The Ring-Artifact Removal removes ring artifacts from CT scans.

The first step of the filter workflow is the definition of the midpoint of the rotation axis and the extent of the artifacts. Based on the midpoint, the filter looks for artifacts at every possible radius (see blue ring in image below). At each of these radii, an extent is examined as given by the Window size (see red ring in image below). Within these extents, the effect of the Ring-Artifacts on the gray values is examined.

However, the algorithm requires a correction wherever multiple phases are affected. Therefore, a Threshold value must be given in order to define a maximum gray value range. That value varies with different materials and scans. It must be smaller than the variance between the ring-artifacts and the average gray value inside a window around the ring artifact.

Please note that there is a significant difference in gray values between, e.g., 8-bit and 16-bit images.

• Center X, Y: Find and insert the center of the Ring-Artifacts (i.e., the rotation axis).
• Window: Determine here the extent in which Ring-Artifacts can be identified (see red ring below).
• Threshold: Determine here the maximum gray value range for the windows. This affects the correction that is applied as averaged gray value.
• Radius: Determine and insert the maximum extent of the Ring-Artifacts.

Click Apply to remove the the Ring-Artifacts according to the defined settings.

In the example below, Ring-Artifact Removal is chosen as Overlay. The Ring-Artifact Removal overlay is displayed with yellow lines. The yellow circle is the overlay and defines the area determined by Center and Radius. It can also be changed by clicking on it and dragging it. Click in the center of the circle to move the center point and click on the outer circle to change the radius.

Here, a red and a blue circle are added to visualize how the Ring-Artifact Removal considers each blue ring within the yellow circle. Each of these rings then builds a center line for a new red Window.

For this single blue ring, the Ring-Artifact Removal filter only examines values within the red area. But considering window after window, the filter checks for all possible rings inside the yellow circle.

The Ring-Artifact Removal calculates the variance of gray values within the blue ring in comparison with the average gray value inside the red window. If the variance is higher than the given threshold, it is assumed that there is a ring present.

Then, the gray values of the identified ring voxels are reduced to match the average gray value in the red window.

You can find more details on the applied algorithm in Sijbers and Postnov (2004).