A phantom and method are provided for co-registering a magnetic resonance image and a nuclear medical image. The phantom includes a first housing defining a first chamber configured to receive a magnetic resonance material upon which magnetic resonance imaging can be performed in order to produce the magnetic resonance image. The phantom also includes three or more second housings configured to be attached to the first housing, where the second housings each define a second chamber configured to receive a radioactive material upon which nuclear imaging can be performed in order to produce the nuclear medical image and upon which the magnetic imaging can be performed in order to produce the magnetic resonance image. The first chamber has a volumetric capacity that is larger than a volumetric capacity of each second chamber.