1. A method for correcting a positron emission tomography (PET) emission image comprising: obtaining a PET emission sinogram of an object; obtaining a computed tomography (CT) image for a scanned portion of the object, the object having a truncated portion outside a field of view (FOV) of a CT image; determining a correction set of CT data based on a measured set of CT data within the CT sinogram; generating modified attenuation correction factors from the measured and correction sets of CT data; and correcting the PET sinogram using the modified attenuation correction factors.