Phantom for verifying the calibration of PET scanners, comprising a substantially cylindrical body having a cross section with a convex curvilinear profile, and a plurality of spheres placed within said body, comprising a solid matrix of germanium-68, the phantom being characterized in that it further comprises a reference sphere filled with a solid matrix of germanium-68, said reference sphere being connected to the phantom in such a way that it can be removed and replaced in the same position.