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Computed Tomography III Reconstruction Image quality - PowerPoint PPT Presentation

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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 l.jpg

Computed Tomography III


Image quality


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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 ray’s 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