Lung cancer is the leading cause of cancer deaths. Safe and specific MRI probes are needed to enable early detection of lesion presence and therapeutic response. Injected PFC nanoemulsion, taken up by tumor associated macrophages (TAMs), can be used as a biomarker to detect metastases using 19F MRI. In a metastatic lung cancer mouse model, we show that PFC is effectively taken up by TAMs and vividly displays lung metastasis using 19F MRI. Validation assays using in vivo bioluminescence and histology support the MRI findings. Overall, 19F hot-spot imaging offers a highly-specific marker of tumor burden in lung parenchyma.