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The
problem of metal
and
high-density objects
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Metallic
materials such as protheses, dental fillings,
surgical clips, and electrodes produce streak and
star pattern artifacts in the CT image. In medical
applications they usually appear together. Streaks
across the image are typically caused by
patient motion and/or small errors of scanner
geometry parameters. Streaking may be absent for stationary objects.
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Metal coil,
aneurysma and blood vessel. Input
radiograph. |
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CT
reconstruction. Note star pattern and streaks.
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Slice
image of metal
screws in cadaver. Note star-pattern artifacts without high spatial
frequency streaks.
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SAMARA:
Streak And
Metal
Artifact Reduction Algorithm
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Exxim
has developed and implemented an iterative
algorithm for 3D reconstruction, which is more
tolerant against input data imperfections than the
standard Feldkamp method. Such data imperfections
can have any nature, e.g. absorption non-linearities
or scanner geometry misalignment. One possible
field of application for the method is
metal artifact reduction. |
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SAMARA
technology is available as an option
in COBRA
starting version 4 |
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Exxim's
Streak And
Metal Artifact Reduction Algorithm (SAMARA) preserves image details
close to the metal and compensates
high-density non-linearities
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Several
methods have been invented to remove the artifacts
coming from metal. The most popular
and well known technique is based on threshold values to define (and
replace by interpolation)
erroneous pixels before image reconstruction. In
practice, because of object motion
(blood pulse, breathing, etc.) such an approach
subjects too much area to interpolation.
Additionally, in many cases (like in the
"coil" case we use for
illustrating SAMARA technology) the metal or
high-density objects do not block the signal
completely. In spite of the non-linearities,
the high frequency and low/medium contrast details
are still observable in the SAMARA reconstructed slices
(blood vessel close to the coil ). In contrast, the
interpolation technique typically is
"smoothing out" everything close to the
metal. SAMARA
technology (available as an option in COBRA
starting version 4) provides significant reduction in
non-linear image distortion preserving image
details close to the metal.
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Metal
coil and blood vessel. Reconstructed CT slice.
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Metal
coil and blood vessel, using a typical MAR
interpolation technique. The details close to the metal
coil are lost.
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Metal
coil and blood vessel. SAMARA (Streak And
Metal Artifact Reduction Algorithm) preserves
image details close to the metal and
compensates high-density non-linearities
(star-pattern artifacts).
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Visit
the GALLERY to see more
examples
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