Cardiac Motion artifacts in PET are a well known problem. The heart undergoes two types of motion, the motion due to respiratory displacement and the motion due to cardiac contraction. These movements lead to blurring of data and to inaccuracies in the quantification. In this study a continuity equation based optical flow method is presented and results on 3D PET patient datasets for cardiac motion correction are presented. The method was evaluated with respect to three criteria: correlation between the images, myocardial thickness and the blood pool activity curves. The results showed that the method was successful in motion correcting the data with high precision.