Computed Tomography III

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					Computed Tomography III

      Image quality
        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 ray’s path
• A characteristic 1/r blurring is a byproduct
• A filtering step is therefore added to correct
  this blurring
       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
           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
    Bone kernels and soft tissue
• 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
 CT numbers or Hounsfield units
• The number CT(x,y) in each pixel, (x,y), of the
  image is:
                                 ( x, y )   water
            CT ( x, y )  1,000
                                    water
• 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
          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
  – Can be used to accurately assess tumor volume
    or lesion diameter
        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
               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
• Contrast resolution of screen-film radiography is
  about 5%; for CT it is about 0.5%
           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):

                       D 3
        Factors affecting spatial
• 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
        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
        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
        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
      Factors affecting contrast
• 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
       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
      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
               Beam hardening
• Like all medical x-ray beams, CT uses a
  polyenergetic x-ray spectrum
• X-ray attenuation coefficients are energy
   – 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
       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
         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
            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
      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
 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

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