This study evaluates the feasibility of performing dynamic [18F]-sodium fluoride (NaF) kinetic studies of the knee using PET/MRI with a focus on determining a robust method to produce an image derived input function (IDIF) using MRI images. Input functions are created using an angiography sequence to isolate the popliteal arteries and create intravascular ROIs centred in the artery to measure blood activity. 12 subjects were given two injections 75 min apart and the derived IDIFs were compared for reproducibility and accuracy. The resulting IDIFs had high reproducibility and gave values matching blood samples and literature values. Given its accuracy and robustness, this technique is well suited for clinical PET/MRI examinations.