introduction to CT

• what is CT

  • computed tomography
  • x ray tube ‘spins around’
  • to produce cross sectional images through anatomy
• advantages of ct


  • images are acquired in slices
  • soft tissue demonstration
  • blood vessels can be visualised
  • large areas can be imaged in a short space of time
  • functional information
  • post processing and reconstruction of images
• what are the principles of CT scanning

  • x ray tube with collimation device
  • tube emits a very narrow, flat fan beam
  • beam is aligned to an array of detectors in the form of an arc
  • each detector is equidistant to the focal spot
  • tube continually rotates around while the patient moves through the gantry
  • x ray beam is attenuated by the patient on the way to the detector
  • the detector will measure the amount of attenuation
• attenuation


  • the denser the tissue the more attenuation will occur
  • bone will attenuate more x rays than fat
• the detectors

  • the array of detectors divide the body into linear strips
  • each detector measures the attenuation that has occurred within its own strip
  • this information is then digitised and fed into a computer
• the computer


  • the computer assigns a numerical value to data from each detector
  • this value is based upon the amount of attenuation that has taken place
• image reconstruction 


  • throughout the examinations many exposures are undertaken as the patient passes through the gantry
  • in order to obtain enough data to reconstruct an image
• data set


  • this data is stored in a matrix
  • a single square within the matrix is called a pixel
  • each pixel corresponds to the volume of tissue
  • the volume of tissue is known as a voxel
• hounsfield units


  • each pixel is assigned a numerical value based on the average attenuation value of the whole voxel
  • this number is compared to the attenuation value of water
• first generation - 1970

  • single x ray source and detector
  • pencil beam geometry
  • rotated 180° 1 degree at a time
  • very long scan and reconstruction times
• second generation - 1972


  • multiple detector array and fan beam
  • first commercially available systems
  • first patient scanned 1972, demonstrating a cystic brain lesion
  • 90 secs per slice
• third generation - 1976


  • tube and detector array rotate 360 degrees
  • wide angle fan beam 50-55 degrees
  • typically 600-900 detectors in an array
  • further reduced scan times (5 secs)
  • whole body scanning achievable
• fourth generation - 1978


  • tube rotates 360° about iso-centre
  • detector array remains stationary - complete ring
  • similar scan times to 3rd gen
  • advancements in slip ring technology and detector means 3rd gen scanners have prevailed
• slip ring technology - 1987


  • slice by slice scanning was very time consuming
  • led to the development of slip ring technology
  • which allows for the patient to move continually through the gantry
• spiral / helical CT - 1989


  • constant x ray production / detection while patient moves through the gantry
  • path of x ray beam describes a ‘spiral’ or ‘helix’
  • produced volumes of data, rather than individual slices
• spiral / helical CT


  • one of the most significant developments in CT
  • decrease in acquisition time, reduction of mis-registration artefacts
  • continuous coverage of anatomy
  • improved reformatted images
  • capable of tracking injected contrast agents
• dual source - 2005


  • siemens medical solutions announced a new type of CT scanner
  • two x ray tubes and two detectors
  • used for cardiac imaging