1. A method of correcting lung density variations in a positron emission tomography (PET) image of a subject comprising: identifying at least an outer extent of a lung cavity in a magnetic resonance (MR) image corresponding to a thoracic region of the subject in response to an intensity associated with pixels forming the MR image, the MR image corresponding to the PET image of the thoracic region of the subject; classifying each pixel within the outer extent of the lung cavity as either pulmonary vasculature or lung tissue; assigning a first specified attenuation coefficient to those pixels classified as pulmonary vasculature and a second specified attenuation coefficient to those pixels classified as lung tissue, wherein the first specified attenuation coefficient is greater than the second specified attenuation coefficient; and reconstructing the PET image using the assigned attenuation coefficients to correct for attenuation of gamma photons.