X – Ray films
a- Intra oral X- ray film:- Chemical composition of X- ray film:- It consist of a sensitized emulsion present on both sides of transparent base. This base made from cellulose acetate while the emulsion made from crystals of silver halides (mainly silver bromides) suspended in gelatin. The silver bromide crystals are sensitive to both light and X- ray photons. (sensitive more to x-ray)Diagram showing the cross-sectional structure of double emulsion radiographic film.
- What will happened during exposure of X- ray film to radiation? X- ray photons interact with electrons of the atoms of the chemical emulsion in the X-ray film so the result is analog image, analog means the image appears identical to the original.The contents of a film packet. A The outer wrapper. B The film. C The sheet of lead foil. D The protective black paper.
Also intra oral film are wrapped by opaque material to prevent light from reaching the film because light photons can activate the silver halides crystals.
We found also a thin sheet of (Lead foil) is usually placed behind the film to *prevent most of secondary radiation that originated in the tissue of the patient behind the film from reaching it. Therefore this lead foil reduce secondary radiation and minimize film fog. In addition *the lead foil absorbs x- ray that have passed through the object and the film so it reduce the exposure of the tissue behind the film. *This foil has a design of (herring bone pattern) which appear on the radiograph when exposed from opposite side.
* X- Ray film type, size and speed:- Intra oral film types:- A - Type I:- Called periapical film used to examine the apical area of the tooth and the surrounding structures, the size of this type include (1.00, 1.0, 1.1 and 1.2).
B – Type II:-Is called bitewing film it used to detect the inter proximal caries and the height of alveolar bone between 2 adjacent teeth.The size include: - (2.00, 2.0, 2.1, 2.2 and 2.3)
C – Type III:-Is called occlusal film that used to demonstrate area larger in dimension than area appear in per apical film. The size include:- is only (3.4).
Screen film:-1. Film emulsion is more sensitive to visible light and more specifically to blue light in the visible light spectrum.2. The size include: - 5Ч7, 8Ч10 and 10Ч12 inches.3. Screen film has 3types:- slow or detail screen, medium or par – speed screen and fast or high – speed screen.4. The screen film placed between 2 fluorescent screen in cassette. These 2 fluorescent screen made from (tiny calcium tungestate crystals). When these crystals exposure to X- Ray the result of this exposure is a creation of blue light this light in turn exposes the screen film to produce the image.
Film-Screen Combinations• Screens and film must be matched• Screens are used in pairs, as film is double-sided• Three types of screens:1. Standard blue light-emitting calcium tungstate2. Rare Earth green light-emitting gadolinium 3. Combination of both or lanthanum3. Combination double-sided
Factors affects film density:- 1. Exposure time, increase exposure time increase the film density.2. Milliampere, increase milliampere value (mA) which is usually ranged from 10 – 15mA cause increasing film density.3. Kilovoltage, increase Kilovoltage, value (kV) cause increasing film density.4. Developing time, developing time usually range from 4 – 5 minutes. Increase developing time cause increasing film density.5. Distance, increase the distance between X-ray tube and the film during exposure cause decrease film density.
Contrast:- It means the graduation of differences in film density at different areas of a radiograph.Long – scale or low contrast:- It means when many different film densities can be seen between totally clear and totally black areas of the radiograph.Short – scale or high contrast:- It means when few different film densities can be seen.
The stepwedge or penetrometer:- Is an object used to show the radiographic contrast its usually made of aluminum and is constructed so that there is a constant increase in thickness of aluminum between the X- ray tube and the film.
Factors affect contrast:- Kilovoltage:- increase kiolovoltage cause increase the (contrast scale) Processing solution temperature since increase the temperature cause decrease of (contrast scale).
Details or definition:- Is the ability to reproduce sharp outlines of the object.Factors affect details:- 1. Focal spot size:- size of focal spot must be as small as possible in order to produce sharp image.2. Size of film crystals which called (film grain):- increase the size of film grain produce less sharp image.3. Movement of patient head or X- ray tube or the film during exposure cause un sharp.4. Target object distance: - which should be as great as possible, other wise the image will be un sharp. 5. Object film distance:- should be as small as possible to produce sharp image. 6. Screen – film contacts:- poor contact between then cause also un sharp image
Developing: - X- ray film is placed in developer for 4 – 5 minutes the action of developing agents are on exposed Ag Br crystals to continue the process of precipitating the specks of silver until all silver is deposit at the site of crystal and the bromine is released into the developing solution this solution is alkaline in its action its temperature is 20C˚. Other action of developing solution is soften the emulsion of the X- ray film. Fixing:- We put the film in fixer solution for 10 – 15 minutes its action is:- 1. Re harden the film emulsion.2. Removed all the unexposed or undeveloped crystals. After fixing washed the film in running water for 20 – 30 minutes.