Radial Difference Filter
For detection of small spherical targets, a radial difference filter is used. This filter is utilized in detection of metastatic lymph nodes  and small liver tumors  from CT images. This filter effectively detects small spherical-shaped targets. However, detection performance of the filter for detection of large targets is not ideal because of shape variations of the targets.
In the detection processes, the value of the radial difference filter is calculated for each voxel in an image. The center voxel in the image is selected. Searching rays are cast from the center voxel. For each searching ray, the different values of intensities of the image are calculated along the searching ray direction. A voxel having a high difference value of the intensity is selected as a point on boundary of the target. If the center voxel is located in a spherical-shaped region, the selected points are located on the border of the spherical-shaped region. The targets including spherical lesions can be detected by checking positions of the selected points.