Computed Tomography II

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					Computed Tomography II

  Detectors and detector arrays
     Details of acquisition
  Tomographic reconstruction
                Detectors
• Xenon detectors
• Solid-state detectors
• Multiple detector arrays
            Xenon detectors
• Use high-pressure (about 25 atm)
  nonradioactive xenon gas, in long thin cells
  between two metal plates
• Very thick (e.g., 6 cm) to compensate in
  part for relatively low density
• Thin metal septa separating individual
  detectors improves geometric efficiency by
  reducing dead space between detectors
       Xenon detectors (cont.)
• Long, thin plates are highly directional
• Must be positioned in a fixed orientation
  with respect to the x-ray source
• Cannot be used for 4th generation scanners
  because those detectors must record x-rays
  as the source moves over a wide angle
         Solid-state detectors
• Composed of a scintillator coupled tightly
  to a photodetector (typically a photodiode)
• Scintillator emits visible light when an x-
  ray is absorbed, similar to an x-ray
  intensifying screen
• Photodetector converts light intensity into
  an electrical signal proportional to the light
  intensity
    Solid-state detectors (cont.)
• Detector size typically 1.0 x 15 mm (or 1.0
  x 1.5 mm for multiple detector arrays)
• Scintillators used include CdWO4 and
  yttrium and gadolinium ceramics
• Better absorption efficiency than gas
  detectors because of higher density and
  higher effective atomic number
    Solid-state detectors (cont.)
• To reduce crosstalk between adjacent detector
  elements, a small gap between detector elements is
  necessary, reducing geometric efficiency
  somewhat
• Top surface of detector is essentially flat and
  therefore capable of x-ray detection over a wide
  range of angles
• Required for 4th generation scanners and used in
  most high-tier 3rd generation scanners as well
       Multiple detector arrays
• Set of several linear detector arrays, tightly
  abutted
• Use solid-state detector arrays
• Slice width is determined by the detectors,
  not by the collimator (although collimator
  does limit the beam to the total slice
  thickness)
  Multiple detector arrays (cont.)
• 3rd generation multiple detector array with 16
  detectors in the slice thickness dimension and 750
  detectors along each array uses 12,000 individual
  detector elements
• 4th generation scanner would require roughly 6
  times as many detector elements; consequently
  currently planned systems use 3rd generation
  geometry
          Slice thickness:
   single detector array scanners
• Determined by the physical collimation of the
  incident x-ray beam with two lead jaws
• Width of the detectors places an upper limit on
  slice thickness
• For scans performed at the same kV and mAs, the
  number of detected x-ray photons increases
  linearly with slice thickness
• Larger slice thicknesses yield better contrast
  resolution (higher SNR), but the spatial resolution
  in the slice thickness dimension is reduced
        Slice sensitivity profile
• For single detector array scanners, the shape of the
  slice sensitivity profile is a consequence of:
   – Finite width of the x-ray focal spot
   – Penumbra of the collimator
   – The fact that the image is computed from a number of
     projection angles encircling the patient
   – Other minor factors
• Helical scans have a slightly broader slice
  sensitivity profile due to translation of the patient
  during the scan
         Slice thickness:
 multiple detector array scanners
• In axial scanning (i.e., with no table movement)
  where, for example, four detector arrays are used,
  the width of the two center detector arrays almost
  completely dictates the thickness of the slices
• For the two slices at the edges of the scan, the
  inner side of the slice is determined by the edge of
  the detector, but the outer edge is determined
  either by the outer edge of the detector or by the
  collimator penumbra, depending on collimator
  adjustment
   Slice thickness: MDA (cont.)
• In helical mode, each detector array contributes to
  every reconstructed image
   – Slice sensitivity profile for each detector array needs to
     be similar to reduce artifacts
• Typical to adjust the collimation so that the focal
  spot – collimator blade penumbra falls outside the
  edge detectors
   – Causes radiation dose to be a bit higher (especially for
     small slice widths)
   – Reduces artifacts by equalizing the slice sensitivity
     profiles between the detector arrays
  Detector pitch/collimator pitch
• Pitch is a parameter that comes into play when
  helical scan protocols are used
• In a helical scanner with one detector array, the
  pitch is determined by the collimator
• Collimator pitch = table movement (mm) per 360-
  degree rotation of gantry / collimator width (mm)
  at isocenter
• Pitch may range from 0.75 (overscanning) to 1.5
  (faster scan time, possibly smaller volume of
  contrast agent)
                 Pitch (cont.)
• For scanners with multiple detector arrays,
  collimator pitch is still valid
• Detector pitch = table movement (mm) per 360-
  degree rotation of gantry / detector width (mm)
• For a multiple detector array scanner with N
  detector arrays, collimator pitch = detector pitch /
  N
• For scanners with four detector arrays, detector
  pitches running from 3 to 6 are used
   Tomographic reconstruction
• Rays and views: the sinogram
• Preprocessing the data
• Interpolation (helical)
                  Sinogram
• Display of raw data acquired for one CT slice
  before reconstruction
• Rays are plotted horizontally and views are shown
  on the vertical axis
• Objects close to the edge of the FOV produce a
  sinusoid of high amplitude
• Bad detector in a 3rd generation scanner would
  show up as a vertical line on the sinogram
             Rays and views
• 1st and 2nd generation scanners used 28,800 and
  324,000 data points, respectively
• State-of-the-art scanner may aquire about 800,000
  data points
• Modern 512 x 512 circular CT image contains
  about 205,000 image pixels
• Number of rays affects the radial component of
  spatial resolution; number of views affects the
  circumferential component of the resolution
            Number of rays
• CT images of a simulated object
  reconstructed with differing numbers of
  rays show that reducing the ray sampling
  results in low-resolution, blurred images
           Number of views
• CT images of the simulated object
  reconstructed with differing numbers of
  views show the effect of using too few
  angular views (view aliasing)
• Sharp edges (high spatial frequencies)
  produce radiating artifacts that become
  more apparent near the periphery of the
  image
             Preprocessing
• Calibration data determined from air scans
  (performed by the technologist or service
  engineer periodically) provide correction
  data that are used to adjust the electronic
  gain of each detector
• Variation in geometric efficiencies caused
  by imperfect detector alignments is also
  corrected
                Interpolation
• CT reconstruction algorithms assume that the x-
  ray source has negotiated a circular, not helical,
  path around the patient
• Before the actual CT reconstruction, the helical
  data set is interpolated into a series of planar
  image sets
• With helical scanning, CT images can be
  reconstructed at any position along the length of
  the scan
           Interpolation (cont.)
• Interleaved reconstruction allows the placement of
  additional images along the patient, so that the
  clinical examination is almost uniformly sensitive
  to subtle abnormalities
• Adds no additional dose to the patient, but
  additional time is required to reconstruct the
  images
• Actual spatial resolution along the long axis of the
  patient still dictated by slice thickness

				
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