RADG316 V: a PowerPoint presentation for Third Year for Radiography Students

Radiographic Film Materials

1. RADG316 V The Recording System: Film Materials

2. Contents Film construction The crossover effect Irradiation films for medical imaging Duplitized X-ray film Single-sided emulsion film

3. Two main groups of film material may be differentiated in a modern imaging department: ( 1) Those films exposed to X-rays alone (direct-exposure film) or to a combination of both X-rays and light (screen - film). These are both known as X-ray films. (2) Those films exposed to light only. Examples of these are fluorographic, duplicating or polaroid films, as well as the JS mm roll films which every photographer knows well. Some of the films in the first group may be duplitized (i.e. having a sensitive emulsion on both sides of the base), whilst others may be single-sided (one emulsion surface only). All of the films in group (2) have a single emulsion surface only.

4. Film base Nowadays, polyester, which is made from polyethylene terephthalate resin, has almost completely replaced cellulose triacetate as the material used in the manufacture of film base. It is extremely strong, virtually untearable and demonstrates high dimensional stability, even under the most rigorous manufacturing or processing conditions. Function of film base (1) To provide a support for the emulsion layer. (2) To transmit light so that an image can be viewed.

5. Magnified cross-section through (a) duplitized film and (b) single-sided emulsion film.

6. Desirable characteristics of film base (I) Transparent, free from any defect or blemish, and uniform m its ability to transmit light. (2) Strong but flexible: (3) Uniform in thickness, otherwise there will be variations in emulsion layer thickness. (4) Dimensionally stable throughout processing. (5) Impermeable to water or processing solutions, (6) Non-flammable: Chemically inactive, (8) Uniform in colour

7. The subbing layer A subbing or 'adhesive' layer is required between the base and the emulsion for two reasons: (1) To ensure that the emulsion layer adheres to the smooth and shiny base material during the coating stage in manufacture; (2) To prevent any separation of the emulsion layer from the base during processing. The preparation used is a mixture of gelatin solution and solvent of the film base. In some instances, coloured dyes may also be added to the subbing layers by certain manufacturers to counteract the crossover or transmission effect

8. The emulsion layer the sensitive emulsion consists of silver halide crystals suspended in gelatin. Film manufacturers are now able to produce flat-shaped halide grains instead of the older rounded-shaped crystals. Kodak led the field with their 'T-Grain' emulsion. Because of their shape, the grains offer a larger surface area to the imaging source. The shape of the grains also means that they are able to lie in close proximity to each other and so minimize the amount of light able to pass through the emulsion to the base and produce the crossover effect, a phenomenon which contributes to image unsharpness

9. The emulsion layer Kodak also add a light-absorbing magenta dye to the surface of the grains, known as an optical sensitizing dye, the effect of which is to increase still further the sensitivity of the emulsion as well as absorbing irradiated light within the emulsion. The supercoat: This is a thin layer of clear gelatin which is applied to the emulsion. It has two prime functions: (1) To protect the sensitive emulsion from the effects of light pressure or abrasion which might occur during use. The super-coating is sometimes referred to as the anti-abrasive layer. (2) To provide suitable surface characteristics.

10. Non-curl hacking This is present only in single-sided emulsion sheet films. In order to prevent or discourage the curling tendency with single-sided emulsion film, the emulsion layer is balanced by coating the base on the opposite side to the emulsion, firstly with a subbing layer and then with a layer of gelatin. The latter layer is not only of similar thickness to the emulsion stratum but also has the same absorbent qualities. This layer is of course not present in roll film.

11. Anti-halation layer When the image is formed by light, some of the light incident on the film will pass through the emulsion layer and reach the base. Depending on the angle at which this light strikes the film base/air interface at the back of the film, it may either pass out of the base or be totally reflected back towards the emulsion. Such reflected light will produce a diffuse image or 'halo' around the proper image. This phenomenon is known as halation and is a cause of unsharpness in the image. Although halation is a phenomenon attributable mainly to single-sided emulsion films, it can also occur with duplitized materials.

12. Halation.

13. Anti-halation layer Manufacturers use one of the following methods in order to prevent halation occurring: ( 1) Adding a dye to the non-curl backing. (2) Adding a dye to the base. (I) Adding a colored dye to the gelatin of the non-curl backing This is the method preferred for most of the single-sided emulsion films used in the radiography department. Examine some of the single-sided films used in your department. Note how the colour of the dye used in the anti-halation backing varies from one type of film to another, depending on the colour of light which is to be used to expose the film (i.e. intensifying screen emission).

14. Anti-halation layer The dye colour selected is always the opposite or 'complementary' colour to the exposing· light, consequently the incident light will be absorbed by the backing and not reflected: e.g. a yellow dye will absorb blue light, a red dye will absorb green light, and so on. For example, subtraction film is green/blue for the yellowish tungsten light used with it, whilst duplication film has a dark blue, almost purple, anti-halation backing for the ultraviolet and tungsten light it is exposed to during the copying of radiographs. These dyes are removed during development. The anti-halation backing is known sometimes as the anti-halo layer or gel coat. (2) Adding a dye to the base itself This is the case with 35 mm still and cine film, for instance. Such a dye cannot be removed during processing. Its density therefore is ,very carefully controlled so that whilst it may function adequately as an anti-halation screen, it none the less transmits sufficient light for viewing.

15. The crossover effect A type of halation known as the crossover effect can occur when film is used with intensifying screens. To prevent this, one manufacturer uses coloured subbing layers which effectively absorb the incident light. Some Kodak X-ray films have a magenta dye added to the surface of the emulsion crystals during manufacture. Magenta is the complementary colour of green. Besides the principal function of increasing the sensitivity of the emulsion, the dye has the effect of reducing light scattering “ within the emulsion and, at the same time, limiting crossover and thus image unsharpness. These dyes are removed from the film during processing and thus do not interfere with the image when viewed by transmitted light.

16. Irradiation The term should not be confused with halation. Irradiation is the sideways scattering of light within the emulsion itself as a result of the light striking the grains of silver halide comprising the emulsion. This scattered light contributes nothing valuable to the image proper and, like halation, is another source of unsharpness (blurring) in the image.

17. Films for medical imaging Films used in medical imaging include: Duplitized emulsion film (1) Direct-exposure (non-screen-type) film: intra-oral dental film, kidney surgery film, radiation-monitoring film. (2) Screen-type film (for use with intensifying screens).

18. Single emulsion film (l) Screen-type film (used with a single intensifying screen). (2) Photofluorographic film: cine film, 70 mm and I 05 mm roll film, 100 mm sheet film. (3) Cathode-ray-tube (CRT) photography: Polaroid film, 10" x 811 and 14" x 11“ sheet film. (4) Duplication film. (5) Subtraction film. (6) Laser imaging film for use with laser imagers.

19. Duplitized X-ray film Direct-exposure-type film: This is sometimes known as envelope-wrapped or non-screen film because of its exposure to X-rays only. Each film is individually folder-wrapped in paper and mounted with a stiff card for support inside a moisture-resistant paper envelope. Direct-exposure film is much slower than a screen-film combination. Nowadays, instead of increasing coating weights as a means of producing greater film blackening for a given exposure, sensitizers are added to non-screen X-ray film emulsions during manufacture.

20. Non-screen films Non-screen films are used because of their superior resolution compared to a screen-film system. They are sometimes used for extremity radiography where fine bony detail is required, or for mammography when microcalcification may be demonstrated. The comparatively high exposure and thus increased radiation dose associated with imaging using these low-speed materials means that the use of this type of film is only rarely justified nowadays.

21. Intra-oral dental film (I) Periapical - 31 x -f 1 mm film for single or groups of teeth. (2) Occlusal - 57 x 76 mm film for imaging mandibles or maxillae in the occlusal plane. (3) Bitewing - a similar film to the periapical but with a custom-made flap which is slid around the film to enable the film to be positioned vertically behind the upper and lower teeth, the patient biting on the flap. Used for demonstrating the crowns.

22. Three types of intra-oral film. (a) P e riapical . (b) Occlusal. (c) Bit e wing.

23. The wrappings of an intra-oral dental film. The wrappings of a dental envelope package. The lead-foil insert acts as an attenuator of back-scattered radiation arising from the intra-oral structures, and because of its presence it is essential that the film back be positioned with the correct side facing the tube. The outer waterproof packet protects the film from moisture, whilst the paper inserts afford some protection from pressure.

24. Kidney surgery film This duplitized non-screen film is designed to enable a radiographic exposure of the kidney to be made extra-abdominally during surgery for the removal of renal stones. The shape facilitates easy placement of the kidney and renal vessels Kidney surgery film. The characteristic shape is to facilitate the placement of the kidney whilst still attached to its renal vessels.

25. Kidney surgery film Each packet contains two films, one with a fast emulsion, the other slow. Should a set of radiographic factors result in overexposure of a small calcareous deposit on one film, then the chances are that it will, however, be demonstrated on the slower one. Thus, the two-film feature allows a wider range of renal stone densities to be demonstrated for any given radiographic exposure, and from the radiographer's point of view allows greater latitude in exposure choice. The films are enclosed in a light-tight, waterproof polythene packet; the whole pack is capable of being cold-sterilized. The film can be processed in an automatic processor.

26. Radiation monitoring film Outwardly similar to dental film in appearance, this film, although duplitized, has one very important difference. On one side of the base is a high-speed emulsion, whilst on the other side is a slower emulsion. This permits a wide range of exposure levels to be recorded. Charac te ristic curves f or radiation monitoring film. The base has a fast emulsion coating on one side and a slow emulsion on the other .. After proc ess in g ( the fast emulsion may be remo ved t o allow measurement of high doses. A large exposure to radiation may result in complete blackening of the fast emulsion, to the extent that exposure will he unmeasurable. Some of this emulsion can then be removecl and radiation exposure estimated from the density of the slower emulsion. It is to be hoped that only the fast emulsion will eyer need to be used when estimating radiation closes received by radiographers!

27. Screen-type film These films arc used in conjunction with pairs of intensifying screens, the latent image being produced mainly by light emission from the screen phosphors. A wide range of different manufacturers' films are available: both blue-sensitive (monochromatic) and green-sensitive (orthochromatic) and varying in speed, contrast, latitude and resolving power. These types of films are sometimes referred to as contrast amplifiers because their average gradient is > 1 Sizes of screen-type X- rav films. 13 x 18cm 15 x 30cm 15 x 40cm 20 x 40cm 18 x 24cm 24 x 30cm 30 x 40cm 35 x 35 cm 35 x 43 cm

28. Single-sided emulsion film This type of film is used in conjunction with a cassette fitted with a single intensifying screen. One particular application is mammography. The films are medium-to-high contrast with high definition, capable of demonstrating microcalcifications in soft tissue.

29. Photofluorographic film These are films used to record the image produced at: (1) The output phosphor of an image intensifier tube; (2) The fluorescent screen of a camera system such as one used to provide miniature radiographs of the chest. All of the films have a single emulsion and an anti-halation layer. The emulsions are predominantly orthochromatic, since most image intensifier output phosphors (e.g. zinc cadmium sulphide) have light emissions containing a strong green component.

30. Photofluorographic film 16mm and 35mm cine film This is used for making a cine recording (cinefluorography) from the output phosphor of an image intensifier. There is a choice of orthochromatic or panchromatic fine grain emulsions, and the film can be supplied in either 85 or 170 m lengths. The reference to 16 mm or 35 mm is an indication of the width of the film Others are 70 mm and 105 mm roll films etc.

31. Films for use with a cathode ray tube or TV monitor These single-emulsion films have applications in the following imaging modalities: • Ultrasound; • Computerized tomography; Magnetic resonance imaging; • Nuclear medicine; • Digital subtraction imaging. The emulsions are orthochromatic, of medium-to-high contrast, and made to match a wide variety of CRT phosphors whose light emissions are known by their 'P' numbers: e.g. P4 - white; P11 - blue; P20, P31 - green. Duplication and subtraction films