Nematodes

Xray

RECORDING OF IMAGE

            X-ray image is the information of pattern that an X-ray beam acquires when it passes through different body tissues after interaction. This image can only be seen with the help of X-ray films. X-ray film also provides permanent record of this information.

X-Ray Film Composition

            Composition of X-ray film is similar to that of photographic film. Radiation sensitive emulsion is coated on both sides of a transparent base (double emulsion film). A thin layer of adhesive is used to attach emulsion and base. The emulsion is protected from scratches, pressure and contamination during use by thin layer of gelatin called supercoating. Thickness of radiographic film is about 0.25 mm.

1. Film Base

            This provides support to fragile photographic emulsion. An ideal film base should have the following qualities;

              I.      Flexible, inert, light in weight and easy to handle

            II.      Relatively transparent

         III.      Undesirable visible pattern should not be produced on it

         IV.      Should not absorb too much light when radiograph is being viewed

           V.      Shape and size must not change during developing process and during storage life

Now a days films have either polyester or cellulose triacetate base. Polyester base has advantage of improved dimensional stability even under conditions of varying humidity and is stronger and thinner than cellulose triacetate base. This is 0.007 inch thick and cellulose triacetate base is 0.008 inch thick.   

2. Film Emulsion

             It is composed of homogenous mixture of gelatin and sliver halide crystals. The gelatin is made from bone and offers following advantages;

It keeps silver halide crystals well dispressed and prevents their clumping

Processing solutions can penetrate gelatin without affecting its strength or performance.

            Variation in speed, contrast and resolution among different X-ray films depend upon the processing of silver halide crystals. This silver halide is in the form of small crystals of silver idobromide (Silver bromide 90-99% and silver iodide 1-10%). The diameter of each crystal ranges from 1-1.5 microns. About 6.3 X 10⁹ crystals are present per cubic centimetre of emulsion. Size, number and quality of the crystals determine the quality of the film emulsion. X-ray image could only be seen after developing of the film. These crystals are reduced to small grains of metallic silver after developing and unreduced crystals are removed by fixer. Remaining metallic silver appears black and a permanent negative is formed.

Types of X-ray Films

1. Screen Films

            These types of films are primarily more sensitive to ultraviolet and blue light range which originates from calcium tungstate crystals of intensifying screens. A screen film requires less exposure time to produce an image as compared to the non-screened films because of intensification factor of intensifying screens.

2. Non-screen Films

            These films are used for direct X-ray exposure i.e. without use of intensifying screens. Emulsion thickness in case of these screens is more than screen films in order to absorb as much X-rays as possible and thus a prolonged processing time is required. Such films are useful in detecting hairline fractures and dental radiography where more details are required.

3. Automatic Processor Films

            These X-ray films have certain characteristics so that rapid processing is possible in an automatic developer. Their increased hardness can bear transportation through roller system. They are less expansive than ordinary X-ray films.

4. Occlusal Films

            These films are used for intra-oral radiography and can be used with or without specially designed cassette.

Handling and Storage of Unexposed Films

¨    Film boxes should be transported to the place of their storage immediately after they are received.

¨    Film storage room should be cool (10-20°C) with low humidity (40-60%).

¨    Film boxes should be kept vertically without any pressure on them.

¨    Films should not be stored near the source of heat, irradiation and water.

¨    Films should not come in contact with gases or vapours from chemical substances as fogging may occur.

¨    Films should be loaded into or unloaded from a cassette on a dry and clean bench inside the dark room under a proper safe light.

¨    Films should be handled delicately and any accidental splashing of processing solution should be avoided.

¨    Shelf life of films is generally less than a year.

Intensifying Screens

            These screens when come in contact with X-ray beam converts most of the radiant energy (95%) into the visible light that has almost same information as the original X-ray beam. The visible light and remaining radiant energy (5%) interacts with the film. The uses of screen intensify the effect of X-ray beam on film as it is far more sensitive to visible light than to X-rays. The film thus allows the reduction in the exposure factors required to obtain a diagnostic radiograph.

           

            An intensifying screen consists of four distinct layers with a total thickness of 0.4 mm. These layers are:

1. Base: this provides the mechanical support to active phosphor layer and is made either high grade cardboard or polyester. It is farthest from the film placement in the cassette. Base should be inert and moisture resistant. It should not suffer damage due to radiation and should not discolour with age.
2. Reflecting Layer: this is spread between the base and phosphor layers. It is made of titanium dioxide or magnesium oxide. The light emitted by the interaction of X-ray and phosphor is directed in all directions including towards the base. The reflecting layer reflects back the light towards the base to the film.
3. Phosphor Layer: it is the active layer of intensifying screen and its main function is to convert X-ray energy into visible light. The material used in the phosphor layer should possess the following qualities:
1. Their should be no phosphorescence
2. Should be off higher atomic number for higher X-ray interaction
3. Must efficiently convert X-rays into visible light
4. Light emitted must be of proper colour to match the sensitivity of X-ray film

            Materials mostly used as phosphor layers are calcium tungstate, barium lead sulphate and zinc-cadmium sulphide.

4. Protective Layer: transparent layer placed close to the film consisting of cellulose compound and serves following purposes:
1. Physical protection to the phosphor layer
2. Prevents static electricity
3. Provides a surface that can be cleaned without affecting phosphor layer.