Latent Image formation in radiographic imaging (0).pptx

How the latent image is being produced after exposure

1. Rad. Imaging PHOTOCHEMISTRY

2. THE FORMATION OF LATENT IMAGE The production of latent image is the 1 st stage of the photochemistry process It is here that all the information eventually seen is recorded as minute changes in the silver bromide crystal These changes are so small that even an electron microscope reveals no difference btw an expose crystal & an unexposed crystal, and only development will show which is which Even with modern computer techniques of image manipulation, if the information is not in the latent image, no amount of further processing will increase or reveal more detail, although it may improve certain areas of particular interest Definition of latent image The literal meaning of latent is ‘hidden’ but ‘hidden image’ is inadequate as definition of ‘latent image’, which is more accurately defined as: The image produced on the film after exposure but prior to development

3. THE FORMATION OF LATENT IMAGE An understanding of latent image formation gives: 1) A full understanding of the development & fixation process 2) A better appreciation of the factors governing quality, e.g . basic fog An increased appreciation of the effects of x-radiation & light in relation to exposure & the final result

4. MECHANISM OF LATENT IMAGE FORMATION There are 2 main theories of latent image formation (1) Gurney Mott (classical) theory (2) Mitchell (modern) theory Both theories are based on the partial reduction of silver bromide crystals during exposure This partial reduction is achieved by the free interstitial silver ions (Ag + ) gaining an electron from some source (usually exposure to light, x-rays or both) EFFECTS OF EXPOSURE The desired result of exposure as far as latent image is concerned is the release of an electron within the silver bromide crystals lattice

5. MECHANISM OF LATENT IMAGE FORMATION Exposure of any substance to a source containing the correct wavelengths and energy produces 3 effects: 1) Compton effect 2) Photoelectric effect 3) Pair production In the x-ray to blue range (1-430nm) used to expose conventional unsensitized film (monochromatic film), only the first 2 effects in the silver bromide crystal Of these 2, the photoelectric effects predominates, as silver bromide is an ionic compound & has very few of the unbound electrons that are necessary for the Compton effect to occur Approx. 96% of the electrons produced are from the photoelectric effect In any case, both effects release a high kinetic energy electron that can be used in latent image formation Unfortunately, not all electrons remain useful

6. CUBICAL SILVER BROMIDE CRYSTAL LATTICE

7. Energy of the released electron The energy of electrons released by exposure is important, as to be useful in image formation, they must be able to enter the conduction band of the silver bromide crystal This band is one of the following 3 energy bands that exist in metallic crystalline solids: 1) Valency band 2) Forbidden gap 3 ) Conduction band

8. THEORIES OF LATENT IMAGE FORMATION The 2 principal theories are: Gurney Mott and the Mitchell Both theories suggest that it is a 2-stage process involving nucleation & growth B oth have general agreement on the growth process but differ on the mechanism of the nucleation process

9. Gurney Mott theory Nucleation Stage 1: An electron that has been released by exposure is captured by a trapping centre & temporarily held A mobile, positively charged silver ion may migrate to this centre & form a silver atom Stage 2: Before the arrival of this silver ion, the electron may escape from the trap due to its vibrational energy & again become free in the conduction band Eventually it becomes trapped long enough to be joined by a silver ion giving a silver atom … Stage 3: Unfortunately, a single silver atom is unstable, separation occurs & stages 1 & 2 happen again Stage 4: fortunately, during its short life the single silver atom can act as a trap for another electron, and if another silver ion is attracted to this site Stage 5: A stable 2-atom speck, called a latent sub-image centre is formed

10. Mitchell theory Nuleation Stages 1 & 2: A free silver ion comes near to a shallow electron trap & deepens it While this trap is deepened, a free electron released by exposure, and another free silver ion approach the trap & immediately form a silver atom This is called pre-image centre Stage 3: Unfortunately, as in Gurney Mott theory, the single silver atom is unstable & it dissociates into a silver ion and an electron Stages 4 & 5: Stages 1 & 2 recur & during its life the single silver atom does not act as a trap for a further electron but must acquire a second silver ion If it is successful and an electron arrives before the escape of this second silver ion, a stable latent sub-image centre forms

11. LATENT IMAGE FORMATION

12. LATENT IMAGE FORMATION

13. LATENT IMAGE FORMATION

14. BOTH THEORIES Growth Both theories consider that this is due to the deepening of the electron traps, because of the presence of the latent sub-image centre, increasing the attraction of that site compared to the shallow trap The increasing electrical attraction of these points in the crystal attracts the electrons from exposure & the silver ions from the crystal lattice, or from the supply of free interstitial ions, causing a build-up of silver atoms Many sub-image centres are formed throughout the crystal on one single exposure, but growth does not proceed in a preferential way as the 1 st formed sub-image centres tend to exhibit greater electrical attraction

15. BOTH THEORIES During their growth, the ever enlarging silver speck start to invade the surface of the crystal This reduces the surface potential barrier by destroying the crystal lattice in the area, therefore rendering the crystal more susceptible to development When growth is complete, the sub-image centres are commonly called development centre

16. FURTHER EXPLANATION ON LATENT IMAGE FORMATION EFFECTS OF EXPOSURE ON SILVER HALIDES Q. What happens when silver halide is irradiated? 1) X-ray photon or visible light penetrating through a crystal of AgBr (silver bromide) 2) The photon is absorbed in the crystal & interacts with a bromine ion causing it to releases electron 3) The electron is then free to move about inside the crystal but within a very short time (10 -11 s) 4) It becomes lodged in a low energy electron trap near the surface of the crystal known as sensitivity speck or sensitivity site

17. FURTHER EXPLANATION ON LATENT IMAGE FORMATION NB: Sensitivity speck are produced by a deliberate introduction of traces of impurities into the silver halide during its manufacture 5) Each sensitivity speck requires more & more negative electron charge & as the charge becomes strong enough, it drives towards it those positively charged silver ions in the crystals which are not firmly held in the crystal 6) As the silver ions arrives they each gains an electron which neutralises their charge & transforms them from silver ion into atoms of metalic silver(Ag ppt )

18. FURTHER EXPLANATION ON LATENT IMAGE FORMATION Significance of the formation of metallic silver In normal circumstances, the number of Ag atoms created amounts only to a few 100/crystal, which is certainly too few to make a noticeable difference to the appearance of the crystals However, this tiny collection of silver atoms has a critical effect on the chemical behaviour of the silver halide crystal In particular, it renders the crystal much more vulnerable than unexposed crystals to attack by the reducing agent in the photographic developer That is why the sensitivity speck or sub-image centre is called development centre in exposed crystals enabling them to be reduced completely to silver during the development process

19. FURTHER EXPLANATION ON LATENT IMAGE FORMATION Difference between silver halide & metallic silver The conversion of metallic Ag of exposed parts of radiographic image is important because the 2 substances have different properties 1) Silver halides can be converted into soluble compound by the action of chemicals known as fixing agent Metallic silver is unaffected by fixing agent & remain insoluble, this provide a means of separating silver halide from metallic silver 2) Metallic silver is opaque to light & its presence can easily be seen as a darkened area against a light background

20. FURTHER EXPLANATION ON LATENT IMAGE FORMATION The blackened part of radiographic image are formed from metallic Ag 3) Silver halides are sensitive to light while metallic silver is not An image which contain traces of silver halide can be expected to undergo changes if exposed to light & such image would not be permanent However, an image formed entirely by the presence of metallic silver will no be affected by light & is essentially permanent