1. A method of processing a set of Positron Emission Tomography (PET) scan images comprising the steps of: administering a PET scan contrast agent and a magnetic resonance imaging (MRI) contrast agent to a subject; simultaneously performing a PET scan and an 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 blood vessels, using data acquired from the MRI scan; and performing pharmacokinetic analysis based on i) the value of V B so derived and ii) the PET scan images.