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Two basic types of film holders are used in veterinary radiography. Screen type film is placed in rigid cassettes with intensifying screens. Non-screen, direct exposure, type film is used in a nonrigid, lightproof paper, card-board holder, or plastic packet (Table 9-1).
Non-screen film does not require use of intensifying screens, and the film holder only protects the film from exposure to light. Reusable cardboard holders are available for this purpose. A more convenient interleaving paper folder sealed in a lightproof envelope with a "rip strip" is also available for use. The ease of use is obvious with no requirement to "reload" the film holder. Each film is purchased in its "holder", which is discarded after use.
Screen type film must be placed in a more rigid film holder since both intensifying screens and film must be protected from physical damage and the film from exposure to light. The heavy rigid cassettes are expensive and contrast markedly with the lightweight cardboard film holder (Figure 9-1).
Cassette construction The first cassettes were made of cardboard with lead backing. Cassettes made of aluminum were standard for many years and are still popular, however, cassettes with plastic fronts (Bakelite®, polycarbonate) have been available for about 40 years. Carbon fiber material has now been introduced for cassette fronts because of its:
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Used with screen film |
Used with non-screen film |
Expensive |
Low cost |
Requires less exposure |
Requires greater exposure |
Must be kept clean |
Usually discarded after use |
Provides protection |
Provides little protection |
for film |
for film |
The cassette front is composed of:
The cassette front must be radiolucent, yet be strong enough to protect the film. Some cassette fronts are color coded to indicate screen speed. They can also have color dots on the edges so that screen speed can be determined when the cassette is on the shelf.
A recent study of the scatter fraction showed the value of the carbon fiber fronts in reducing scatter fractions by 50% or more. The carbon fiber fronts will also reduce patients’ exposure when compared with the aluminum cassette by amounts ranging from 28% at 60kVp to 17% at 120kVp (Schmidt et al, 1983).
Several types of hinges are available to provide tight closure for the cassette back. One type of closure consists of stainless steel hinges with small slide catches. A more common type of hinge consists of three steel crossbars that pivot on a shouldered rivet and nylon washer in the middle of the cassette. The back of the cassette can be lead-lined to absorb secondary radiation (back scatter) that would otherwise reach the film and cause fogging.
A pad of felt is placed between the cassette edges and the hinged back and forms a light-tight seal. The felt strips that provide the lightproof character of the cassettes are generally the same, regardless of type of construction.
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Cassettes can be purchased with a photo-timed capability. The photo-timed cassette has a back that absorbs a minimal amount of radiation. Therefore, the radiation level that penetrates the cassette can be easily detected and used to terminate the exposure by activating a sensing device. This is generally not a provision that can be used by most veterinary practices.
Sizes of the cassettes are the same as the film sizes. They are available in sizes in both English and metric systems.
Cost of cassettes varies with the quality of construction and the size. Prices are only for the cassettes and do not include the price of the pair of intensifying screens.
A large part of veterinary radiography must be performed with the use of cassettes and intensifying screens. Radiography of extremities, however, can be easily performed with non-screen film and does not require expensive cassettes.
Cassettes should be numbered particularly when there are many in use. Defects noted on a radiograph can then be easily traced to the cassette used in making the faulty radiograph. Most intensifying screens have a serial number stenciled along the top front edge. These numbers are small and it is difficult to read them on a film. It is easier to write a number near the top of the intensifying screen with a heavy black felt tipped pen. This number then appears on each radiograph exposed in this cassette. The same number is then written on the back of the cassette. It is possible to write the number on all four sides of the screen so that if an edge is collimated out of the field another marker could be seen.
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