Background: The measurement of the concentration of theranostic agents in vivo is essential for the assessment of their therapeutic efficacy and their safety regarding healthy tissue. To this end, there is a need for quantitative T 1 measurements that can be obtained as part of a standard clinical imaging protocol applied to tumor patients. Purpose: To generate T 1 maps from MR images obtained with the magnetization-prepared rapid gradient echo (MPRAGE) sequence. To evaluate the feasibility of the proposed approach on phantoms, animal and patients with brain metastases. Study Type: Pilot. Phantom/Animal model/Population: Solutions containing contrast agents (chelated Gd 3+ and iron nanoparticles), male rat of Wistar strain, three patients with brain metastases. Field Strength/Sequence: A 3-T and 7-T, saturation recovery (SR), and MPRAGE sequences. Assessment: The MPRAGE T 1 measurement was compared to the reference SR method on phantoms and rat brain at 7-T. The robustness of the in vivo method was evaluated by studying the impact of misestimates of tissue proton density. Concentrations of Gd-based theranostic agents were measured at 3-T in gray matter and metastases in patients recruited in NanoRad clinical trial. Statistical Tests: A linear model was used to characterize the relation between T 1 measurements from the MPRAGE and the SR acquisitions obtained in vitro at 7-T. Results: The slope of the linear model was 0.966 (R 2 = 0.9934). MPRAGE-based T 1 values measured in the rat brain were 1723 msec in the thalamus. MPRAGE-based T 1 values measured in patients in white matter and gray matter amounted to 747 msec and 1690 msec. Mean concentration values of Gd 3+ in metastases were 61.47 μmol. Data Conclusion: The T 1 values obtained in vitro and in vivo support the validity of the proposed approach. The concentrations of Gd-based theranostic agents may be assessed in patients with metastases within a standard clinical imaging protocol using the MPRAGE sequence. Evidence Level: 2. Technical Efficacy: Stage 1.