Markers, Phantoms and Associated Methods for Calibrating Imaging Systems

A marker for calibrating both an MRI system and an x-ray system comprises a first non-magnetic component 12 of a first mass density and a first hydrogen proton density and a second non-magnetic component 12 of a second mass density and a second hydrogen proton density. The first mass and proton densities are different from the second mass and proton densities. The first component may contact the second component such that a boundary between the components is visible in both MRI and x-ray images. The first component may be a ceramic and the second component may be a liquid. The first component is visible in MRI images and the second component is visible in x-ray images. Several markers may be incorporated into a phantom for registering the coordinate systems of an MRI and x-ray scan. The marker may be used in a combined radiotherapy and imaging system.