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Computed Tomography III. Reconstruction Image quality Artifacts. 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

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computed tomography iii

Computed Tomography III


Image quality


simple backprojection
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
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
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 kernels
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
ct numbers or hounsfield units
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
ct numbers cont
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
digital image display
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
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%
image quality cont
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):
factors affecting spatial resolution
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
factors affecting spatial resolution cont
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 cont19
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 cont20
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 resolution
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
factors affecting contrast resolution cont
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 cont23
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
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
beam hardening cont
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 cont28
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
  • 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
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
partial volume averaging cont
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