Computed%20Tomography%20III - PowerPoint PPT Presentation

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Computed%20Tomography%20III

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Compared with x-ray radiography, CT has significantly worse spatial resolution ... Like all medical x-ray beams, CT uses a polyenergetic x-ray spectrum ... – PowerPoint PPT presentation

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Title: Computed%20Tomography%20III


1
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

6
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

9
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

10
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

11
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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15
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

16
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)

17
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

18
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

19
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

20
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

21
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

22
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

23
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

24
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

31
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
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