Radiation-sensitive polymer gels have been adapted for making test objects that can be used to assess the imaging characteristics of magnetic resonance imaging (MRI) systems. The polymer gels contain acrylic monomers within a gel matrix, and when irradiated with x rays the constituents polymerize to produce highly cross-linked microparticles that dramatically affect water NMR relaxation rates where they form. The size of these effects depends on the radiation dose and composition of the mixture irradiated, while the spatial pattern of relaxation time changes can be precisely controlled by spatial modulation of the x-ray exposure. This permits the manufacture of complex test patterns free of susceptibility or edge effects, and overall image performance can be assessed by constructing contrast-detail diagrams using a singly irradiated gel containing areas of different sizes and contrasts. Polymer gels are stable and a variety of different tests objects can be constructed inexpensively. Such materials and test phantoms may find widespread application in diagnostic MRI quality assurance and testing programs.