Abstract:
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Purpose: 16-slice CT suffers from windmill artifacts in helical scans due to
under sampling along the axis of rotation in the presence of partial volume
data of a scanned object. The windmill artifact appears as black and white
pattern rotating around high contrast features that change along the
longitudinal axis. In this work we study the dependence of the windmill
artifact on the scan and reconstruction parameters.
Methods and Materials: A phantom containing an 80 mm thick, 5 mm diameter metal
disc perpendicular to the axis of rotation was used for this study. The
frequency of the white and black pattern was measured on the axial images as a
function of the pitch and of the number of slices. The windmill artifact level
was quantified on MPR images using mean square differences method. The
dependence of the artifact level on the pitch, the distance from the center of
rotation, and on the number of slices was investigated. A comparison of the
windmill artifacts in true 3D Cone Beam and standard 2D back-projection schemes
was performed. A schematic model explaining the observed differences has been
developed.
Results: The set of measured data was reconstructed in standard 2D (fan beam)
and true 3D (cone beam) reconstruction algorithms. The dependence of the
windmill artifact level on the scan pitch at the center of rotation is similar
for 2D and 3D back projection and is a wave-like function. The windmill
artifact level at varying distances from the center of rotation was measured.
It is shown that at large distance from the center of rotation the artifact
level is higher for 2D compared to 3D back projection. The windmill artifact
level is reduced for large z-filtering broadening factor used for spiral
interpolation. The Frequency of the windmill pattern increases with the pitch
and depends on the number of slices. Some of the parameter-combinations were
qualitatively verified on a head phantom with a cast human skull.
Conclusion: The windmill artifact level and frequency depend strongly on the
pitch and on the distance from the center of rotation. At a large distance from
the center of rotation the windmill artifact is reduced using true 3D back
projection compared to 2D back projection. It is therefore possible to minimize
windmill artifacts by optimizing the scan and reconstruction parameters. (E.S.,
G.S., A.A., D.B. are employees of Philips Medical Systems & Technologies.)
Questions about this event email: efrat.shefer@philips.com
Shefer PhD, E,
Windmill Artifacts in 16-slice CT. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3104065.html
