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Introduction
As we have previously known, X-rays are a modern and innovative technology used in medicine, especially in the field of medical diagnosis. One of the old methods of X-ray imaging is the use of X-ray film technology. X-ray film records information related to the body (tissues) through which X-rays pass, thus greatly helping in diagnosing and treating the patient's problem. In the previous article (Part 1) we learned about the X-ray film imaging technique, its working principle, as well as the reactions that occur during imaging, and how we can choose the film. In this article we will talk about the basic components of X-ray film as well as its layers, explaining how to prepare silver halide crystals and how the latent image is produced in addition to the additional components of the film.
Composition of X-ray Film
The following diagram shows the main components of an X-ray film:
X-ray Film Layers
The following diagram shows the layers of X-ray film:
I) Base of X-ray Film
The base of the film is a transparent supporting material made from polyester polyethylene terephthalate resin, with a thickness of 0.18 mm. Its primary functions include providing support for the emulsion layer and allowing the transmission of light.
Ideal Characteristics for Base Material:
a) Flexible, thick, and durable.
b) Non-flammable.
c) Provides structural support for delicate emulsion.
d) Low light absorption: Should not create a visible pattern on the radiograph.
e) Dimensional stability: Retains size and shape during processing, handling, and storage.
In 1933, a blue tint was introduced to X-ray films, which were originally made from clear and colorless triacetate and polyester. This adjustment was made to create a film that was easier to view, reducing eye strain. The blue tint can be added to either the base or the emulsion of the film. As a result, all modern X-ray films now feature this blue tint.
II) Adhesive Layer
The adhesive layer, also known as the subbing layer or substratum layer, is composed of a mixture of gelatin solution and a solvent of the film base. Its primary function is to ensure that the emulsion layer and the base remain securely adhered to each other throughout the coating stage and subsequent processing. Additionally, it provides a uniform surface that allows for the even coating of the emulsion layer.
III) Emulsion Layer
The emulsion layer has two principal components: silver halide grains and a vehicle matrix. It consists of a homogeneous mixture of gelatin and silver halide crystals, with a typical emulsion containing 90 to 99% silver bromide (AgBr) and about 1 to 10% silver iodide (AgI). The inclusion of silver iodide significantly enhances the sensitivity of the emulsion compared to a pure silver bromide emulsion. Additionally, the emulsion contains traces of sulfur, specifically allylthiourea.
Preparation of Silver Halide Crystals
How silver halide crystals are made?
Silver halide in an emulsion exists as small crystals that can vary in shape, including tabular, globular, polyhedral, or irregular forms. The size of these crystals typically ranges from 1.0 to 1.5 microns in diameter, with an estimated 6.3 × 10^10 grains per centimeter of emulsion. Interestingly, an imperfect crystal is necessary for photographic sensitivity, as a perfect crystal exhibits almost no such sensitivity.
Crystal Defects:
1- A point defect occurs when a silver ion moves out of its normal position in the crystal lattice, creating interstitial ions.
2- A dislocation is a line imperfection within the crystal.
3- These defects cause a strain in the crystal structure.
4- An iodine ion can strain the crystal structure in a similar manner.
Chemical Sensitization
Chemical sensitization of crystals involves adding allyl thiourea, a sulfur-containing compound, to the emulsion. This compound reacts with silver halide to produce silver sulfide, which typically forms on the surface of the crystals, creating what is known as a "sensitivity speck". The sensitivity speck is crucial as it traps electrons, initiating the formation of latent image centers.
The Latent Image
Remnant radiation interacts with silver halide crystals primarily through photoelectric interaction. The energy deposited in the film mirrors the pattern of the subject exposed to radiation, creating an invisible image called the latent image. This latent image, produced by the exposure of the film to light or radiation, becomes visible through chemical processing, transforming into what is known as the manifest image.
Formation of Latent Image
Metallic silver, which is black, produces the black areas seen in developed films. The exposure of silver-iodo-bromide grains to light photons, either from screens or direct X-ray exposure, initiates the formation of atomic silver, which then forms a visible pattern on the film.
Latent Image Formation (Gurney-Mott Hypothesis)
According to the Gurney-Mott hypothesis, energy absorbed from light photons ejects a bromine electron, which gets trapped at a sensitivity speck. This trapped electron attracts an interstitial silver ion (Ag+). The silver ion and electron combine to form neutral (black) silver (Ag). If more than 6^-10 Ag° accumulates at the speck, it becomes a latent image center, making it developable.
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Grain Size and Distribution
Grain size and distribution significantly impact the film's characteristics. A larger average grain size results in higher film speed. The size distribution affects the contrast; a greater availability of grains leads to lower contrast. Graininess, which is the apparent clumping of crystals visible on the radiograph, increases with larger crystal sizes, resulting in a grainier film appearance.
Gelatin
Gelatin serves as the suspending medium and binding agent for silver halide particles in photographic film. It is derived from collagen, which is primarily sourced from the cartilage, skin, and the protein matrix (ossein) found in animal bones. Gelatin is used as a binder because it allows silver nitrate and sodium bromide to react efficiently, resulting in finely dispersed and suspended silver bromide (AgBr). When heated, gelatin can be easily spread over the film base, and it sets firmly as a gel upon cooling. Its flexibility prevents cracking when bent, and it remains optically transparent. Gelatin does not chemically react with silver halide, yet its porous nature allows processing chemicals to reach the silver halide crystals. Additionally, certain constituents in gelatin enhance the activity of silver bromide and act as antifogging agents.
The above figure shows the decomposition of gelatin and the release of silver. (a) Cuttings of the untreated X-ray film (control). (b) Edges of film lose color after a few minutes of exposure. (c) Clean bluish polyester sheets were obtained after the enzyme treatment of X-ray sheets.
Supercoat (Overcoat)
The supercoat, or overcoat, is a protective layer of gelatin that adds durability to unexposed radiographic film. It is anti-static and helps prevent damage from scratches, pressure, or contamination during storage, handling, and processing.
Few Additives
Several additives are used in the photographic emulsion:
1- Preservative: Phenol is used as a bactericide.
2- Silver iodide: Extends sensitivity towards the blue range of light.
3- Dyes: May be added to further extend color sensitivity.
4- Glycerin: Makes the emulsion pliable.
5- Saponin: Enhances the emulsion’s receptivity to processing chemicals.
6- Alcohol: Prevents frothing during the coating process.
Conclusion
Understanding the composition and structure of X-ray film is crucial for appreciating its role in medical imaging. From its supportive base and adhesive layers to the sensitive emulsion containing silver halide crystals, each component plays a vital role in capturing and processing the latent image formed during exposure. This foundational knowledge not only enhances our grasp of X-ray technology but also underscores its enduring significance in modern healthcare diagnostics.
Here (Part 2) of this topic ends, continue reading in (Part 3).
#xray #xray_film #components_of_xray_film #medical_imaging #radiographic_film #radiography
