CT image of the CTP657 phantom with tubes numbered for identification. The z-axis of the phantom is parallel to the z-axis of the CT scan, which is perpendicular to the page. The axes of Tubes 1, 2, 3, and 4 are aligned with the phantom z-axis, while Tubes 1t and 3t are tilted 30° with respect to the phantom z-axis. The foam also contains multiple holes, including larger openings for inserts of any materials of interest.