In this work we focus on the imaging of magnetic nanoparticles in tumor tissue. The spatial distribution of magnetic nanoparticles in tumor tissue is an essential parameter in the evaluation of the efficiency of magnetic drug targeting (MDT). We developed a volume-based imaging method for x-ray-micro tomography calibrated by magnetorelaxometry to determine the concentration of magnetic nanoparticles in a certain tissue. With this technique, the x-ray absorption information contained in a mu-CT image can be directly related to an absolute mass of magnetic nanoparticles in a certain volume element of the tumor. In contrast to other related methods reported in the literature, the procedure described here is capable of a mass resolution of 0.044 mu gmm(-3), making it possible to map the particle distribution in tumors with extremely low magnetic particle content, such as are usually found in mouse experiments on MDT.