Ring-Artifact Removal removes ring artifacts from CT scans. Ring artifacts in CT images appear as concentric circular patterns centered on the rotation axis and are typically caused by defective, unstable, or improperly calibrated detector elements during scanning. In CT scans these artifacts can obscure internal features and interfere with accurate microstructural analysis. The applied algorithm is based on the work of Sijbers and Postnov, 2004.

The first step in the filtering process is defining the midpoint of the rotation axis and the extent of the artifacts. Based on this midpoint, the filter searches for artifacts at every possible Radius. At each radius, the extent examined is given by the Window size (see the Red Ring in the image below). The effect of the ring artifacts on the gray values is examined within these extents.

However, the algorithm requires correction wherever multiple phases are affected. Therefore, a Threshold value must be provided to define the maximum gray value range. This value varies depending on the material and scan. The threshold value must be smaller than the variance between the ring artifacts and the average gray value inside a window around the ring artifact. Note that there is a significant difference in gray values between 8-bit and 16-bit images, for example.

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

Click Apply to remove the ring artifacts according to the defined settings.

In the example below, Ring-Artifact Removal has been selected as the Overlay. The Ring-Artifact Removal overlay is displayed with yellow lines. The intersection of the straight yellow lines indicates the specified Center. The yellow circle indicates the specified Radius. You can also change both directly via the overlay. Click on the center of the circle to move the center point, and click on the outer circle to change the radius.

Below, red and blue circles are added to illustrate how the Ring-Artifact Removal considers each blue ring within the yellow circle. These rings then form a center line for a new red Window. The Ring-Artifact Removal filter only examines values within the red area for this single blue ring. However, the filter checks for all possible rings within the yellow circle. The filter calculates the variance of gray values within the blue ring compared to the average gray value inside the red window. If the variance is higher than the given threshold, a ring is assumed to be present. Then, the gray values of the identified ring voxels are adjusted to match the average gray value in the red window.