1. An apparatus for calibrating an x-ray computed tomography device, the apparatus comprising: first, second, and third spheres formed from a material that is visible to x-rays, the three spheres being configured to receive x-rays without changing shape, each of the three spheres having a sphere attenuation value to x-rays; a base at least in part supporting the three spheres so that each of the spheres contacts at least one of the other spheres, the base having a base attenuation value to x-rays, the sphere attenuation value being greater than the base attenuation value, each of the spheres being locked in place on the base in a manner that prevents sphere translational movement, the second sphere contacting the first and third spheres, the first sphere having a first center, the second sphere having a second center, the third sphere having a third center, the first and second centers forming a first line, the first and third centers forming a second line, the first and second lines diverging.