The optical properties of a receptor-targeted probe designed for dual-modality mapping of the sentinel lymph node (SLN) was optimized. Specific fluorescence brightness was used as the design criterion, which was defined as the fluorescence brightness per mole of the contrast agent. Adjusting the molar ratio of the coupling reactants, IRDye 800CW-NHS-ester and tilmanocept, enabled us to control the number of fluorescent molecules attached to each tilmanocept, which was quantified by H1 nuclear magnetic resonance spectroscopy. Quantum yields and molar absorptivities were measured for unconjugated IRDye 800CW and IRDye 800CW-tilmanocept (800CW-tilmanocept) preparations at 0.7, 1.5, 2.3, 2.9, and 3.8 dyes per tilmanocept. Specific fluorescence brightness was calculated by multiplication of the quantum yield by the molar absorptivity and the number of dyes per tilmanocept. It predicted that the preparation with 2.3 dyes per tilmanocept would exhibit the brightest signal, which was confirmed by fluorescence intensity measurements using three optical imaging systems. When radiolabeled with Ga68 and injected into the footpads of mice, the probe identified SLNs by both fluorescence and positron emission tomography (PET) while maintaining high percent extraction by the SLN. These studies demonstrated the feasibility of 800CW-tilmanocept for multimodal SLN mapping via fluorescence and PET-computed tomography imaging.