Brightness and Density

• Absorption


  Process in which x-ray photons are attenuated by the body and do not reach the image receptor
• Bit Depth


  The number of bits stored per pixel; defines the shades of gray available for each pixel
• Brightness


  Amount of light emission (luminance) on a display monitor
• Density


  Overall blackness of a processed image
• Dynamic Range


  The range of exposure intensities an image receptor can accurately detect
• Exposure Indicator


  Numerical representation of the amount exposure; usually the mean value
• Grid


  Device used to reduce the intensity of scatter radiation in the remnant (exit) x-ray beam; placed between the object imaged and the image receptor
• Grid Ratio

  Ratio of the height of the grid line to the width of the interspace material; ex 5:1
• Inverse Square Law


  The intensity of the x-ray beam is inversely proportional to the square of the distance from the source
• Matrix


  Rectangular or square table of numbers that represent the pixel intensity to be displayed on the monitor
• Object-to-Image Distance


  The distance between the object being radiographed and the image receptor
• Post-processing


  Manipulating an exposed radiograph to alter the appearance of the image
• Pixel


  Single brightness level; basic picture element on a display
• Source-to-Image Distance


  The distance between the x-ray tube and the image receptor
• Transmission


  Process in which x-ray photons pass through matter without having any interactions before reaching the image receptor
• Window Level


  Image manipulation parameter that changes screen image contrast; usually via a mouse
• As brightness increases

  Density decreases
• As the matrix size decreases

  Visible detail decreases
• As bit depth increases

  The number of shades of gray increases
• As window level is decreased


  Brightness is decreased
• As mAs increases

  Patient dose increases
• As mAs decreases

  Quantum noise increases
• As OID increases


  Beam intensity decreases
• As field size decreases

  Scatter radiation increases
• As field size increases

  Radiographic contrast decreases
• As part thickness increases

  Scatter increases
• As SID decreases

  Radiation intensity increases
• Structural details in a radiograph

  • Density
  • Contrast
• Absorption is shown on an image as light areas
• Transmission is shown on an image as dark areas
• The primary factor affecting brightness or density is the amount or quantity of radiation reaching the IR
• An overexposed image will show insufficient density
• When too few photons reach the IR, the image will be underexposed
• Digital systems have a wide dynamic range
• The numerical value assigned to each pixel is based on the attenuation or Transmission of x-ray photons
• Highly attenuating tissues are assigned a low numerical value because of their higher brightness resultant
• Bit depth is determined by the Analog-to-Digital Converter (ADC)
• Adjusting the window level allows image brightness to be increased or decreased
• The three primary factors

  • Milliamperage
  • Time
  • Kilovoltage
• Milliamperage (mA) and Time (s) control radiation quantity and density