Basics of X-ray and Mammographic Systems

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Special Considerations for Mammography Systems

  • In addition to tungsten, molybdenum and rhodium used as target materials
K-shell e- binding energy (keV)
K-shell characteristic x-ray (keV)
Tungsten 69.5 59.32
Molybdenum 20.0 17.48
Rhodium 23.2 20.22
  • Molybdenum & rhodium used as filters
  • Take advantage of filter's k-edge to shape spectrum for optimum subject contrast
  • Mammography tubes usually have beryllium windows
    • Less filtering
    • Reduces beam energy increase for improved subject contrast of breast
  • Many mammography systems calculate and insert filtration required for optimum beam spectrum
Tube port (window)
  • Beryllium replaces glass
  • Reduced
    • attenuation of beam to maximize radiation output
    • filtering of beam to improve subject contrast
Exposure parameters
  • Tube current
    • significantly less than radiography
      • smaller focal spots used
    • further reduction in mA for magnification (small focal spot)
  • Kilovoltage
    • significantly less than radiography
      • improves subject contrast
  • Source to image distance (SID)
    • significantly less than radiography
      • compensates for low beam intensity due to smaller focal spot size
  • Small focal spot used
    • recovers spatial resolution lost by magnification of 1.2-2.0
    • reduces penumbra
    • requires lower mA resulting in longer exposure time
  • Scatter radiation reduced using air gap instead of grid
  • Increases dose
  • Reduced spatial resolution
  • Improved effective resolution
Heel Effect
  • Highest radiation output to thickest part of breast (chest wall)
  • Anode-cathode axis oriented in anterior-posterior direction
    • Cathode side of tube closest to patient
  • Decreases scatter
  • Decreases geometric blurring
  • Decreases superimposition
  • Decreases patient motion
  • Decreases exposure time
  • Decreases dose
  • Increases uniformity of breast thickness
  • Lower grid ratios used
  • Typically single emulsion film
  • Single screen above film
Digital Mammography
  • Direct: amoryphous selenium
  • Indirect: CsI, CR