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Radiographic Film: Composition,
Characteristics, and Handling
Radiographic Film
Composition and Structure
Radiographic film is an emulsion-gelatin containing radiation-sensitive silver halide crystals. The film consists of several key components:
Base : The foundation of the film, providing a rigid structure for the emulsion. The base is flexible yet fracture-resistant, allowing easy handling while maintaining dimensional stability during use and processing. Emulsion : The heart of the x-ray film, containing a homogeneous mixture of gelatin and silver halide crystals. The gelatin acts as a support medium for the silver halide crystals, allowing the processing chemicals to easily penetrate and interact with them. Silver Halide Crystals : The primary light-sensitive component, typically composed of 98% silver bromide and 2% silver iodide. These crystals can have various shapes, with the tabular shape being most commonly used for general radiography.
The manufacturing of radiographic film is a precise procedure requiring tight quality control. During the 1960s, when nuclear weapons testing was at its peak, film manufacturers took extraordinary precautions to ensure that contamination from radioactive fallout did not invade the manufacturing environment, as this could seriously fog the film.
Latent Image Formation
When imaging x-rays emitted by a patient and incident on an x-ray intensifying screen, the energy is deposited in the emulsion, primarily through photoelectric interactions with the atoms of the silver halide crystals. This interaction is sometimes referred to as the photographic effect, and the resulting invisible image is called the latent image.
The formation of the latent image involves the following steps:
Light photons interact with the silver halide crystals, releasing electrons. These electrons migrate to the sensitivity centers within the crystal lattice. At the sensitivity centers, atomic silver is formed by the attraction of an interstitial silver ion. This process is repeated many times, resulting in the build-up of silver atoms.
The remaining silver halide is converted to silver during the processing stage, creating the visible radiographic image.
The Gurney-Mott theory is currently an acceptable explanation of the photographic effect, although the process is not yet fully understood and is still under research.
Types of Radiographic Film
There are two main types of radiographic film:
Screen Film : This is the most commonly used type of film, which is used in conjunction with radiographic intensifying screens. Several characteristics must be considered when selecting screen film, such as contrast, speed, and spectral matching. Contrast : Screen films are available in multiple contrast levels, with high-contrast films producing a very black and white image and low- contrast films appearing more gray. Speed : The sensitivity of the screen-film combination to x-rays and light, with different speed options available.
Crossover : The exposure of an emulsion caused by light from the opposite radiographic intensifying screen, which is reduced in modern films due to the use of tabular silver halide crystals.
Direct Exposure Film (or "Non-Screen Film"): This type of film is exposed directly by the x-ray beam, without the use of intensifying screens.
The variety of films available in medical imaging has increased significantly, with manufacturers offering a wide range of options to meet the diverse needs of radiologists and healthcare professionals.
Tabular Grain Emulsions and Crossover
Control
Tabular Grain Emulsions
Tabular grain emulsions reduce crossover because the covering power is increased. The addition of a light-absorbing dye in a crossover control layer reduces crossover to near zero. This has three critical characteristics:
It absorbs most of the crossover. It does not diffuse into the emulsion but remains as a separate layer. It is completely removed during processing.
Spectral Matching
The most important consideration in the selection of modern screen film is its spectral absorption characteristics. Since the introduction of rare Earth screens in the early 1970s, radiologic technologists must be particularly
screen is coated with a special light-absorbing dye to reduce reflection of screen light, which is transmitted through the emulsion and base. Antihalation coating is used on all single-emulsion screen film, not just mammography film.
Laser Film
Laser film is the traditional mode of printing radiographs. Laser film is silver halide film that has been sensitized to the red light emitted by the laser in much the same way that blue and green sensitive screen film is sensitized. Different types of lasers are used in laser printers, and laser film is light sensitive; therefore, laser film must be handled in total darkness.
Specialty Film
Cinefluorography is a special examination that is reserved almost exclusively for the cardiac catheterization laboratory. The radiologic technologist who becomes involved in such procedures uses cine film. Spot films from 70 to 105 mm in width are used in a number of different types of spot film cameras. Roll-type film usually can be adequately processed in the automatic processor used for conventional radiographs.
Handling and Storage of Film
Radiographic film is a sensitive radiation detector and must be handled accordingly. Improper handling and storage result in poor radiographs with artifacts that interfere with diagnosis. Radiographic film is pressure sensitive, so rough handling can cause artifacts. Creasing of the film before processing can produce a line artifact, and dirt on the hands or on radiographic intensifying screens can produce specular artifacts. In a dry environment, static electricity can cause characteristic artifacts.
Radiographic film is sensitive to the effects of elevated temperature and humidity, especially for long periods. It should be kept in refrigerated storage, and before use, it should be stored in a cool, dry place, ideally in a climate-controlled environment. Storage in an area that is too dry can be equally objectionable, as static artifacts are possible when the relative humidity dips to below about 40%. Heat increases the fog of a radiograph and therefore reduces contrast.
Radiographic film must be stored and handled in the dark, as any light can expose the emulsion before processing. Low-level diffuse light can increase fog, while bright light causes gross exposure. Ionizing radiation, other than the useful beam, can create an image artifact by increasing fog and reducing contrast.
Most radiographic film is supplied in boxes of 100 sheets, and each box contains an expiration date, which indicates the maximum shelf life. Aging results in loss of speed and contrast and an increased fog. It is always wise
to store boxes of film on edge rather than laying them flat, and the storage of film should be sequenced so that the oldest film is used first.
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