Abstract:
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Purpose: This study investigates the dependence of the slice sensitivity
profile (SSP) on the distance from the center of rotation for 2D and 3D back
projection.
Methods and Materials: The slice width profile for helix scans was measured
using an 80mm thick metal disc perpendicular to the axis of rotation. The slice
width profile (SSP) was determined by the HU value of the metal disc as a
function of the image location along the axis of rotation. The reconstructed
images were located at intervals smaller than the measured slice width. The SSP
for axial scans was measured using a metal plate located at 450 to the axis of
rotation. The slice width was given by the HU value profile of the axial
projection of the metal plate, corrected for the known angle. The measurements
were performed for 2D (fan beam) and true 3D (cone beam) back projection at
varying distance from the center of rotation.
Results: The SSP was measured for axial scans using an 8x3mm mode. For 2D back
projection the slice width of the external slice, corresponding to a cone angle
of 1.20, varied from 3mm at the center of rotation to 3.8mm at 18-cm off-center
location. No such variation was measured for the 3D back projection. For the
central slices, with zero cone angle, the slice width did not depend on the
distance from the center of rotation for both 2D and 3D back projection. Slice
width of 0.8mm was measured in helix scans. For 2D back projection, an increase
of 80% was observed at a distance of 16 cm from iso-center relative to the SSP
at the iso-center itself. For 3D back projection the slice width was constant
as a function of the distance from the center of rotation. Using the 3D back
projection, each voxel is reconstructed from rays that truly traverse it. This
is not the case for 2D back projection as it neglects the cone angle, resulting
in a broadening of the slice width away from the center of rotation.
Conclusion: The true 3D (cone beam) back projection is superior to the 2D (fan
beam) back projection as it results in a slice width profile that does not
depend on the distance from the center of rotation. The increasing of the slice
width away from the center-of-rotation depends on the cone angle and is
inherent to the 2D back projection. (E.S., A.A. are employees of Philips
Medical Systems & Technologies.)
Questions about this event email: efrat.shefer@philips.com
Shefer PhD, E,
Comparison of CT Slice-width Profile in 3D and 2D Back Projection. Radiological Society of North America 2003 Scientific Assembly and Annual Meeting, November 30 - December 5, 2003 ,Chicago IL.
http://archive.rsna.org/2003/3104040.html
