How to create an atlas based on PET image

Hello TTK community,
Here a little bit of a technical question. The goal is to create an atlas image based on several whole body PET image (starting from 100 ot finally 10000) in an efficient way.
Now i register the image with a multi resolution rigid transform one by one and calculate the mean of all image.
Is there a more accurate way ?
Thank you,
Cyril