Computed Tomography III - PowerPoint PPT Presentation

Computed Tomography III

Reconstruction

Image quality

Artifacts

• 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

• 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

• 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 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

• 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 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

• 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

• 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%

• 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):

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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 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

• 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

• 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