Synopsis: Determination of human cardiac Pi using 31P-MRS is challenging as the resonance frequency of Pi is concealed by a close resonating 2,3-DPG signal originating from blood. Long TR acquisition using adiabatic excitation at 7T can compensate for the rapid blood signal replacement in partially-saturated short TR scans. In order to quantify Pi concentration in vivo, knowledge about longitudinal relaxation of Pi is still required. We have measured the T1 of Pi in 4 healthy volunteers at 7T using dual-TR method and used this value to quantify cardiac Pi concentration in 8 healthy volunteers.