Computed Tomography III

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Computed Tomography III

• Reconstruction
• Image quality
• Artifacts

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Simple backprojection

• Starts with an empty image matrix, and the ?
  value from each ray in all views is added to each
  pixel in a line through the image corresponding
  to the rays path
• A characteristic 1/r blurring is a byproduct
• A filtering step is therefore added to correct
  this blurring

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Filtered backprojection

• The raw view data are mathematically filtered
  before being backprojected onto the image matrix
• Involves convolving the projection data with a
  convolution kernel
• Different kernels are used for varying clinical
  applications such as soft tissue imaging or bone
  imaging

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Convolution filters

• Lak filter increases amplitude linearly as a
  function of frequency works well when there is
  no noise in the data
• Shepp-Logan filter incorporates some roll-off at
  higher frequencies, reducing high-frequency noise
  in the final CT image
• Hamming filter has even more pronounced
  high-frequency roll-off, with better
  high-frequency noise suppression

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Bone kernels and soft tissue kernels

• Bone kernels have less high-frequency roll-off
  and hence accentuate higher frequencies in the
  image at the expense of increased noise
• For clinical applications in which high spatial
  resolution is less important than high contrast
  resolution for example, in scanning for
  metastatic disease in the liver soft tissue
  kernels are used
• More roll-off at higher frequencies and therefore
  produce images with reduced noise but lower
  spatial resolution

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CT numbers or Hounsfield units

• The number CT(x,y) in each pixel, (x,y), of the
  image is
• CT numbers range from about 1,000 to 3,000
  where 1,000 corresponds to air, soft tissues
  range from 300 to 100, water is 0, and dense
  bone and areas filled with contrast agent range
  up to 3,000

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CT numbers (cont.)

• CT numbers are quantitative
• CT scanners measure bone density with good
  accuracy
• Can be used to assess fracture risk
• CT is also quantitative in terms of linear
  dimensions
• Can be used to accurately assess tumor volume or
  lesion diameter

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Digital image display

• Window and level adjustments can be made as with
  other forms of digital images
• Reformatting of existing image data may allow
  display of sagittal or coronal slices, albeit
  with reduced spatial resolution compared with the
  axial views
• Volume contouring and surface rendering allow
  sophisticated 3D volume viewing

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Image quality

• Compared with x-ray radiography, CT has
  significantly worse spatial resolution and
  significantly better contrast resolution
• Limiting spatial resolution for screen-film
  radiography is about 7 lp/mm for CT it is about
  1 lp/mm
• Contrast resolution of screen-film radiography is
  about 5 for CT it is about 0.5

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Image quality (cont.)

• Contrast resolution is tied to the SNR, which is
  related to the number of x-ray quanta used per
  pixel in the image
• There is a compromise between spatial resolution
  and contrast resolution
• Well-established relationship among SNR, pixel
  dimensions (?), slice thickness (T), and
  radiation dose (D)

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Factors affecting spatial resolution

• Detector pitch (center-to-center spacing)
• For 3rd generation scanners, detector pitch
  determines ray spacing for 4th generation
  scanners, it determines view sampling
• Detector aperture (width of active element)
• Use of smaller detectors improves spatial
  resolution
• Number of views
• Too few views results in view aliasing, most
  noticeable toward the periphery of the image

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Factors affecting spatial resolution (cont.)

• Number of rays
• For a fixed FOV, the number of rays increases as
  detector pitch decreases
• Focal spot size
• Larger focal spots cause more geometric
  unsharpness and reduce spatial resolution
• Object magnification
• Increased magnification amplifies the blurring of
  the focal spot

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Factors affecting spatial resolution (cont.)

• Slice thickness
• Large slice thicknesses reduce spatial resolution
  in the cranial-caudal axis they also reduce
  sharpness of edges of structures in the
  transaxial image
• Slice sensitivity profile
• A more accurate descriptor of slice thickness
• Helical pitch
• Greater pitches reduce resolution. A larger
  pitch increases the slice sensitivity profile

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Factors affecting spatial resolution (cont.)

• Reconstruction kernel
• Bone filters have the best spatial resolution,
  and soft tissue filters have lower spatial
  resolution
• Pixel matrix
• Patient motion
• Involuntary motion or motion resulting from
  patient noncompliance will blur the CT image
  proportional to the distance of motion during
  scan
• Field of view
• Influences the physical dimensions of each pixel

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Factors affecting contrast resolution

• mAs
• Directly influences the number of x-ray photons
  used to produce the CT image, thereby influencing
  the SNR and the contrast resolution
• Dose
• Dose increases linearly with mAs per scan
• Pixel size (FOV)
• If patient size and all other scan parameters are
  fixed, as FOV increases, pixel dimensions
  increase, and the number of x-rays passing
  through each pixel increases

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Factors affecting contrast resolution (cont.)

• Slice thickness
• Thicker slices uses more photons and have better
  SNR
• Reconstruction filter
• Bone filters produce lower contrast resolution,
  and soft tissue filters improve contrast
  resolution
• Patient size
• For the same technique, larger patients attenuate
  more x-rays, resulting in detection of fewer
  x-rays. Reduces SNR and therefore the contrast
  resolution

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Factors affecting contrast resolution (cont.)

• Gantry rotation speed
• Most CT systems have an upper limit on mA, and
  for a fixed pitch and a fixed mA, faster gantry
  rotations result in reduced mAs used to produce
  each CT image, reducing contrast resolution

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Beam hardening

• Like all medical x-ray beams, CT uses a
  polyenergetic x-ray spectrum
• X-ray attenuation coefficients are energy
  dependent
• After passing through a given thickness of
  patient, lower-energy x-rays are attenuated to a
  greater extent than higher-energy x-rays are
• As the x-ray beam propagates through a thickness
  of tissue and bones, the shape of the spectrum
  becomes skewed toward higher energies

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Beam hardening (cont.)

• The average energy of the x-ray beam becomes
  greater (harder) as it passes through tissue
• Because the attenuation of bone is greater than
  that of soft tissue, bone causes more beam
  hardening than an equivalent thickness of soft
  tissue

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Beam hardening (cont.)

• The beam-hardening phenomenon induces artifacts
  in CT because rays from some projection angles
  are hardened to a differing extent than rays from
  other angles, confusing the reconstruction
  algorithm
• Most scanners include a simple beam-hardening
  correction algorithm, based on the relative
  attenuation of each ray
• More sophisticated two-pass algorithms determine
  the path length that each ray transits through
  bone and soft tissue, and then compensates each
  ray for beam hardening for the second pass

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Motion artifacts

• Motion artifacts arise when the patient moves
  during the acquisition
• Small motions cause image blurring
• Larger physical displacements produce artifacts
  that appear as double images or image ghosting

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Partial volume averaging

• Some voxels in the image contain a mixture of
  different tissue types
• When this occurs, the ? is not representative of
  a single tissue but instead is a weighted average
  of the different ? values
• Most pronounced for softly rounded structures
  that are almost parallel to the CT slice

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Partial volume averaging (cont.)

• Occasionally a partial volume artifact can mimic
  pathological conditions
• Several approaches to reducing partial volume
  artifacts
• Obvious approach is to use thinner CT slices
• When a suspected partial volume artifact occurs
  with a helical study and the raw scan data is
  still available, additional CT images may be
  reconstructed at different positions