Detection and removal of sentinel lymph nodes (SLN) is important in the diagnosis and treatment of cancer. The SLN is the first regional lymph node draining the primary tumor, and if the cancer has spread, it is most likely to find metastases in the SLN. In this study, we have for the first time been able to image the very same contrast agent, superparamagnetic iron oxide nanoparticles (SPIO-NPs), in rat SLNs by using both our frequency- and phase-gated magnetomotive ultrasound (MMUS) algorithm and conventional magnetic resonance imaging (MRI); MMUS post mortem, MRI in vivo. For both higher NP-concentration and smaller NPs, we found that the MMUS data showed a larger magnetomotive displacement (1.56 ± 0.43 and 1.94 ± 0.54 times larger, respectively) and that the MR-images were affected to a higher degree. The MMUS displacement also increased with lower excitation frequency (1.95 ± 0.64 times larger for 5 Hz compared with 15 Hz) and higher excitation voltage (2.95 ± 1.44 times larger for 30 V compared with 10 V). The results show that MMUS has potential to be used as bedside guidance during SLN surgery, imaging the same particles that were used in prior staging with other imaging techniques.