Author: Marc Kachelrieß
To provide a phantom that reveals partial volume artifacts as well as stepping artifacts in three dimensions.
The Artifact Phantom consists of several parts. All phantom parts have a density of twice the density of water. The complete phantom is contained in a infinitely long water cylinder with a diameter of 200 mm. It consists of 12 objects which are labeled in the images below with numbers 1 through 12.
- All cylinders are parallel to the z-axis.
- The bottom is made of three transaxial cylinders with 160 mm diameter and 4 mm height, separated by a gap of 1 mm. Cylinders (1) and (2) are solid, cylinder (3) is hollow and has a wall thickness of 10 mm. The cylinders shall provoke artifacts in non-exact cone-beam algorithms (similar to the Feldkamp Killer Phantom).
- After another 1 mm gap follows the hollow cylinder (4), 160 mm in diameter with a thickness of again 10 mm. It is clipped at its top by a plane tilted by half the cone angle of the scanner. Here we have taken 5° as the tilt angle. The clipping with the tilted plane allows to provoke partial volume artifacts not only for transaxial rays but for all rays with angles between -5° and 5°, depending on what angular position the source has when passing through the clip plane.
- On the other side of the said clip plane there is a 20mm solid cylinder (5). This cylinder serves together with cylinder (4) to provoke nonlinear partial volume artifacts for all available rays. It is clipped at a height of 20 mm with a second plane that is parallel to the first plane.
- On top of the second plane are five clipped cylinders (8)...(12) centered around the corners of a symmetrical pentagon. These centers lie at a distance of 50 mm away from the z-axis, i.e. the five pentagon cylinders lie on a circle of 100 mm diameter. These five cylinders end at different z-positions where they merge in five cones (same label as the five cylinders), as becomes clear from the figures and from the phantom definition file. The tips of these cones lie at z=200 mm. The cones serve to study stepping artifacts. Each cone has a different slope, depending on its length.
- In the middle of the five cylinders/cones are located two hollow spheres (6) and (7) with outer diameters of 20 mm and 40 mm respectively. Their wall thickness is 1 mm and 2 mm respectively. They allow to provoke stepping artifacts and to study the reconstruction of thin and arbitrarily located plane objects (the surfaces are of low curvature, hence they can locally be regarded as being plane). Since the spheres touch at z=100 mm and thus form a three dimensional wedge, they also allow to study (more or less quantitatively) the spatial resolution of a given reconstruction algorithm.
The images presented below are of size 240 mm x 240 mm.
Figure 1: Axial cuts through the artifact phantom. The phantom is shown at four different view angles such that four of the five cones are cut by the view plane. The dashed yellow lines show intersection of the two clip planes with the image planes. The red arrows depict the z-positions of the three transaxial views shown in figure 2.
Figure 1: Transaxial
images at the z-positions depicted in figure 1.
The dashed yellow line shows the intersection of the bottom clip plane with the image plane.
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