 Mudevan and Dr. Indanil, associate professor in GSM Madhav College, Rajmande. Today, I will be discussing about the Gandhi-Mott theory that explains how a latent image is formed on an X-ray film. So much has developed in the field of radiology for the past two decades. There has been drastic improvement, evolution. And even in the field of radiographic film as well, there has been drastic improvements. Previously, we used to have these conventional X-ray films wherein the image processing, the film processing has to be done by wet chemicals and which then forms a macroscopic visible image on the film. But before these macroscopic image information is obtained on the film, there is a process called latent image formation where when the X-ray protons, they strike the patient and then strike the image receptor and form the image on the receptor, it is not immediately visible to our naked eye. So, the information is stored on the film image receptor but it is not visible to the naked eye. This has to be processed either wet or dry processing in order for the macroscopic image to be seen. Why we are going to learn about this process is the film is the major source of archiving which is given to the patient predominantly. Now with the advent of CRMDM, archiving is done in the department in the computer itself. But still the record, the hard copy has to be handed over to the patient. So, it is important to learn how the latent image is formed in order to understand how the macroscopic image is processed. So, today I am going to explain about the Garni-Mott hypothesis which explains how the latent image or the microscopic image which stores the information of the patient is formed on the film. Today we will be discussing the Garni-Mott theory which explains the formation of latent image on the radiographic film after it is exposed to X-rays or the light photons. Before explaining the details, I would like to introduce a little bit, I would like to talk a little bit about the silver halide crystals that are present in the radiographic emulsion. So, these are the crystals that are responsible to form the latent image and participate in the chemical reactions. The silver halide emulsion which is present in the X-ray film consists of 90 to 99 percent silver bromide and a mixture of 1 to 10 percent silver iodide. Why silver iodide is added? Because it increases the sensitivity of the emulsion. These crystals of silver halide are formed with a polar covalent bond and they have strong ionic character. The size of the crystals ranges from 1 to 1.5 microns and there are various shapes that can be manufactured. They may be either tabular octahedral polyhedral but out of all these tabular crystals are more preferred because they have larger surface area and they increase the covering power. This silver halide crystal is not a perfect crystal and defects are present inside the crystal. This is what makes the crystal sensitive to the latent image formation. This is what if it's a perfect crystal it does not participate in the reactions. These defects may be a point defect also called as a Frankel's defect wherein the silver ions which are present inside the cubic lattice they may dislodge, dislocate from their place and move a Rome about freely within the cubic lattice. Another defect is called a dislocation defect where the structure of the bromide and iodide ions they are of unequal sizes and so the construction of the wall may not be regular so I will show you in the diagram what it means. This crystal is further sensitized by process called chemical sensitization by adding a sulfur containing compound called allyl thiouria. This reacts with the silver halide and forms silver sulfide which sits on the surface of the crystal. This is the main basis for the formation of what is called the sensitivity spec. This is where the whole reaction or the process takes place. This is the cubic structure of the silver iodobromide crystal. The small black pearls are the silver ions the reddish ones the reddish brown ones are the bromide ions and you can see the bigger iodide ions and the silver sulfide sensitive spec which is sitting on the surface of the crystal. You can notice here that the size in the line suppose imagine that this is a brick wall the size of these bricks are not equal the iodide ion is a little bigger than the bromide ion. So this is what makes the wall weaker the construction is a little weaker and this makes it more susceptible to the reaction it makes it more sensitive. This is called dislocation defect and point defect as I will show you now. You can see the silver ion has moved away from its place and it is roaming freely it is called the interstitial silver ion which is roaming freely inside the cubicle lattice. So these two dislocation and point defects are the things that are responsible for the increased sensitivity of the crystal. So it is not a perfect crystal. Now observe the processor of the Garni theory how the latent image takes place formation takes place. When a light photon or x-ray photon bombards the bromide ion this bromide ion has an extra electron within it and this surplus electron is dislodged on the bromide ion and this electron roams inside this free electron roams inside the crystal until it is trapped by the sensitivity spec. The sensitivity spec acts as an electron trap and it gets lodged there. After giving away this electron this bromide ion which has a negative charge now transforms into a neutral bromide atom and this gets dissolved within the solution processing solution. Now you can see that the electron is trapped inside this sensitivity spec this imparts a net negative charge to this sensitivity spec center here. This has got a net negative charge here due to the lodging of the electron. Previously we talked about the roaming silver ion which is free to roam inside this crystal. This silver ion has got a positive charge. So as you know unlike charges attract each other this positive silver ion gets attracted to the electron which is lodged within the sensitivity spec. Immediately as it combines with the negative electron this forms a neutral silver atom. So this positive silver ion combines with the electron to form a one neutral atom of silver. Again this process repeats itself as another photon strikes another bromide ion and dislodges the electron which again roams inside the lattice until again it is attracted or lodged or caught by the electron trap that is the sensitivity spec. Again there is a net negative charge imparted here and another silver interstitial silver ion which is roaming inside gets attracted again into the sensitivity spec and again forms a neutral atom of silver. Thus here we have seen that there are two neutral atoms of silver formed due to this procedure. Practically we need at least three to four atoms of silver in order for the latent latent image center to be effective for the further processing and developing. So as you can see this is another graphic representation. This is a imagine this as a crystal silver idobromide crystal and there is a sensitivity spec here. The outer surface of the cubic lattice has a net negative charge because the bromide ions are more concentrated on the outer surface and this is the sensitivity spec sitting on the surface of the crystal. After two three or four clumps of silver atoms are formed here neutral silver atoms as it combines with the electron it performs a small microscopic clump of silver. This is called latent image because it cannot be seen with the naked eye even with the routine microscopes also it is not visible. So it is the information is hidden there but it cannot be seen with the naked eye from this stage it has to be processed called as chemical processing. In conventional films we use the wet processing technique and nowadays we are using a dry processing in the printers. So this step is again amplified further in the factor of about 100,000 times to get the visible image formation. So this microscopic clump of silver grows and grows until it becomes a macroscopic clump of silver. Now you should know that pure silver is black pure metallic silver is black. So this is the mid contrary to the jewelry which we see which shines brightly that is due to the polishing that is given to the jewelry it appears so bright and white but pure metallic silver appears black. So this is the blackness imparted to the radiographic film which gives rise to that property of the film. So the blackness is due to the pure metallic silver clumped silver that grows after the development process. Thank you.