Image Characteristics

• When X rays hit the film,the Ag halide get exposed in that area.When this film is processed(developer solution) the Exposed Ag halide gets converted into Ag granules.These granules block the passage of light from the View box,making this area radiolucent.
• This overall degree of darkening of exposed area of film is called as Radio graphic density.
• Radio graphic density depends on-

1. Number of photons hitting the exposed area of film-more number of photons means more darkness,more radio graphic density.
2. Subject thickness-If a kid is exposed to the same amount of tube current,voltage and exposure time as used to take cardiographs for an adult than the radio graphic density would be more.
3. subject density-Enamel,dentin,cementum,bone,muscle,fat,air

• Radio graphic contrast is a term that describes the range of radio graphic densities on a radio graph.
• Radio graphic Speed refers to the amount of exposure needed to produce a standard amount of radio graphic density.Film speed is controlled largely by the size of the silver halide grains and their silver content.
• Radio graphic speed is resembled by letters C;D;E;F.F is the ultra speed films requiring least amount of radiation for producing same amount of radio graphic density as that produced in C at the same exposure level.
• Although film speed can also be increased by processing films at slightly higher temperature
• When an unexposed film is processed than it also show some amount of radio graphic density( due to the thickness of  base and development of unexposed Ag halide crystals.This is called as Gross fog.Its equal to 0.2 to 0.3.

X ray films

• Composition-Base and Emulsion
• Emulsion consist of silver halide basically Silver Bromide
• Sometimes to increase the speed of the film (discussed latter) Iodide is also added.
• Iodide increases speed of the film because it has got larger size than bromide and therefore it can disrupt the crystal structure of AgBr more easily and quickly
• Direct Exposure films are the ones that are made to get exposed to X rays.Screen films are the ones that are made to get exposed to the visible light(used in extra oral radiography)
• Screen films are used in extra oral radiography.They are placed in between two Intensifying screens which absorbs Xrays.These Intensifying screens contain phosphor layer which absorbs Xrays and than emit the visible light that exposes Ag Halide crystals in the film.

X-Ray TUBE

• An X ray tube is composed of a cathode and an anode situated within an evacuated(vacuum) glass envelope.
• Cathode- Negative charged focusing cup having tungsten filament
• Anode-Copper stem having tungsten target acting as focal spot.Its positively charged.
• Cathode releases electrons towards anode, which is directly proportional to the temperature of the tungsten filament of cathode.
• Current is passed through the cathode tungsten filament to heat it up,this heating produces production of electrons from the tungsten filament.The tube voltage difference between the cathode and anode causes movement of electrons from cathode to anode with a kinetic energy.This electrons hit the anode at focal spot and lose there kinetic energy which forms the X ray photons.
• Focal Spot-Its the area of anode on which electrons from cathode collide.Area of focal spot size is inversely proportional to the image sharpness.To increase the image sharpness therefore area of focal spot should be decreased,But if it is done than area of heat dissipation also decreases.So to overcome this focal spot is bent to an angle of 20 degree due to this actual focal spot size becomes 1mm* 3mm (increases area for heat dissipation) & effective focal spot size becomes 1mm*1mm (increases sharpness) .
• When electrons in the tube flows from cathode to anode than there is a flow of current in side tube called as tube current.
• But to make electrons move from cathode to anode there need to be a voltage difference between anode and cathode,this voltage difference across anode & cathode is called as tube voltage.Tube voltage is amplified by high voltage transformer i.e. 110 V of outside voltage is converted to 60 to100 KV,this causes electron kinetic energy raised up to 60 to 100 Kev.
• Both the tube current and tube voltage can be changed from outside.
• Timer-The electronic timer controls the length of time that high voltage is applied to the tube and therefore the time during which tube current flows & X rays are produced.
• BREMSSTRAHLUNG RADIATION-This radiation is produced when a fast moving electron head on collides with the nucleus of tungsten atom and losses its all kinetic energy (equivalent to the potential difference  between cathode and anode) to produce a photon of maximum energy.More frequently some electrons don't collide target atom head on,rather high speed electrons have a near or wide misses with atomic nuclei. During deflection these electrons loose some of there kinetic energy that is released as photon.These photons produced have energy below that the one produced by head on collision.Thus Bremsstrahlung interaction produces a continuous spectrum of energy.
• Reasons for continuous spectrum-

1. Continuously varying Voltage difference between the target and filament(Half wave rectification).
2. Bombarding electron passes target nucleus at varying distances and thus releasing photon of different energy.
3. Many electrons participate in many bremsstrahlung interactions in target before loosing all energy.Electrons with higher kinetic energy produces more number of photons too.

• CHARACTERISTIC RADIATION-These radiations are produced when electron strike inner orbital electron of target atom.This knock out the target atom electron and at the same time electron from outer orbit jumps to inner orbit after releasing photon,which forms the Characteristic radiation.
• FACTORS CONTROLLING X RAY BEAM-

1. Exposure time-If exposure time is increased than current passed through cathode is more therefore electron produced are more from the filament which produces more photons on collisions with anode.When exposure time is doubled,number of photons produced are also doubled,but the range of photons energy remain unchanged.
2. Tube Current-As the current passing through the cathode is increased,the heat produced also increases that produces more electrons(tube current),which in turn increases the number of photon produced on collision to anode.Tube current is directly proportional to the number of photons produced,keeping other parameter constant .The quantity of radiations produced is equivalent to the product of tube current and exposure time.as far as this product remains constant number of electrons produced will be constant at a given tube voltage.
3. Tube Voltage-As the tube voltage is increased the number of photons produced and the energy of photons produced,both increases.The electrons emitted from cathode move with more kinetic energy when higher tube voltage is used.Thus more photons are produced by bremsstrahlung radiations.

• FILTRATION-Filtration is a process of preferentially removing some low energy photons from the x ray beam,which only lead to undue patient exposure. Filtration is done by using aluminium layer is called as extrinsic filtration.Inherent filtration is provided by insulating oil.Total filtration should be equal to 1.5mm of aluminium layer.
• COLLIMATION-Collimation is a metallic barrier with an aperture in middle used to reduce the size of X ray beam and thereby reducing the patient exposure.When patient gets exposed to x ray than 10% xray reaches the film rest is absorbed by the oral tissues and after absorption they are scattered back.These scattered photons affect the quality of X ray.Thus by collimation production of scattered photons also get reduced,thereby increasing the quality of X ray.Lead is used for collimation.