1. A method of correcting lung density variations in a positron emission tomography (PET) image of a subject comprising: identifying a pulmonary vasculature and 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 the pixels within the outer extent of the lung cavity as corresponding to the pulmonary vasculature or the lung tissue; and applying attenuation coefficients to correct for attenuation of gamma photons associated with the lung tissue and pulmonary vasculature in a reconstruction of the PET image based on the classification of the pixels within the outer extent of the lung cavity.