1. A method of processing a set of Positron Emission Tomography (PET) scan images comprising the steps of: administering a suitable PET scan contrast agent and a suitable MRI contrast agent to a subject; simultaneously performing a PET scan and a Magnetic Resonance Imaging (MRI) scan on the subject to generate PET scan images and corresponding data from the MRI scan; calculating V B the fraction of pixel volume occupied by the blood vessels using the data acquired during the MRI scan and performing pharmacokinetic analysis based on the value of V B so derived and the PET scan images.