Radial/Spiral ArtifactsWhat about wrap-around artifacts on radial or spiral imaging? it seems like they should always be present because phase-encode goes in every direction.
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Wrap-around and other types of aliasing artifacts do occur in radial/spiral methods, but they have forms different than those seen on rectilinear (Cartesian) scanning.
In most radial and spiral imaging methods, frequency- and phase-axes do not maintain fixed relationships to the cardinal anatomic directions (left-right, superior-inferior, etc.) Instead, phase- and frequency-encoding directions change continually as the trajectory sweeps or rotates around the center of k-space. When aliasing occurs, it usually takes the form of curvilinear bands diffusely distributed across the image, although discrete radially arranged "ghosts" may also appear.
PROPELLER acquisition shows how frequency- and phase-encoding directions change depending on orientation of each blade.
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Aliasing from red point (lower left) outside the circular FOV is replicated as an arc across the image.
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When relatively a few blades and small FOV are used, several discrete aliased images from back of head are noted
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When many blades are used, the aliasing artifacts become more widely dispersed.
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A somewhat different form of artifact is commonly seen in the background of PROPELLER images consisting of a diffuse "halo" of fine lines or streaks. These are reconstruction artifacts resulting from gridding, the process where data points acquired in radial/spiral mode must morphed/interpolated to fit into an evenly spaced rectangular array for data processing. With linear interpolation, simple shading occurs across the image, but with higher order interpolation schemes more complex oscillations are aliased into the image. Occasionally these may resemble bright streaks from the edges of high signal objects, similar to those seen around bone or metal on CT.
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The "gridding" problem in spiral/radial/PROPELLER imaging - acquired data points do not match the rectangular array required for discrete Fourier transformation, so they must be morphed/interpolated to fit. This introduces ringing and aliasing artifacts into the final image.
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Diffuse radial reconstruction artifact with insufficient data points in this so-called "top of head" artifact
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Fine streaks from high signal region on right cheek on this PROPELLER image.
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References
Pipe JG. Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging. Magn Reson Med 1999; 42:963-969.
Patch SK. k-Space data preprocessing for artifact reduction in MR imaging. Multidimensional Image Processing, Analysis, and Display: RSNA Categorical Course in Diagnostic Radiology
Physics 2005; pp 73–87.
Rasche V, Proska R, Sinkus R, et al. Resampling of data between arbitrary grids using convolution interpolation. IEEE Trans Med Imaging 1999;18:385–392.
Pipe JG. Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging. Magn Reson Med 1999; 42:963-969.
Patch SK. k-Space data preprocessing for artifact reduction in MR imaging. Multidimensional Image Processing, Analysis, and Display: RSNA Categorical Course in Diagnostic Radiology
Physics 2005; pp 73–87.
Rasche V, Proska R, Sinkus R, et al. Resampling of data between arbitrary grids using convolution interpolation. IEEE Trans Med Imaging 1999;18:385–392.
Related Questions
How does PROPELLER reduce motion artifacts?
How does PROPELLER reduce motion artifacts?