<p>Immune activity within tumors and secondary lymphoid organs critically influences cancer progression, metastasis, and treatment response. We present a dual-nucleus (¹H/¹⁹F) molecular MRI platform for non-invasive, high-resolution profiling of the tumor-immune microenvironment in immunocompetent mouse models. Tumor viability was visualized by engineering breast cancer cells to express a novel mouse-derived ¹H MRI reporter gene, enabling in vivo differentiation of viable and necrotic tumor regions. Concurrently, ¹⁹F MRI using perfluorocarbon (PFC) nanoemulsions enabled longitudinal tracking of immune cell infiltration, extending beyond conventional tumor-associated macrophage–focused approaches. Ex vivo analyses confirmed PFC uptake across diverse immune subsets, with tissue- and context-specific variations in ¹⁹F signal driven by differences in cell abundance and labeling efficiency. Notably, ¹⁹F MRI revealed a predominantly myeloid signature within tumors, a mixed myeloid/lymphoid profile in the spleen, and a lymphoid-skewed signal in tumor-draining lymph nodes. By integrating tumor-specific ¹H imaging with immune-resolving ¹⁹F imaging, this single-modality platform offers a comprehensive view of tumor architecture and immune cell presence within tumors. Our imaging approach enables discrimination of immunologically dense tumors and offers critical insights into the interpretation of ¹⁹F MRI signals within immune competent animal models, providing an immunoimaging tool that increases the translational relevance of preclinical therapeutic insights.</p>