The radiographThe radiograph

Photographic principlesPhotographic principles

ObjectivesObjectives

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

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

The photographic effectThe photographic effect

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

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

These changes are not immediately These changes are not immediately visible, but, visible, but,

They are associated with an alteration in They are associated with an alteration in the chemical behaviour of the substance.the chemical behaviour of the substance.

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

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

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

is known as is known as “Photographic effect” of “Photographic effect” of radiationradiation

The chemical processing necessary to The chemical processing necessary to make invisible changes visible is called make invisible changes visible is called ‘photographic development’‘photographic development’

Photosensitive materialsPhotosensitive materials

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

Radiography is a specialized Radiography is a specialized application of the photographic application of the photographic processprocess

Therefore in radiography too silver Therefore in radiography too silver halides are used as photosensitive halides are used as photosensitive material to record the imagesmaterial to record 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 combined with atoms of halogen elementselements– 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 Physical properties of silver halideshalides

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

Crystal /lattice of AgXCrystal /lattice of AgX

Chemical properties of Silver Chemical properties of Silver halidehalide

Pure silver halide crystals are relatively Pure silver halide crystals are relatively stable and do not suffer chemical stable and do not suffer chemical breakdownbreakdown

But under certain conditions it is possible But under certain conditions it is possible to convert silver ions to atoms of metallic to convert silver ions to atoms of metallic silver by donating electronssilver by donating electrons

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

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

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

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

Effects of exposure on silver Effects of exposure on silver halidehalide

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

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

The electron moves around the The electron moves around the crystalcrystal

In a short time it In a short time it logesloges in a low in a low energy energy electron trapelectron trap near the surface near the surface of the crystalof the crystal

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

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

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

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

Significance of formation of metallic Significance of formation of metallic silversilver

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

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

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

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

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 is said to constitute a 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 formed on a film as a result of exposure to radiation and which may be made to radiation and which may be made visible by photographic developmentvisible by photographic development

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

Differences between silver halide Differences between silver halide & metallic silver& metallic 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 Image containing

traces of silver halide traces of silver halide undergo changes undergo changes when exposed to light when exposed to light terfore, Image terfore, Image containing silver containing silver halide is not permanethalide is not 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 visible as darkened area against a light area 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 Can be transformed from one state to

the otherthe other

Functions of gelatin as the emulsion Functions of gelatin as the emulsion binderbinder

Acts as the medium for formation of silver Acts as the medium for formation of silver halide crystals during chemical productionhalide crystals during chemical production

Maintains the uniform distribution of silver Maintains the uniform distribution of silver halide in the liquid emulsionhalide in the liquid emulsion

Does not react chemically with silver Does not react chemically with silver halideshalides

It allows the film base to be coated evenly It allows the film base to be coated evenly with warm liquid emulsion, which is then with warm liquid emulsion, which is then chilled and allowed to set and driedchilled and allowed to set and dried

In solid state , it can be wetted and then In solid state , it can be wetted and then allows penetration by the chemical agents.allows 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 chemical reaction between silver nitrate and an alkali halide (potassium bromide)an alkali halide (potassium bromide)

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

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

Rigorous quality control is maintainedRigorous quality control is maintained

Steps in productionSteps in production

1.1. 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 controlled rates and in measured quantities to liquid gelatin.quantities to liquid gelatin.

2.2. On mixing, it produces potassium nitrate On mixing, it produces potassium nitrate (in solution) and a precipitate of tiny (in solution) and a precipitate of tiny insoluble crystals or insoluble crystals or grainsgrains of silver of silver bromide dispersed uniformly through the bromide dispersed uniformly through the gelatingelatin

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

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

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

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

5.5. 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.;

sensitizerssensitizers

antifrothing agentsantifrothing agents

plasticizers, hardeners, wetting plasticizers, hardeners, wetting agents, agents, antifoggants, bacterizides, antifoggants, bacterizides, fungicidesfungicides

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

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

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

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

is allowed to set firmlyis allowed 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 Usually for x-ray films the emulsion is

coated on both sides of the basecoated on both 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 Finally it is cut into different sizes and

packedpacked

Light sensitive & x-ray Light sensitive & x-ray sensitive emulsionssensitive emulsions

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

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

Films exposed solely to direct x-Films exposed solely to direct x-radiationradiation

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

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

1.1. Close packing of silver halide grainsClose packing of silver halide grains

2.2. Increasing the size of the halide grainsIncreasing the size of the halide grains

3.3. Increasing the thickness of the emulsion Increasing the thickness of the emulsion layerlayer

4.4. Modifying the shape of the halide grainsModifying the shape of the halide grains

(Methods 2 & 3 has limitations – producing (Methods 2 & 3 has limitations – producing graininess and reducing resolution)graininess 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 Emulsion is coated on both sides of the

basebase Has the similar benefit as gained by Has the similar benefit as gained by

increasing the emulsion thicknessincreasing the 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 Emulsion is coated on one side of the basebase

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 Disadvantages associated with

duplitized emulsion is eliminated by duplitized emulsion is eliminated by using single emulsionusing single emulsion

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

They are known as They are known as mmononochroochromaticmatic emulsionsemulsions

The sensitivity can be changed and The sensitivity can be changed and extended by using sensitizers. – extended by using sensitizers. – orthochromaticorthochromatic & & papancnchrhromomataticic

Effect of x-rays on screen type Effect of x-rays on screen type filmfilm

The sensitivity of screen type film to x-rays The sensitivity of screen type film to x-rays is minimalis minimal

Can be seen by a test – Can be seen by a test – – cover one half of the film with a black cover one half of the film with a black

paper on either side.paper 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-attributable to the direct exposure to x-raysrays

Other types of light sensitive Other types of light sensitive filmfilm

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

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

Laser imager filmsLaser imager films

X-ray sensitive (direct X-ray sensitive (direct exposure) filmsexposure) films

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