CT Scans the principles

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CT Scans - the principles

• Craig Douglas

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General Info.

• It is an X-ray imaging method
• Formation of an image is a distinct two stage
  process
• Principles described by Dr. Alan Cormack
• First system devised by Dr. Godfrey Hounsfield

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The Process

• Step 1- Scanning
• - A very thin X- ray beam is passed through
  the edges of a slice of body tissue
• - The beam is rotated around the body ( takes
  up to 15 seconds )
• - Radiation penetrates to different degrees,
  depending on the density of the tissues they pass
  through ( dependent on the atomic numbers of the
  elements within the tissues )
• - Electronic radiation detectors detect
  penetration
• Step 2- Image Re-Construction

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Scanner
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Still Scanners!

• Third Generation
• -X-ray tube and radiation sensors both rotate
  in step with each other
• Fourth Generation
• -X-ray tube rotates and radiation sensors form
  a continuous circle
• No real indications of superiority

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Image Formatting

• A CT image appears to be a continuous display of
  a slice of tissue
• But, No!
• A CT scan is actually a matrix of individual
  elements

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Image Formatting

• The slice of tissue is formatted as an array of
  small volumes of elements, these are known as
  VOXELS
• Each pixel on the monitor will represent a VOXEL
• One pixel is displayed as uniform brightness, the
  elements within it are blurred together

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Voxels and Pixels

• The operator selects the number of pixels in an
  image, therefore this dictates the size of the
  voxels
• As pixel number increases, voxel size decreases,
  allowing for better quality images.

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CT Numbers

• Each pixel can be represented by a CT number
• The value is related to the physical density of
  the tissue in the corresponding voxel
• Measured in Hounsfield Units
• Water is the reference and assigned a value of
  zero

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CT Numbers

• Tissues more dense than water have a positive
  number
• Same for opposites
• CT Number density (tis) density (H2O)
• density (H2O)
• a X 1000

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Image Display

• Areas of high density, with high CT numbers
  appear white on the grey-scale
• Areas of low density, with low CT numbers appear
  darker on the grey-scale

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Windowing

• Relationships between the CT numbers and the
  grey-scale can be adjusted by the system viewer
• Viewer controls can set upper and lower CT number
  window limits
• CT numbers gt upper limit white
• CT numbers lt lower limit black
• Easier distinction between soft tissues

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Conversions
1000
0
-1000
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Image Quality

• Greater contrast sensitivity than plain
  radiography because
• All tissues are viewed directly, as opposed to
  looking at deep tissues obscured by superficial
  tissues
• The relatively thin X-ray beam produces
  relatively less scatter
• The ability to express a small range of CT
  numbers over the entire grey scale

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Noise

• This is a mottled appearance on CT
• Its presence decreases the visibility of low
  contrast features
• Produced due to the random manner in which x-ray
  photons are absorbed in the tissue
• Decrease by gt x-ray dose (careful)
• or, gt voxel size (poor)

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Artifacts

• Most commonly appear as streaks
• Causes
• - patient moving during scanning
• - metal objects in field of view

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Radiation Dose

• Measure absorbed radiation dose to the tissue
  within the scanned slice
• Rad 100 ergs per 1g tissue
• Gray 1 joule per 1kg tissue
• Represents radiation at a specific point, not
  total radiation applied
• Different areas record different values

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Dose Distribution

• Direct exposure is limited to the tissue within
  the slice
• A small amount will scatter to adjacent tissues
• Total radiation energy increases with the no. of
  slices imaged and this must be considered
• Generally even distribution, due to rotation of
  X-ray beam