nitrate is removed by allowing it to set, shredding it and by allowing it to set, shredding it and washing in waterwashing in water

1.1. The gel is re-liquified and repeatedly The gel is re-liquified and repeatedly heated and cooled (ripening & digestion) heated and cooled (ripening & digestion) to grow in size and to allow sensitivity to grow in size and to allow sensitivity specs to form. The size of grains specs to form. The size of grains determines the speed of the emulsion.determines the speed of the emulsion.

2.2. Finally, prior to coating onto the film Finally, prior to coating onto the film base, other agents are added. E.g.;base, other agents are added. E.g.;

sensitizerssensitizersantifrothing agentsantifrothing agentsplasticizers, hardeners, wetting agents, plasticizers, hardeners, wetting agents, antifoggants, bacterizides, fungicidesantifoggants, bacterizides, fungicides

The coating processThe coating process The liquid emulsion is coated onto a transparent The liquid emulsion is coated onto a transparent

polyester base (film)polyester base (film) To aid adhesion the base is pre-coated with a To aid adhesion the base is pre-coated with a

thin subbing layer (substratum)thin subbing layer (substratum) Constant thickness emulsion should be Constant thickness emulsion should be

maintainedmaintained Once the liquid emulsion has been applied it is Once the liquid emulsion has been applied it is

allowed to set firmlyallowed to set firmly Then the thin protective supercoat of pure Then the thin protective supercoat of pure

gelatin is appliedgelatin is applied Usually for x-ray films the emulsion is coated on Usually for x-ray films the emulsion is coated on

both sides of the baseboth sides of the base Coating is a continuous process and the Coating is a continuous process and the

completed product is wound onto large rollscompleted product is wound onto large rolls Finally it is cut into different sizes and packedFinally it is cut into different sizes and packed

Light sensitive & x-ray sensitive Light sensitive & x-ray sensitive emulsionsemulsions

There are two types of films according to There are two types of films according to the exposure conditionsthe exposure conditions

Films exposed to light emitted from Films exposed to light emitted from intensifying screens or cathode ray tubes or intensifying screens or cathode ray tubes or image intensifiersimage intensifiers

Films exposed solely to direct x-radiationFilms exposed solely to direct x-radiation

Light - sensitive filmsLight - sensitive films(All films are sensitive to light. This term refers to the (All films are sensitive to light. This term refers to the

films exposed with light). They are constructed to films exposed with light). They are constructed to obtain maximum absorption of light photons. This obtain maximum absorption of light photons. This is achieved by,is achieved by,

2.2. Close packing of silver halide grainsClose packing of silver halide grains3.3. Increasing the size of the halide grainsIncreasing the size of the halide grains4.4. Increasing the thickness of the emulsion layerIncreasing the thickness of the emulsion layer5.5. Modifying the shape of the halide grainsModifying the shape of the halide grains(Methods 2 & 3 has limitations – producing graininess (Methods 2 & 3 has limitations – producing graininess

and reducing resolution)and reducing resolution)

Light - sensitive filmsLight - sensitive films

There are two typesThere are two types Duplitized filmsDuplitized films Single-coated (single sided) filmsSingle-coated (single sided) films

Duplitized filmsDuplitized films Emulsion is coated on both sides of the baseEmulsion is coated on both sides of the base Has the similar benefit as gained by increasing Has the similar benefit as gained by increasing

the emulsion thicknessthe emulsion thickness Used with two intensifying screensUsed with two intensifying screens AdvantagesAdvantages are: are:

– increased sensitivityincreased sensitivity– Increased image contrastIncreased image contrast

DisadvantagesDisadvantages are: are: – loss of sharpness due to cross over effectloss of sharpness due to cross over effect– significant parallax effectsignificant parallax effect– higher costhigher cost

Single-coated (single sided) Single-coated (single sided) emulsion filmsemulsion films

Emulsion is coated on one side of the baseEmulsion is coated on one side of the base Used with one intensifying screenUsed with one intensifying screen Small notch is cut on one edge to aid Small notch is cut on one edge to aid

identification of the emulsion sideidentification of the emulsion side Disadvantages associated with duplitized Disadvantages associated with duplitized

emulsion is eliminated by using single emulsion is eliminated by using single emulsionemulsion

Spectral sensitivitySpectral sensitivity

The absorption of lights by the emulsion is The absorption of lights by the emulsion is different for various colours in the spectrum. different for various colours in the spectrum. This is referred to as spectral sensitivityThis is referred to as spectral sensitivity

Normal silver bromide emulsion is more sensitive Normal silver bromide emulsion is more sensitive (absorb readily) to blue, violet & ultra violet (absorb readily) to blue, violet & ultra violet light.light.

They are known as They are known as mmononochroochromaticmatic emulsions emulsions The sensitivity can be changed and extended by The sensitivity can be changed and extended by

using sensitizers. – using sensitizers. – orthochromaticorthochromatic & & papancnchrhromomataticic

Effect of x-rays on screen type filmEffect of x-rays on screen type film The sensitivity of screen type film to x-rays is The sensitivity of screen type film to x-rays is

minimalminimal Can be seen by a test – Can be seen by a test –

– cover one half of the film with a black paper cover one half of the film with a black paper on either side.on either side.

– Load it into a cassetteLoad it into a cassette– Give a suitable exposureGive a suitable exposure– Process the filmProcess the film

Less than 5% of the image density is Less than 5% of the image density is attributable to the direct exposure to x-raysattributable to the direct exposure to x-rays

Other types of light sensitive filmOther types of light sensitive film

Films to be exposed with the light emitted Films to be exposed with the light emitted (image) from a Cathode Ray Tube (CRT) (image) from a Cathode Ray Tube (CRT) screenscreen

Fluorographic films recording image on Fluorographic films recording image on image intensifier output phosphorimage intensifier output phosphor

Laser imager filmsLaser imager films

X-ray sensitive (direct exposure) X-ray sensitive (direct exposure) filmsfilms

Image is produced direct exposure of the Image is produced direct exposure of the film to x-rays onlyfilm to x-rays only

Sensitivity is increased bySensitivity is increased by– Duplitized emulsionDuplitized emulsion– Increased emulsion thicknessIncreased emulsion thickness– Using sensitizersUsing sensitizers

Used in Used in – dental radiographydental radiography– Radiation monitoring –film badgeRadiation monitoring –film badge

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