The radiograph Photographic principles Objectives n To examine

The radiograph Photographic principles

Objectives n To examine how invisible x-ray image has to be recorded and converted into a visible form n To examine in detail the photographic effect of radiation on sensitive emulsion

The photographic effect n n 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.

§ The exposed materials respond differently in n n 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 n In photography photosensitive materials used are Silver halides. n Radiography is a specialized application of the photographic process n Therefore in radiography too silver halides are used as photosensitive material to record the images

Silver halides n 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 n White or pale-yellow crystalline salts similar in appearance to common salt n The links between silver and halogen atoms are ‘ionic bonds’. n 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

Crystal /lattice of Ag. X

Chemical properties of Silver halide n n n 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 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

n Some X-ray photons pass through without any interaction n Some photons interact with bromine ions and release an electron n The electron moves around the crystal n In a short time it loges in a low energy electron trap near the surface of the crystal n This low energy electron trap is called a ‘sensitivity spec’

n Sensitivity specs are formed by deliberate introduction of ‘impurities’ into the crystal during its manufacture n As it collects more and more electrons sensitivity specs acquire a negative charge n When this negative charge becomes strong enough some positively charge silver ions are drawn towards it. n As they come the silver ions each gain an electron to become a neutral silver atom (metalic silver)

Significance of formation of metallic silver n n 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

The latent image n The existence on a film of numbers of silver halide crystals possessing development centres is said to constitute a latent image. n 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)

Differences between silver halide & metallic silver Silver halide n Can be converted to soluble compounds by fixing agents n Sensitive to light n Image containing traces of silver halide undergo changes when exposed to light terfore, Image containing silver halide is not permanet Metallic silver n Unaffected by fixing agents n Not sensitive to light n Image containing only metallic silver is permanent n Opaque to light and is visible as darkened area against a light background

Photographic emulsion n Photosensitive layer containing silver halide suspended in gelatin (on an x-ray film) is called a photographic emulsion n It is coated on a transparent base Properties of gelatin n Exists either as a liquid or as a solid jelly n Can be transformed from one state to the other

Functions of gelatin as the emulsion binder n n n n Acts as the medium 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 n n 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

Steps in production Solutions of silver nitrate and potassium bromide (and other halides) are added at controlled rates and in measured quantities to liquid gelatin. 2. 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) 1.

(If mix rapidly all the grains produced will be of roughly equal size – narrow grain size distribution. It produces high contrast characteristics. ) n (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. ) 3. Unwanted potassium nitrate is removed by allowing it to set, shredding it and washing in water n

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. 5. Finally, prior to coating onto the film base, other agents are added. E. g. ; sensitizers antifrothing agents plasticizers, hardeners, wetting agents, antifoggants, bacterizides, fungicides 4.

The coating process n n n n 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 n Duplitized films n Single-coated (single sided) films

Duplitized films n n 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: – increased sensitivity – Increased image contrast n Disadvantages are: – – – loss of sharpness due to cross over effect significant parallax effect higher cost

Single-coated (single sided) emulsion films n Emulsion is coated on one side of the base n Used with one intensifying screen n Small notch is cut on one edge to aid identification of the emulsion side n Disadvantages associated with duplitized emulsion is eliminated by usingle emulsion

Spectral sensitivity n n 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

Effect of x-rays on screen type film The sensitivity of screen type film to x-rays is minimal n 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 n Less than 5% of the image density is attributable to the direct exposure to x-rays n

Other types of light sensitive film n Films to be exposed with the light emitted (image) from a Cathode Ray Tube (CRT) screen n Fluorographic films recording image on image intensifier output phosphor n Laser imager films

X-ray sensitive (direct exposure) films n Image is produced direct exposure of the film to x-rays only n Sensitivity is increased by – Duplitized emulsion – Increased emulsion thickness – Using sensitizers n Used in – dental radiography – Radiation monitoring –film badge

END Next lesson Describing photographic performance