Abstract: Children with metastatic solid tumors remain essentially incurable with conventional chemotherapy and radiation, with no significant improvement in survival over the past three decades. Another major hurdle is the life threating toxic effects of chemoradiotherapy to the patient. Therefore, more “tumor-specific” therapies are needed to improve survival andminimize toxicity. Presently at the University of Wisconsin Carbone Cancer Center (UWCCC), we are one of the few FDA-approved centers to administer natural killer (NK) cells after haploidentical (from mother or father) hematopoietic stem cell transplant (haploSCT) to children with progressive or refractory, metastatic solid tumors. The anti-tumor effects of this, however, have not been sufficient to cure these patients. Some patients develop graft-versus host disease (GVHD), which is when donor cells attack the host tissues. The development of GVHD impedes to conclusive results on the efficacy of NK cell infusions. In addition, it is not known if NK cells traffic directly to tumors and mediate cytotoxicity, or if they traffic to other tissues and stimulate other effector cells, thus indirectly eliminating the tumor. However with current imaging technology, we are unable to ascertain the fate of the NK cells after they are infused. Previous studies have shown the use of an isotope of fluorine, 19F, as a noninvasive method of tracking murine and human dendritic cells. In this study, we aim to optimize the non-toxic tracer agent 19F using magnetic resonance imaging (MRI) to track NK cells in vivo. This method not only successfully tracks but also quantifies the number of apparent cells in a region of interest. This study will enable us to monitor the fate of NK cells after their infusion allowing us to determine how they induce their anti-tumor effects.