Hi, Dear Kris and STIR users, I have a question about the format of 3d sinogram. When I was trying to bin my raw data into a 3d sinogram, I don't know what's the exactly definition of positive and negative ring difference. For example, if there's a crystal pair between crystal_i and crystal_j with the axial position of z_i and z_j. From the raw data, I could get two combinations of the same coincidence pair as (C_i, C_j) or (C_j, C_i). Obviously these two type of combinations should goes to the same oblique sinogram. But how to calculate the ring difference in this case? Ring difference could be either (z_i-z_j) or (z_j-z_i). Which one is correct? If I choose one of them, for example, z_i-z_j, and with the value of z_i=1, z_j=2, then the ring_difference = -1, if I choose the other, the ring_difference = +1. I think the difference of these two definitions are the same as switch the cells in the Michelogram between up-left corner and bottom-right corner. I have tried to bin the sinogram in both way and do the reconstruction by OSEM program, and the results looks almost the same. Does that mean the absolute direction of the ring difference is not important? or there's something else I misunderstood? Thanks. Yuxuan