The dispersion (frequency dependence) of the spin-lattice relaxation time in the rotating frame, T1 rho, is considered for tissue characterization. Methods for the volume-selective determination of the proper T1 rho dispersion and for imaging of parameters characterizing this frequency dependence are described. On- and off-resonance versions of the techniques are demonstrated. In vitro studies of excised rat tissues and in vivo applications to mice with implanted adenocarcinoma are reported. T1 rho dispersion images show clear contrasts of the malignant tissue, whereas muscle tissue is completely suppressed. No contrast agent is required. The measuring time is only twice as long as that for conventional magnetic resonance images. The results suggest that the T1 rho dispersion is less susceptible to the biological variability than the absolute values of the relaxation times.