• Created by Godfrey Hounsfield in 1972. He won a Nobel prize in 1979
• CT image is created by revolving an XR tube around the patient at high speed, opposite a stationary ring of detectors of very high sensitivity.
  • Because of the sensitivity of the detectors a wide range of densities can be obtained
  • XR beam is fan shaped
• range of densities in a normal film is around 20. In a CT image it is 2000
• Hounsfield units
  • This is a measure of attenuation in CT scans
  • Air -1000 units
  • Water 0 units
  • Cortical bone +1000 units
• range of Hounsfield units displayed can be manipulated by choosing a particular window width. Certain tissues can be highlighted by the appropriate window width; hence “bony windows” & “soft tissue windows”

Plane of imaging

• image is obtained in the axial plane, but images can be reconstructed in any desired plane