Radiographic Film Structure and Function

Radiographic Film

• Radiographic film displays a radiographic image.
• It consists of two parts: the base & emulsion.
• Most film has double emulsion, containing two layers.
• Adhesive layer is between the base and emulsion.
• Thickness is approximately 150 to 300 µm.

Film Structures

• Adhesive layer
  • Located between the emulsion and the base, ensuring adhesion.
  • Maintains contact and integrity during use and processing.
• Base
  • Foundation of the film, providing surface and support for the emulsion.
  • Thickness of 150 to 300 µm, semirigid, lucent, and made of polyester.
  • Polyester withstands high temperatures and is fireproof.
  • Must be strong but flexible.
  • Tinted blue to reduce glare.
• Emulsion
  • The heart of the film, interacting with x-rays or light to transfer information.
  • A mixture of gelatin and silver halide crystals, about 3 to 5 µm thick.
  • Holds silver halide crystals uniformly dispersed in place.
  • Overcoat protects the emulsion from scratches, pressure, and contamination.

Silver Halide Crystals

• Active ingredient of the emulsion, composed of 98% Silver Bromide (AgBr) and 2% Silver Iodide (AgI).
• Atoms have high atomic numbers (Br = 35; Ag = 47; I = 53), compared to gelatin and base (Z ≈ 7).
• Interaction of x-rays/light photons with these high-Z atoms results in a latent image.
• Shape can be tabular, cubic, octahedral, polygonal.
• Tabular shape is most common for general radiography.
• Approximately 0.1µm thick and 1µm in diameter.

Latent Image

• Invisible image formed on the film after exposure, before development.

Cassette Construction

• Consists of a front and a back, hinged at one edge.
• A thin sheet of lead foil is attached to the back side, connecting to a plastic foam pressure pad and an intensifying screen (I.S.).
• The cassette wall (front) contains the front I.S. and a short lead blocker for patient identification.
• Locking methods are used to prevent light and ensure close contact between film and screens.
• Internal metal/plastic surfaces are black-coated to prevent light reflections.
• A slightly curved cassette back is used for good film-screen contact, minimizing unsharpness.

Cassette Materials

• Carbon fiber is used for its advantages:
  • Strength
  • Light weight
  • Low beam absorption
• Carbon fiber cassettes significantly reduce patient dose by absorbing half the x-rays compared to aluminum/plastic cassettes.

Cassette Back

• Construction is metal or plastic, lined with lead foil to protect from backscattered radiation.
• Should have at least 0.12 mm lead equivalence when used with 150 kV equipment.

Cassette Fittings

• Clips and fasteners: usually stainless steel.
• Hinges: metal or plastic.
• Pressure pad: plastic foam sponge.

Care Of Cassettes

• Treat gently, limit the number carried at a time.
• Carry securely between the body and arm, holding the bottom edge.
• Store upright, avoid contact with fluids.

Intensifying Screens

• Part of the cassette, converting x-ray energy into visible light.
• Developed by Thomas Edison in 1897.
• Initial screens used Calcium Tungstate (CaWO4).

Advantages of Calcium Tungstate (CaWO4)

• Emits light in violet-to-blue region, where conventional radiographic film is most sensitive.
• Other screen phosphors (green/red) would require more light photons to produce a latent image, reducing their Intensification Factor (IF).
• Light emitted by calcium tungstate screens is readily absorbed by radiographic film.

Advantages of Calcium Tungstate (CaWO4)

• Reduces x-ray dose to the patient.
• Short exposure time, reducing motion blur.

Rare Earth Screens

• Newer phosphor materials used for most radiographic applications.
• Refers to elements in Group IIIa of the periodic table, with atomic numbers 57 to 71.
• These transitional metals are scarce.
• Faster than calcium tungstate screens, making them better for most radiographic imaging.
• Result in lower patient dose, less thermal stress on the tube, and reduced shielding.
• Faster speed due to characteristics:
  • Higher percentage of x-rays absorbed by the screen (Detective Quantum Efficiency, DQE).
  • Higher amount of light emitted for each absorbed x-ray (Conversion Efficiency, CE).

Materials Used in Rare Earth Screens

• Gadolinium
• Lanthanum
• Yttrium

Rare Earth Phosphor Compositions

• Terbium-activated gadolinium oxysulfide (Gd2O2S: Tb)
• Terbium-activated lanthanum oxysulfide (La2O2S: Tb)
• Terbium-activated yttrium oxysulfide (Y2O2S: Tb)
• Lanthanum oxybromide (LaOBr)

Layers of the Intensifying Screen

• Protective Coating
  • Closest to the radiographic film, approximately 10 to 20 µm thick.
  • Makes the screen resistant to abrasion and damage.
  • Eliminates static electricity buildup and provides a cleaning surface.
  • Transparent to light.
• Phosphor
  • The active layer, emitting light during x-ray stimulation.
  • Thickness varies from 50 to 300 µm.
  • Converts x-ray beam into light.
  • Before 1980, most phosphors used crystalline calcium tungstate.
  • Newer, faster screens use gadolinium, lanthanum, and yttrium.
  • Emits visible light, observed in a dark room.
• Reflective Layer
  • Located between the phosphor and base, approximately 25 µm thick.
  • Made of a shiny substance (e.g., magnesium oxide, titanium dioxide).
  • When x-rays interact with the phosphor, light is emitted isotropically (equally in all directions).
  • Less than half of this light reaches the film.
  • The reflective layer redirects light towards the film, nearly doubling the number of light photons.
• Base
  • The layer furthest from the film, approximately 1 mm thick.
  • Provides support for the active phosphor layer.
  • Made of polyester.

Screen Characteristics

• Any material that emits light in response to stimulation is a phosphor.
• Visible light emitted is called luminescence.
• Stimuli can include electric current, biochemical reactions, visible light, and x-rays.

Types of Luminescence

• Fluorescence: Visible light emitted only during stimulation.
• Phosphorescence: Phosphor continues to emit light after stimulation.

Primary Characteristics of Radiographic Intensifying Screens

• Screen speed
• Image noise
• Spatial resolution

Speed

• Ranges from 50 (slow, detail) to 1200 (very fast).
• Describes the efficiency of x-ray conversion into light.
• Determined by the number of x-rays that interact with the phosphor and the efficiency of conversion into visible light.

Properties of Radiographic Intensifying Screens that Affect Speed

• X-ray beam quality, image processing, and temperature are controlled by the technologist.

Image Noise

• Speckled background on a radiograph.
• Occurs with fast screens and high-kVp techniques.
• Reduces image contrast.
• Higher conversion efficiency increases noise.

Spatial Resolution

• Refers to the ability to distinguish small objects.
• A focused photograph has good spatial resolution, while a blurred one has poor resolution.
• Direct-exposure radiographs have better spatial resolution compared to intensifying screens.
• Screen phosphor interaction results in a larger area of activation on the film emulsion than direct x-ray exposure, leading to reduced spatial resolution (image blur).
• High-speed screens have low spatial resolution, while fine-detail screens have high resolution.
• Improved with smaller phosphor crystals and thinner layers.

Care of Screens

• Do not slide film into cassettes to prevent scratching.
• Place the film inside.
• Do not remove film with fingernails, use the hinged edge.
• Do not leave cassettes open, screens can be damaged by dust or chemicals.
• Clean screens periodically, frequency determined by usage and environment.
• Use specialized screen cleaning materials.
• Maintain good screen-film contact.

This quiz explores the essential components of radiographic film, including the base, emulsion, and adhesive layer. You'll learn about their roles in image quality and processing, as well as the importance of their materials and thickness. Test your understanding of film structures used in radiography!