Radiographic Film: Composition, Latent Image, and Characteristics
Radiographic Film: Structure and Function
Radiographic film captures the remnant radiation, allowing visualization of the patient’s internal structures. It consists of several key components:
Base
Provides support, resistance, and a rigid structure. It is typically transparent, with a bluish tint, and made of polyester.
Emulsion
A composite of silver halide crystals suspended in gelatin. This porous material acts as a physical support for the silver halide crystals. The crystals have a lattice-like structure, and imperfections or sensitivity particles (Ag2S) are present within the crystal lattice. The arrangement, shape, and size of these crystals determine the film’s contrast and sensitivity.
Supercoating
A protective layer, similar to varnish, designed to reduce scratches.
Latent Image Formation
The latent image begins with photoelectric interactions between X-ray photons and atoms within the silver halide crystals. This interaction creates a pattern representative of the object being imaged. When the crystal lattice is deconstructed, silver ions (Ag+) are reduced to metallic silver (Ag), which is black.
(X-ray source – Chassis – Intensifying Screen – Film – Intensifying Screen – Chassis)
Formation of the Latent Image: This occurs within the film and is not visible until the film is processed. To become manifest, the latent image must be processed. Silver ions migrate to one side, and bromine ions to the other. Areas exposed to X-rays turn black, while unexposed areas remain transparent. For example, bone attenuates more X-rays, preventing photons from reaching the film, resulting in a white or clear area on the processed image.
Characteristics of Radiographic Film
Spectral Correspondence
The film’s sensitivity to specific wavelengths of light. The chassis color (blue or green) is chosen to match the emission spectrum of the intensifying screens. Orthochromatic film requires a corresponding wavelength spectrum; poor spectral correspondence necessitates increased patient radiation exposure.
Rare Earth
Elements that emit light when they absorb X-rays. Used in intensifying screens to enhance image formation.
Speed
The film’s sensitivity to light photons. Higher sensitivity (faster film) requires less radiation. Film speed is typically considered in conjunction with its intensifying screens. Larger grain size generally correlates with increased sensitivity.
Latitude
The range of exposure factors that produce an acceptable image. Wide latitude allows for a margin of error (around 15%) in technical factors. Latitude is inversely proportional to contrast (Latitude = 1/Contrast). High contrast films (predominantly black and white) have smaller grains and less diffusion of photons. Low contrast films (more shades of gray) have larger, more heterogeneous grains, leading to greater photon diffusion.
Double-Emulsion Film
Crossover: To minimize the printing of photons from one emulsion layer to the other, tabular grains (larger and wider, with minimal space between them) are used. An anti-crossover filter is placed behind the film to prevent photons from affecting the opposite emulsion layer, avoiding unwanted artifacts.
Law of Reciprocity
This law applies to direct exposure X-ray films, such as those used in intraoral radiography. Films used with intensifying screens receive many photons, requiring lower X-ray exposure. Direct exposure film should not be overexposed.
Safe Lights
Used in darkrooms during film processing. The color of the safe light is chosen based on the film’s spectral sensitivity. Safe lights typically emit wavelengths longer than 600nm, as most films are not sensitive to these wavelengths. Panchromatic films are highly sensitive to all wavelengths, and therefore, no safe light can be used. Energy-efficient yellowish-white bulbs, which emit a broad spectrum of wavelengths, are *not* suitable for use as safe lights. (Note: These types of bulbs are obsolete for this application.)