Rabi , Bloch , and Purcell observed that magnetic nuclei , like and , could absorb RF energy when placed in a magnetic field and when the RF was of a frequency specific to the identity of the nuclei .
Rabi , Bloch , and Purcell noticed that magnetic nuclei , like 1H and 31P , could absorb RF energy when placed in a magnetic field of a strength specific to the identity of the nuclei .