Computed Tomography

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Computed Tomography

• Kristie Powell

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History

• J Radon
• Predicted that through his mathematical
  projections a three dimensional image could be
  produced
• Godfrey Hounsfield
• Credited with the invention of computed
  tomography in 1970 1971
• Built the first scanner on a lathe bed
• Took nine days to produce the first image

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Computed Tomography

• Medical imaging technique employing tomography
• Tomography is imaging by sections
• Digital geometry processing generates a 3D image
• Comprised of a series of 2D x-ray images
• Uses x-rays equipment to obtain cross sectional
  pictures of the body

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X ray Projection
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X ray Projection
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Process (1)

• x-ray principle
• x-rays pass through the body
• Absorption or attenuation produces a matrix
  profile
• CT scanner is a rotating frame
• x-ray tube mounted on one side
• x-ray detector positioned across
• Fan beam of x-ray produced as frame spins
• One revolution scans a horizontal slice
• Detector creates profiles of attenuated x ray
  beam

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Process (2)

• Data slices accumulate as the body passes through
  the gantry
• Tomographic reconstruction
• Combines each slice
• Fast Fourier Transform technique
• Increase the voxel (pixel) resolution
• Back projection
• Produces 2D data that can be displayed

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Process(3)

• CT Images
• Pixels are displayed in terms of radiodensity
• Based on the mean attenuation of the tissue
• -1024 to 3071 Hounsfield scale
• Voxel is based on slice thickness
• Windowing
• Calculating Hounsfield units(HU) to make an image
• Overview of structures that attenuate the beam

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Properties

• Specifications
• 512 x 512 12 bit gray levels
• Resolution 0.5 mm
• Slice interval 1-10mm depending on anatomy
• All digital
• Printed on x-ray films
• Hounsfield Units
• Grey levels
• Air is 1000HU
• Fat is 100 to 300HU
• Water is 0Hu

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3D Images (1)

• Multiplanar Reconstruction (MPR)
• Volume built by stacking the axial slices
• Software then cuts slices through the volume in a
  different plane (usually orthogonal)
• Examining the spine
• Reconstruction of slices
• Maximum Intensity Projection (MIP)
• Enhance areas of high radiodensity
• Minimum Intensity Projection (mIP)
• Enhance air spaces so are useful for assessing
  lung structure

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3D Image (2)

• 3D rendering techniques
• Surface rendering
• A threshold value of radiodensity is chosen
• A threshold level is set, using edge detection
• 3D model can be constructed and displayed on
  screen
• Volume rendering
• Transparency and colors are used to allow a
  better representation of the volume
• The bones of the pelvis could be displayed as
  semi-transparent

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Image Quality

• Artifacts
• Abrupt transitions between low and high density
  materials
• Partial volume effect
• Unable to differentiate between a small amount of
  high-density material (e.g. bone) and a larger
  amount of lower density (e.g. cartilage)
• Partially overcome by scanning using thinner
  slices
• Noise
• Appears as graining because of low signal to
  noise ratio

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Image Quality (2)

• Motion
• Blurring caused by movement of the object
• Windmill
• Streaking occur when the detectors intersect the
  reconstruction plane
• Reduced with filters or a reduction in pitch
• Beam Hardening
• A cupped appearance because more attenuation in
  the center of the object than around the edge
• Corrected by filtration and software

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Image Quality (3)

• Spatial Resolution
• Ability to distinguish between 2 high contrast
  objects
• Edge of bone
• Tumor margins
• Limited by number of detectors and number of
  projects
• Improvements
• Decrease slice thickness
• Use smaller display field of view

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Image Quality (4)

• Contrast Resolution
• Ability to distinguish a low contrast object from
  its background
• Liver tumors
• Soft tissue
• Improvements
• Increase section thickness
• Decrease matrix size
• Result in less noise

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CT Accquistion

• Axial
• Each slice/volume is taken
• Table is incremented to the next location
• Tomographic reconstruction is used to generate
  images
• Spiral
• Gantry holding the source and detector array
  rotates
• Patient is translated along the axis of rotation
• The volume is scanned very quickly
• Table is in constant motion as the gantry rotates
  continuously

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Advantages

• CT scanning is
• Painless
• Noninvasive
• Accurate
• Simultaneously able to image
• Bone
• Soft tissue
• Blood vessels
• Examinations are quick

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Disadvantages

• Slight chance of cancer from radiation
• Benefit of an accurate diagnosis far outweighs
  the risk
• Not recommended for pregnant women because of
  potential risk to the baby
• Serious allergic reaction to contrast materials
  that contain iodine is rare
• Radiology departments are equipped to deal with
  it

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Diagnostic Use

• Cancer
• Follow the course of cancer treatment
• Cardiac
• Each portion of the heart is imaged more than
  once
• ECG trace is recorded
• ECG is then used to correlate the CT data with
  corresponding phases of cardiac contraction
• Extremities
• Fracture injuries and dislocations can easily be
  recognized with a 0.2 mm resolution

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References

• "Computed Tomography." 3 June 2008
  ltwww.wikipedia.orggt.
• Gonzalez, Carlos F. Computed Brain and Orbital
  Tomography. Canada John Wiley and Sons, 1976.
• New, Paul F., and William R. Scott. Computed
  Tomography of the Brain and Orbit. Baltimore
  Maryland The Williams and Wilkins Company, 1975.