Objectives: Hybrid cardiac imaging using combined positron emission tomography (PET) and magnetic resonance (MR) systems is an upcoming imaging technology, currently under evaluation for clinical use. The aim of this work is to intraindividually compare results from left ventricular function and left ventricular volumes evaluated by an integrated PET-MR system using a dual tracer protocol.Methods: Dual tracer [13N]ammonia ([13N]NH3) (750 ± 230MBq) and 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) (310 ± 92MBq) cardiac PET-MR was performed in 13 patients for evaluation of cardiac viability in ischemic heart disease. Left ventricular ejection fraction, endiastolic volumes (EDV), and endsystolic volumes (ESV) were derived from 3 Tesla cardiac MR as well as from the simultaneously acquired [13N]NH3 and [18F]FDG PET images. MR data were calculated from short axis cine images in the endiastolic and endsystolic phase during breath hold. PET data were derived from the myocardial phase during free breathing using 16 gates in a time frame of 15 minutes for [13N]NH3 and 10 or 20 minutes for [18F]FDG.Results: Mean values for LVEF derived from MR, [13N]NH3 PET and [18F]FDG PET were 45 ± 16%, 48 ± 17% and 46 ± 17%. Mean values for EDV were 188 ± 66ml for MR, 171 ± 85ml for [13N]NH3 PET and 159 ± 86ml for [18F]FDG PET. For ESV MR derived values were 122 ± 68ml; [13N]NH3 PET values were 101 ± 76ml and [18F]FDG PET values were 96 ± 78ml respectively. LVEF showed an excellent correlation between MR and [13N]NH3 PET (R = 0.96; mean difference 4.6%; range 0-14%) as well as between MR and [18F]FDG PET (R = 0.89; mean difference 6%; range 1-17%). In addition, significant correlations have been observed between EDV obtained from MR and by [13N]NH3 PET (R = 0.86; mean difference 39ml; range 7-106ml), and between EDV measured by MR and [18F]FDG PET (R = 0.83; mean difference 43ml; range 0-109ml). Similarly, the comparison of ESV values acquired from MR with those acquired from [13N]NH3 PET and [18F]FDG PET revealed an excellent correlation (R = 0.94; mean difference 29ml; range 7-60ml) and (R = 0.88; mean difference 32ml; range 13-82ml) respectively; all P < 0.001.Conclusion: Left ventricular volumes and left ventricular function simultaneous derived from an integrated PET-MR do show an excellent correlation between the data derived from the MR and the PET system with slightly better results for [13N]NH3 PET compared to [18F]FDG PET when considering MR as the gold standard.