A phantom has a casing in which vials are arranged, preferably in rows and columns. The vials are filled with solutions of a substance which appears in the imaging modality to be tested. The solutions are of different concentrations; for example, the concentration can increase row by row. The solutions can contain two substances which appear in the imaging modality, in which case the concentration can increase row by row for one and column by column for the other. The phantom can be used to test the linearity of the response of a DCE-MRI or other medical imaging device and to determine whether the fault lies with the coil or the pulse sequence.