A hybrid attenuation correction technique (ACT) is under development for 18F-FDG total body positron emission tomography (PET). With a short transmission scan of the thorax, any time within a few days of the imaging session, this technique can correct for attenuation in the entire body. Segmentation, registration, and active contour finding techniques are applied to both emission and short transmission images to locate and map the major attenuating structures in the body. This technique eliminates the need for the patient to remain still from the start of the transmission scan to the end of the emission scan without the added noise of simultaneous or post emission transmission scan measurements. The results of volunteer studies are comparable to standard measured ACT, both visually and quantitatively. The efficient use of scanner time and improved patient comfort make this technique particularly suitable for clinical imaging.