Computed tomography

• source of computed tomography:

  • ionizing radiation
  • collimated X-Ray beam; X-Ray tube rotates around the patient (through the gantry)
• In computed tomography, narrow x-ray beam are aimed at the patient and quickly rotates around the body of the patient producing signals and processed by the machine to produce cross-sectional images (tomograms)
• early sensors for computed tomography were scintillation detectors with photomultiplier tubes excited by sodium iodide (NaI) crystals
• modern detectors are solid state scintillators coupled to photodiodes or are filled with low-pressure xenon gas
• An image is obtained in CT by computer processing of the digital readings of the detectors.
• 1. motorized x-ray tube that rotates around the circular opening (gantry) together with collimator
  2. patient lies on the patient couch that slowly move through the gantry
  3. while x-ray tube rotates, it shoots narrow beam/collimated beam throughout the body
  4. ct scanner uses specialized detectors (scintillators with PMT), which is directed opposite to the x-ray tube
  5. detectors pick-up x-rays that pass through the patient
  6. in one full rotation, CT generates 2D images
  7. image processing
  8. 3D
• image attributes of computed tomography: thin transverse sections of the body are acquired representing an absorption pattern or X-ray attenuation of each tissue
• Absorption values are expressed as Hounsfield Units (HU)
• CT number for dense bone: 3000
• CT number for muscle: 50
• CT nmber for white matter: 45
• CT number for gray matter: 40
• CT number for blood: 20
• CT number for cerebrospinal fluid: 15
• CT number for water: 0
• CT number for fat: -100
• CT number lungs: -200
• CT number for air: -1000
• computed tomography has good contrast resolution allowing differentiation of tissues with similar physical densities
• tomographic acquisition eliminates the superposition of images of overlapping structures
• advanced scanners can produce images that can be viewed in multiple planes or as volumes. Any region of the body can be scanned
• computed tomography has become diagnostic modality of choice for a large number of disease entities
• computed tomography is useful for tumor staging
• computed tomography has high cost of equipment and procedure
• computed tomography has high dose of ionizing radiation per examination
• CT produces artifacts from high contrast objects in the body such as bone or devices
• table increment/pitch ratio: movement of patient couch
• slow pitch ratio = higher patient dose, good contrast resolution
• fast pitch ratio = lower patient dose, poor contrast resolution