I want to calculate mean

i have pet dicom file of cubic , in this image there is lattice and also air i put FDG and sent for pet , after pet scan i want to calculate the mean in air and lattice.
any one can guide me how i can calculate the mean from pet image

Define the air and lattice regions using Segment Editor. Then use the Segment Statistics module to get all kinds of statistics including the mean of the image content under these segments.