This is a non-attenuation corrected (NAC) PET scan. The lungs and skin are bright while the heart and central areas of the body are dark. CT scans are obtained simultaneously with PET scans to gather tissue density information. The PET scan is then "attenuation-corrected" for differential absorption of pair-production photons. The NAC PET images are used by nuclear medicine physicians to confirm certain findings. Attenuation correction can lead to pitfalls in diagnosis. In the explanation images, notice that the central portion of each NAC scan is cold due to attenuation of photons from pair production. The PET-AC images are corrected based on attenuation calculated from the CT data. The lungs appear hot due to lower density and therefore significantly less attenuation relative to the surrounding tissues.