The invention provides for a method of operating a medical instrument (100, 400, 500, 600), with magnetic resonance imaging system (102). The method comprises acquiring (202) equilibrium magnetization magnetic resonance imaging data (148) by controlling the magnetic resonance imaging system according to a T1 measuring magnetic resonance imaging protocol and calculating an equilibrium magnetization baseline image (156). The method further comprises repeatedly acquiring (206) the dynamic PRFS magnetic resonance data according to a proton resonance frequency shift magnetic resonance imaging protocol. The method further comprises repeatedly acquiring (208) magnetic resonance data portions (152) according to the T1 measuring magnetic resonance imaging protocol with a saturation preparation (804) at the start of the acquisition. The acquisition of the dynamic PRFS magnetic resonance data and the magnetic resonance data portions are interleaved. The method further comprises repeatedly reassembling (212) the resonance data portions into dynamic T1 magnetic resonance data. The method further comprises repeatedly calculating (214) a T1 map (158) using the reassembled dynamic T1 magnetic resonance data and the equilibrium magnetization image. The method further comprises repeatedly calculating (216) a PRFS phase calibration (160) using the dynamic PRFS magnetic resonance data and the T1 map. The method further comprises repeatedly calculating (218) a PRFS temperature map (162) using the dynamic PRFS magnetic resonance data and the PRFS phase calibration if the PRFS phase calibration has been calculated.
