In order to study the relationship between the concentration of Gd-DTPA in tissue, and the resulting changes in relaxation times and signal intensity, a phantom material that has similar relaxation times to tissue and that can be doped with Gd-DTPA is required. The "tissue-equivalent" material should not contain Gd; nor should it alter the relaxivities of Gd-DTPA from their values in aqueous solution (R1 = 4.5 sec-1 mM-1; R2 = 5.5 sec-1 mM-1 at 1.5 T). Conventional materials, based on CuSO4-, MnCl2-, or GdCl3/LaCl3-agarose mixtures, are not suitable, since Gd is displaced from the Gd-DTPA chelate. The new material, consisting of Ni-DTPA dissolved in agarose, is easy to prepare and does not interact with Gd-DTPA. Its relaxation times are stable; relaxivity R1 was within 4% of its aqueous value over 109 days. T1s have low dependence on temperature (0.2-1.0%/degrees C at 21 degrees C) and on field strength, allowing the material to be used as a relaxation time standard for quality assurance. Equations giving the concentration of Ni-DTPA and agarose to produce a required T1 and T2 are provided.