In the cone-beam composite-circling mode, the trajectory is composited by two circular motions. An x-ray focal spot within an
x-ray source is rotated at constant or variable speeds on a plane facing a short object to be reconstructed, while the x-ray tube is rotated around the object on the gantry plane. In this paper, we describe an algorithm for determining the boundary surface of the region for PI-line-based exact reconstruction using composite-circling curves. Such an exact reconstruction region consists of all PI-segments on the scanning curve. For different combinations of scanning parameters, we find the radius of the largest sphere centered at the origin that can be embedded in the region. The exact
reconstruction region is found to be the largest, reaching the radius of the focal spot circling facing the object, when the ratio of the focal spot circling frequency to the x-ray tube rotation frequency is two. The numerical results are also compared with those of standard saddle curves.