A phantom 10, and associated method of calibration for use in both medical resonance (MR) imaging and radiographic (x-ray) imaging systems, comprises a first component 12 having a first outer shape, a portion of which defines part of at least one pocket； and a second component 14 coupled to the first component and having a second outer shape, a portion of which defines another part of the at least one pocket [28, Fig 2] (into which a marker, for example a metal or ceramic ball bearing, can be placed). At least one of the first and second components comprises a reservoir [20, Fig 2], the reservoir having a shape at least a portion of which locates a centre of the at least one pocket. In use the reservoir is filled with a liquid, such as water or a water-based solution. The pocket may be spherical in shape and the/each portion of the first and second outer shapes are hemispherical. The first and second components may comprise at least one elongate member 16b, 18a, an end of which comprises the portion which defines part of at least one pocket.
