In a method or system for determining a radiation dose of a radiopharmaceutical, magnetic resonance image data of an object under examination are acquired by operation of a magnetic resonance image data acquisition unit. At least one target area and/or at least one area at risk for accumulation of the radiopharmaceutical is/are segmented in the magnetic resonance image data. Molecular image data of the object under examination by operation of a molecular image data acquisition unit during the accumulation of the radiopharmaceutical in the at least one target area and/or the at least one area at risk. A radiation dose of the radiopharmaceutical is determined in the at least one target area and/or the at least one area at risk using the molecular image data.
