1. A method, comprising: accessing a set of known magnetic resonance fingerprinting (MRF) signal evolutions, where a member of the set of known MRF signal evolutions was produced by combining data associated with nuclear magnetic resonance (NMR) signals associated with a plurality of resonant species, where combining the data involves using a forward operation for which an inverse operation that identifies relative fractions of resonant species associated with the member is available; accessing an acquired NMR signal, where the acquired NMR signal was produced by a volume that contains one or more resonant species, where the one or more resonant species simultaneously produced individual NMR signals in response to MRF excitation produced by an MRF pulse sequence; and performing the inverse operation on the selected entry to determine relative fractions of the one or more resonant species for the volume.