FIGURE 4. Average attenuation of 2 oral CT contrast agents at 3 different concentrations measured in CT and with PET 511-keV transmission scans. CT attenuation increases with increasing concentration of contrast agent. PET attenuation, however, depends minimally on concentration of contrast agent and is only slightly higher than attenuation of water. Transformation applied by scanner software to convert CT numbers into PET attenuation coefficients in Discovery LS scanner is indicated as dashed line (3). If it is applied to measured CT numbers, too high PET attenuation coefficients are calculated ().