Photographic Characteristics of X-ray Film

Photographic Characteristics of X-ray Film

• Photographic density is the film's response to incident radiation, dependent on tissue absorption.

• Tissue absorption is influenced by:

  • Patient factors:
    • Composition
    • Thickness
  • Beam factors:
    • Energy spectrum
      • kVp
      • Phase
      • Filtration

• Photographic/Optical Density (OD) measures film blackness/opacity.

  • OD is calculated as log₁₀(Io/It), where Io is the incident light and It is the transmitted light.
  • Logarithms are used to easily represent wide ranges of light intensities.

• Opacity is the ability of film to block light (Io/It).

• Transmittance is the ability of film to transmit light (It/Io).

  • Higher OD values mean darker film, less light transmitted.

• Base + Fog:

  • Unexposed film has a minimum OD of ~0.12.
  • Base is the plastic material absorbing a small amount of light (blue dye), OD ~0.07.
  • Fog is the development of unexposed silver halide grains, OD ~0.05.

• Sensitometric Curve/Characteristic Curve (H&D Curve):

  • Shows the relationship between exposure intensity and resulting film blackness.
  • Key parts:
    • Toe: low exposure, low density.
    • Linear Region: mid exposure, linear change.
    • Shoulder: high exposure, high density, little density change.
    • Base + fog: initial optical density of the film.
    • Sensitivity (speed): reciprocal of exposure required for OD 1.0.
    • Gamma (contrast): average gradient of the curve.
    • Latitude: range of exposures producing acceptable densities.

Radiographic Contrast

• Radiographic contrast is the difference in density between image areas.

• Factors affecting contrast:

  • Subject contrast:
    • Thickness
    • Atomic differences
    • Energy spectrum (kVp)
    • Contrast material
    • Scatter radiation
  • Film contrast:
    • Film's characteristic curve
    • Film density
    • Screen/direct exposure
    • Film processing

• Film Gamma:

  • Maximum slope of the characteristic curve.
  • Gamma = (D₂ - D₁) / (log E₂ - log E₁)
  • Shows change in film density with exposure.
  • Ranges from 2.0-3.5.

• Film Latitude:

  • Range of exposure producing acceptable densities (usually 0.25-2.0).
  • Inversely related to contrast: high contrast = low latitude, low contrast = high latitude.

• Film Speed (Sensitivity):

  • Reciprocal of exposure required to produce density of 1.0 above base + fog.
  • Ability of receptor to respond to low x-ray exposure.

• Film A vs. Film B: Film A is faster than film B, but they have the same contrast.