Magnetic nanoparticles have been used in clinical and animal studies to generate localized heating for tumor treatments when the particles are subjected to an external alternating magnetic field. One approach to deliver the nanoparticles is via directly injecting the nanoparticles in the extracellular space of the tumor. Its advantage is that multiple-site injections can be exploited to cover the entire target region in the case of an irregularly shaped tumor. Currently since most tissue is opaque, the detailed information of the nanoparticle spreading after the injection can not be visualized directly and it is often quantified by indirect methods such as temperature measurements to inversely determine the distribution. In this study, we use a high-resolution microCT imaging system to investigate the nanoparticle concentration distribution in a tissue equivalent agarose gel. The preliminary results are promising to obtain a 3-D distribution of the ferrofluid in tissue. The local density variations induced by the nanoparticles in the vicinity of the injection site can be detected and analyzed by the microCT system. Experiments are performed to study how the injection amount, gel concentration, and nanoparticle concentration in the ferrofluid affect nanoparticle spreading in the gel. The obtained quantified information is important for future studies of temperature elevations in opaque tumor to design optimized treatment protocols.