Example apparatus and methods concern determining whether a target material appears in a region experiencing nuclear magnetic resonance. One method acquires a baseline value for a magnetic resonance parameter (MRP) while the region is not exposed to a molecular imaging agent that affects the MRP, acquiring a non-specific uptake value for the MRP while the sample is influenced by a non-specific molecular imaging agent and acquiring a specific uptake value for the MRP while the sample is influenced by a specific molecular imaging agent. The non-specific masking problem is solved by characterizing the region as a function of the baseline value, the non-specific uptake value, and the specific uptake value. The function relies on the similarities and differences between non-specific uptake of the non-specific molecular imaging agent, non-specific uptake of the specific molecular imaging agent, and specific uptake of the specific molecular imaging agent.