While PET with [15O]H2O is the gold standard for imaging CBF, quantification requires measuring the arterial input function (AIF), which is an invasive and noisy procedure. ASL is an attractive alternative, however, its accuracy is limited by low SNR. Considering these limitations, we propose a hybrid PET/MRI approach using global CBF measurements from phase contrast MRI to convert [15O]H2O PET data into CBF maps. To test this method, using a large animal model, CBF was measured by this hybrid approach and by PET only, where the AIF was measured. Good agreement was found over a CBF range (20-100 ml/100g/min).