COMPUTED TOMOGRAPHY

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COMPUTED TOMOGRAPHY
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GENERAL CONSIDERATIONS

• Geoffrey Hounsfield
• Developed first working model in 1970.
• Developments and Advancements
• Fourth generation scanners
• Helical/Spiral CT
• EBCT (Electron Beam Computed Tomography) EBT

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GENERAL CONSIDERATIONS

• The CT Image
• Shows a thin slice of anatomy obtained from
  multiple measurements.
• Different from conventional tomography.

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GENERAL CONSIDERATIONS

• Components of a CT scanner
• Scanning gantry
• Detector array
• X-ray generator
• Computerized processing unit

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IMAGE PRODUCTION

• The formation of a CT image is a distinct three
  phase process.
• The scanning phase.
• The reconstruction phase.
• Digital-to-analog conversion phase.

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IMAGE PRODUCTIONTHE SCANNING PHASE

• Fourth generation scanners place the detector
  array in a fixed position.
• The tube rotates around the patient.
• Collimators determine slice thickness and reduces
  patient exposure.
• Table moves.

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IMAGE PRODUCTIONTHE SCANNING PHASE

• Scan and Step Scanning
• One complete scan around to body is made while
  the body is not moving. Then the body is moved to
  the next slice position.

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IMAGE PRODUCTIONTHE SCANNING PHASE

• Spiral/ Helical Scanning
• The patients body is moved continuously as the
  x-ray beam is scanned around the body.
• The data set is not broken into slices.
• The volume data can be used to reconstruct 3-D
  images.

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IMAGE PRODUCTIONTHE RECONSTRUCTION PHASE

• Anatomic information received from the detector
  array is process by computer algorithms to
  produce the digitized axial tomographic image.
• The photon beam is differentially attenuated by
  the exposed tissues.
• Coefficient of attenuation (CT number)

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IMAGE PRODUCTIONDIGITAL-TO-ANALOG PHASE

• Responsible for the visible and displayed analog
  image (shades of gray)
• There are adjustable factors associated with each
  of the phases that can have an effect on the
  image.
• Window level and width
• Reformatting

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IMAGE PRODUCTIONDIGITAL-TO-ANALOG PHASE

• Window level and window width
• Manipulated by the technologist or radiologist to
  change the characteristics and highlight specific
  tissue attributes prior to printing the images.
• Soft tissue windows and bone windows

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IMAGE PRODUCTIONDIGITAL-TO-ANALOG PHASE

• Reconstruction or reformatting the image
• CT information is typically acquired in the axial
  plane.
• Sagittal and coronal images can be created by
  manipulation of the axial images.
• 3D manipulation.

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ADDITIONAL CT RELATED PROCEDURES

• Contrast Computed Tomography
• Intrathecal, intradiscal or intravenous injection
  of radiopaque contrast before CT.
• Multidetector Computed Tomography
• Helical/Spiral CT
• EBCT

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ABSORBED DOSE AND CONTRAINDICATIONS

• Contraindications
• Related to ionizing radiation.
• Hypersensitivity to iodine products.
• Children should not be scan without strong
  indications.
• Absorbed dose
• Typical lumbar scan (30-36 slices) is 3-5 rads

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INDICATIONS FOR CT

• Trauma
• Vascular
• Aneurysms, heart
• Neoplasms
• Arthritides
• Chest and Abdomen
• Bone densitometry