An apparatus for calibrating an x-ray computed tomography device has a plurality of objects formed from a material that is visible to x-rays, and a base at least in part fixedly supporting the plurality of objects so that each of the plurality of objects contacts at least one of the other objects. Each one of the plurality of objects: 1) is configured to receive x-rays without changing shape, 2) has substantially the same shape and size as the other objects, 3) has an attenuation value to x-rays (&ldquoobject attenuation value&rdquo), and 4) is symmetrically shaped relative to its center point. Like the objects, the base also has an attenuation value to x-rays (the &ldquobase attenuation value&rdquo). The object attenuation value is greater than the base attenuation. Each of the plurality of objects is kinematically locked in place on the base.
