A phantom according to the invention features a radiotransmissive vessel and a removable closure member, which together define a sealed chamber. A non-magnetic support member extends through a wall of the chamber--preferably through the closure member--and supports a hollow container within the sealed chamber. The support member is pivotally supported through the wall of the chamber by means of a non-leak pivot joint. The support member supports a magnetically drivable element--either a magnet or a ferromagnetic mass--on the opposite end that is not disposed within the chamber. By moving a co-acting magnetically driving element, movement of the support member, and hence the container within the chamber, can be controlled.